Impedance-derived phase angle is associated with muscle mass, strength, quality of life, and clinical outcomes in maintenance hemodialysis patients.

INTRODUCTION: We aimed to evaluate the association between the phase angle and muscle mass, muscle strength, physical performance tests, quality-of-life scales, mood scales, or patient and hospitalization-free survival rates in hemodialysis (HD) patients.
METHODS: We included 83 HD patients. The patients were divided into tertiles based on phase angle value. The phase angle was measured using a bioimpedance analysis machine. Thigh muscle area per height squared (TMA/Ht2), handgrip strength (HGS), nutritional indicators, physical performance, quality-of-life, depression or anxiety status, and the presence of hospitalization or death regardless of cause were evaluated.
RESULTS: In our study, no significant differences were observed in the serum albumin level and body mass index according to tertiles of phase angle. The phase angle tertiles were associated with TMA/Ht2 and HGS. The phase angle was also associated with physical performance measurements and depression or anxiety status. Subgroup analyses according to sex, age, and diabetes mellitus showed similar trends to those of the total cohort. Furthermore, the hospitalization-free survival rate and patient survival rate were favorable in patients with high values for the phase angle.
CONCLUSION: The present study demonstrated that the phase angle is associated with muscle mass, strength, physical performance, quality-of-life scale, and hospitalization-free survival in maintenance HD patients.

Introduction

Chronic kidney disease is one of the most important global health problems with increasing prevalence [1]. It can progress to end-stage renal disease requiring renal replacement therapy. Hemodialysis (HD) is the most commonly used modality among renal replacement therapies [2]. Patients undergoing HD have a high risk of developing chronic pathologies such as insulin resistance and/or chronic inflammation, which lead to accelerated aging [3]. Consequently, HD patients have a high prevalence of malnutrition, protein-energy wasting, or frailty [4]. The evolution of HD techniques has increased the survival of HD patients; however, their complications are yet to be resolved, and lead to decreased quality of life and poor patient survival [1]. Therefore, identification of early indicators or interventions for these patients is needed to overcome those complications.

Bioimpedance analysis (BIA) is a popular method for estimating body composition in clinical practice. The BIA machine is an easy, safe, and inexpensive tool to use. It was originally designed to measure the impedance of the human body, which led to the development of specific regression equations using impedance estimates of body composition [5]. Aside from body composition measurements, BIA can determine the phase angle, which is the ratio of resistance to capacitive reactance of electrical current [6]. The specific equations for predicting body composition are not accurate in the presence of various conditions. However, the phase angle is a raw parameter without modification from specific equations. Although the accurate meaning of the phase angle is not completely understood, previous studies have shown that the phase angle is associated with nutritional status and survival in HD patients [7-9]. However, only a few studies provide comprehensive data including accurate measurements of muscle mass, muscle strength, various physical performance tests, quality-of-life scales, mood scales, and patient and hospitalization-free survival rates. In this study, we aimed to evaluate the association between the phase angle and these variables in HD patients.

Patients and methods

Study population

The study participants were initially enrolled in a previous study [10]. Briefly, this study was performed in a tertiary medical center between September 2012 and March 2015. We included all patients undergoing HD with age ≥ 20 years, dialysis duration ≥ 6 months, ability to ambulate without the use of an assistive device, ability to communicate with the interviewer, and no hospitalization within the last 3 months before enrollment. This study was approved by the institutional review board of CHA Gumi Medical Center (No. 12–07). Written informed consent was obtained from all subjects involved in the study. Consent was obtained from each participant because all participants had the ability to communicate with the interviewer and did not include minors. None of the patients were taking opioids, antihistamines, or antidepressants, which are drugs associated with decreased physical activity and cognitive function. A total of 84 patients were enrolled and 1 patient was excluded owing to lack of phase angle data. Finally, 83 patients were included in our analysis. The patients were divided into tertiles based on the phase angle value as follows: low tertile, middle tertile, and high tertile.

Baseline variables

The collected baseline data were sex, age, presence of diabetes mellitus (DM), dialysis vintage, hemoglobin (g/dL), high-sensitivity C-reactive protein (mg/dL), blood urea nitrogen (mg/dL), creatinine (mg/dL), aspartate transaminase (U/L), alanine transaminase (U/L), calcium (mg/dL), phosphorus (mg/dL), sodium (mEq/L), potassium (mEq/L), chloride (mEq/L), intact parathyroid hormone (pg/mL), total cholesterol (mg/dL), albumin (g/dL), and Single-pool Kt/Vurea (spKt/Vurea). DM was defined as a patient-reported history and a medical record of a DM diagnosis or medication. spKt/Vurea was calculated using Daugirdas’ formula [10, 11].

Assessment of phase angle, muscle mass or strength indices, and subjective global assessment score

In our study, all patients underwent three HD sessions per week. All measurements, including BIA, muscle mass, strength, and physical performance, were performed on the day after the midweek HD session. Therefore, all measurements were performed regardless of fluid status between the intracellular and extracellular compartments or influence of HD sessions.

The phase angle was measured using a multifrequency BIA system (InBody, Seoul, Korea). The value was calculated using the angle value of the time delay between the voltage waveform at 50 kHz and the current waveform. Briefly, eight electrodes were placed (two on each foot and two on each hand) with the patient in an erect position. Using the reactance (Xc) and resistance (R) values obtained from the BIA system at 50 kHz, the phase angle was estimated using the follow formula: phase angle (°) = arctangent (Xc/R) × (180/π).

Body mass index (BMI, kg/m2) was calculated as body weight per height squared. Handgrip strength (HGS) was measured in all patients. Each patient performed three trials with the dominant hand using a manual hydraulic hand dynamometer (Jamar®; Sammons Preston, Chicago, IL, USA). The maximum value among the three trials was selected. Subjective global assessment (SGA) was calculated using scores from seven items (weight loss, dietary intake, gastrointestinal symptoms, functional capacity, comorbidity, decreased fat, and decreased muscle) [12]. The thigh muscle area (TMA, cm2) was calculated using midthigh computed tomography (CT) with a 320-slice CT scanner (Aquilion ONE; Toshiba Medical Systems Corp., Tokyo, Japan). An axial image was obtained at the midpoint of a line extending from the superior border of the patella to the greater trochanter (3-mm thickness, five slices). The images were analyzed using an image analysis software (ImageJ 1.45S; National Institutes of Health, Bethesda, MD, USA). Finally, TMA was adjusted using height squared.

Assessment of physical performance, health-related quality of life, hospitalization, and survival

Gait speed (GS, m/s) was evaluated using the time (s) for 4-m walking. The low GS group was defined as those with a speed of ≤ 1 m/s [13]. For the five times sit-to-stand test (5STS, s), each patient was seated on a chair with the arms crossed and the hands touching the shoulders [14]. The patients were asked to stand up and sit down five times as quickly as possible, and the time taken in seconds was recorded. For the 30 s sit-to-stand test (STS30), the patients were seated on a chair with the arms crossed and the hands touching the shoulders. Scores were defined as the number of stands a patient could complete in 30 s without using the arms as support [15]. For the 6-min walk test (6-MWT, m), the patients were asked to walk at their usual pace for 6 min, and the distance covered was recorded in meter [16]. For the timed up-and-go test (TUG, s), the patients were instructed to stand up from an armchair, walk 3 m, turn around, return to the chair, and sit down [17]. The time in seconds was recorded. The results of the Short Physical Performance Battery test (SPPB) were determined using the GS, 5STS, 6-MWT, and balance test results, which were scored between 0 and 12 [18].

The presence of frailty was defined using Johansen’s method [19]. Briefly, slowness, poor endurance, physical inactivity, and unintentional weight loss were defined as components of frailty. The presence of each frailty component was scored as 1, and the scores of all components were summed. Patients scoring ≥ 3 points were defined as having frailty. HRQoL was assessed using the Korean version of the Kidney Disease Quality of Life Short Form version 1.3 (KDQOL-SFTM 1.3) [20]. Briefly, KDQOL- SFTM 1.3 includes the Short Form-36 scale (36 items) and the kidney disease-specific scale (11 items). The total score (from 0 to 100) was calculated for each domain. A low score means a low quality of life. The scores of the physical component scale (PCS) and mental component scale (MCS) were calculated according to previous reports [21, 22]. The kidney disease component scale (KDCS) was evaluated using the sum of scores from 10 kidney disease-specific items except sexual function. The Beck Depression Inventory (BDI) and Beck Anxiety Inventory (BAI) were evaluated as previously reported, for which a high score indicates severe depression or anxiety status [23]. Questionnaires were completed during the dialysis sessions. In addition, we determined whether the patient had limitations in performing vigorous or moderate physical activity. Vigorous or moderate physical activity was defined based on the World Health Organization guidelines [24]. The patients selected one among the following three answers: severe limitation, some limitation, or no limitation. The presence of hospitalization regardless of cause and survival at the end point of follow-up were evaluated.

Statistical analysis

Data were analyzed using the statistical software IBM SPSS Statistics version 25 (SPSS Inc., Chicago, IL, USA). Categorical variables are expressed as counts (percentages). Continuous variables are expressed as mean ± standard deviation or standard error. For continuous variables, means were compared using one-way analysis of variance, followed by post-hoc Tukey comparison, and analysis of covariance for multivariate analysis. The correlation between two continuous variables was assessed using Pearson’s or partial correlation analysis. Linear regression analysis was performed to assess the independent predictors of TMA/Ht2, HGS, or GS. The results of multivariate analysis were adjusted for age, sex, and DM. Kaplan-Meier analysis was used to plot survival among the groups, and the Beslow method was used to determine statistical significance. We calculated the sensitivity, specificity, and probability of area under the receiver operating characteristic curve (AUROC) to predict frailty or low GS using phase angle. The level of statistical significance was set at P < 0.05.

Results

Patients’ clinical characteristics

The phase angle value in the low, middle, and high tertile was 3.89 ± 0.45° (2.43–4.39), 4.70 ± 0.19° (4.40–4.98), and 5.85 ± 0.56° (5.06–7.01), respectively. The mean age in the low, middle, and high tertile was 59.1 ± 9.9, 60.3 ± 12.3, and 50.3 ± 11.4 years, respectively (Table 1). Patients in the high tertile were younger than those in the other tertiles. The proportion of male patients in the low, middle, and high tertiles was 48.1%, 39.3%, and 67.9%, respectively, whereas the proportion of patients with DM was 37.0%, 50.0%, and 53.6%, respectively. No significant differences were observed in dialysis vintage and baseline laboratory findings among the three groups. Dry or achieved weight immediately after the HD session in the low, middle, and high tertiles was 58.6 ± 10.7, 62.2 ± 8.4, and 64.9 ± 13.3 kg, respectively (P = 0.107). Body weight at BIA measurements in the low, middle, and high tertiles was 58.9 ± 11.2, 62.4 ± 8.8, and 65.2 ± 13.9 kg, respectively (P = 0.128). Difference between dry weight and weight at BIA measurements in the low, middle, and high tertiles was 0.3 ± 0.9, 0.2 ± 1.0, and 0.3 ± 0.9 kg, respectively (P = 0.912).

Table 1

Clinical characteristics of patients.

	Total (n = 83)	Low T (n = 27)	Middle T (n = 28)	High T (n = 28)	P -value
Sex (male, %)	43 (51.8%)	13 (48.1%)	11 (39.3%)	19 (67.9%)	0.091
Age (years)	56.5 ± 12.0	59.1 ± 9.9	60.3 ± 12.3	50.3 ± 11.4*+	0.002
Diabetes mellitus (%)	44 (53.0%)	10 (37.0%)	14 (50.0%)	15 (53.6%)	0.436
Dialysis vintage (years)	4.6 ± 5.2	5.7 ± 5.3	4.3 ± 5.2	3.9 ± 5.0	0.412
Hemoglobin (mg/dL)	10.9 ± 0.6	10.8 ± 0.4	11.0 ± 0.7	11.0 ± 0.6	0.269
C-reactive protein (mg/dL)	0.4 ± 0.6	0.4 ± 0.6	0.3 ± 0.4	0.5 ± 0.8	0.605
Blood urea nitrogen (mg/dL)	59.6 ± 14.7	57.8 ± 17.4	58.9 ± 11.3	62.1 ± 14.9	0.529
Creatinine (mg/dL)	10.3 ± 2.6	9.8 ± 2.5	10.2 ± 2.0	10.9 ± 3.2	0.290
Aspartate transaminase (U/L)	17.9 ± 5.9	18.2 ± 7.1	18.1 ± 5.8	17.4 ± 4.8	0.581
Alanine transaminase (U/L)	15.8 ± 7.6	17.4 ± 9.3	14.2 ± 6.3	15.9 ± 6.8	0.302
Serum calcium (mg/dL)	8.4 ± 0.7	8.6 ± 0.7	8.3 ± 0.5	8.2 ± 0.9	0.054
Serum phosphorus (mg/dL)	5.4 ± 1.2	5.3 ± 1.2	5.4 ± 1.1	5.6 ± 1.4	0.593
Serum sodium (mEq/L)	138 ± 2.8	138 ± 3	138 ± 3	137 ± 2	0.881
Serum potassium (mEq/L)	5.0 ± 0.6	4.9 ± 0.7	5.1 ± 0.4	5.1 ± 0.5	0.570
Serum chloride (mEq/L)	98.5 ± 3.4	98 ± 4	98 ± 3	99 ± 3	0.845
Intact parathyroid hormone (pg/mL)	263 ± 185	276 ± 220	254 ± 155	259 ± 180	0.903
Total cholesterol (mg/dL)	154 ± 34	151 ± 33	153 ± 37	156 ± 34	0.872
Single-pool Kt/Vurea	1.4 ± 0.3	1.4 ± 0.2	1.3 ± 0.3	1.4 ± 0.3	0.831

Data are expressed as mean ± standard deviation for continuous variables and as number (percentage) for categorical variables. P-values were tested using one-way analysis of variance, followed by a post-hoc Tukey comparison for continuous variables and Pearson’s χ2 or Fisher’s exact tests for categorical variables.

*P < 0.05 compared with Low T and

+P < 0.05 compared with Middle T. Abbreviations: Low T, low tertile; Middle T, middle tertile; High T, high tertile.

Association between phase angle and various indices

On univariate analyses, HGS, SGA score, TMA/Ht2, GS, SPPB, 5STS, STS30, 6-MWT, and TUG were better in patients in the high tertile than in those in the other tertiles (Table 2). The phase angle as a continuous variable was associated with HGS, SGA score, TMA/Ht2, GS, SPPB, 5STS, STS30, 6-MWT, and TUG (Table 3). The correlation coefficients between the phase angle and TMA/Ht2, HGS, and GS were 0.517, 0.485, and 0.463, respectively (Fig 1). No significant association was observed between the phase angle and serum albumin levels or BMI. The results of multivariate analyses were similar to those of univariate analyses. Table 4 shows the results of logistic regression analyses using TMA/Ht2, HGS, and GS as important variables for muscle mass, muscle strength, and physical performance, respectively. On univariate and multivariate analyses, the phase angle was positively associated with these indices.

Table 2

Comparison of muscle mass indices, nutritional markers, and physical activity markers according to the tertiles of phase angle.

	Univariate				Multivariate
	Low T	Middle T	High T	P-value	Low T	Middle T	High T	P-value
Handgrip strength (kg)	23.0 ± 5.5	24.6 ± 5.9	30.4 ± 8.5*+	<0.001	23.8 ± 1.1	25.9 ± 1.1	28.3 ± 1.1*	0.023
SGA score	5.1 ± 0.9	5.7 ± 1.0*	6.3 ± 0.9*	<0.001	5.1 ± 0.2	5.8 ± 0.2*	6.1 ± 0.2*	0.001
Serum albumin (mg/dL)	3.9 ± 0.3	3.8 ± 0.3	3.8 ± 0.2	0.557	3.9 ± 0.1	3.8 ± 0.1	3.8 ± 0.1	0.301
Body mass index (kg/m2)	22.8 ± 4.1	24.4 ± 3.1	24.1 ± 3.6	0.232	22.6 ± 0.7	24.4 ± 0.7	24.2 ± 0.7	0.150
TMA/Ht2 (cm2/m2)	32.6 ± 4.4	36.9 ± 5.9*	40.9 ± 8.0*	<0.001	32.9 ± 1.1	37.7 ± 1.2*	39.9 ± 1.2*	<0.001
Gait speed (m/s)	0.83 ± 0.16	0.89 ± 0.20	1.04 ± 0.17*+	<0.001	0.85 ± 0.03	0.91 ± 0.03	1.00 ± 0.03*	0.015
SPPB	10.4 ± 1.4	10.7 ± 2.2	11.5 ± 1.0*	<0.001	10.6 ± 0.3	10.8 ± 0.3	11.2 ± 0.3	0.305
5STS (sec)	9.4 ± 2.3	8.4 ± 2.4	6.7 ± 1.9*+	0.045	9.3 ± 0.4	8.3 ± 0.4	7.0 ± 0.5*	0.003
STS30 (sec)	15.3 ± 4.2	16.6 ± 5.5	21.6 ± 5.6*+	<0.001	15.7 ± 1.0	17.0 ± 1.0	20.8 ± 1.0*	0.003
6-MWT (meters)	413 ± 94	441 ± 128	519 ± 90*+	<0.001	426 ± 19	456 ± 19	493 ± 20	0.065
Timed up-and-go test	8.2 ± 1.9	7.8 ± 2.1	6.1 ± 1.5*+	0.001	8.0 ± 0.3	7.6 ± 0.3	6.5 ± 0.3*	0.015

Data were expressed as mean ± standard deviation for univariate analysis or mean ± standard errors for multivariate analysis. P-values were tested using one-way analysis of variance, followed by a post-hoc Tukey comparison for univariate analysis and analysis of covariance for multivariate analysis. The results of multivariate analysis were adjusted for age, sex, and presence of diabetes mellitus.

Abbreviations: SGA, subjective global assessment; TMA/Ht2, thigh muscle area per height squared; SPPB, Short Physical Performance Battery; 5STS, five times sit-to-stand test; STS30, 30-s sit-to-stand test; 6-MWT, 6-min walk test; Low T, low tertile; Middle T, middle tertile; High T, high tertile.

Table 3

Correlation between phase angle and various indices.

	Univariate		Multivariate
	r	P-value	r	P-value
Handgrip strength (kg)	0.485	<0.001	0.320	0.004
SGA score	0.431	<0.001	0.353	<0.001
Serum albumin (mg/dL)	–0.049	0.657	–0.119	0.293
Body mass index (kg/m2)	0.164	0.137	0.211	0.060
TMA/Ht2 (cm2/m2)	0.517	<0.001	0.434	<0.001
Gait speed (m/s)	0.463	<0.001	0.372	0.001
SPPB	0.266	0.015	0.173	0.129
5STS (sec)	–0.405	<0.001	–0.316	0.005
STS30 (sec)	0.441	<0.001	0.342	0.002
6-MWT (meters)	0.321	0.003	0.159	0.166
Timed up-and-gotest	–0.332	0.002	–0.205	0.072

Correlations were analyzed using Pearson’s correlation for univariate analysis and partial correlation for multivariate analysis. The results of multivariate analysis were adjusted for age, sex, and presence of diabetes mellitus.

Abbreviations: SGA, subjective global assessment; TMA/Ht2, thigh muscle area per height squared; SPPB, Short Physical Performance Battery; 5STS, five times sit-to-stand test; STS30, 30-s sit-to-stand test; 6-MWT, 6-min walk test.

Fig 1

Correlation between phase angle and TMA/Ht2 (A), HGS (B), and GS (C). Abbreviations: TMA/Ht2, thigh muscle area per height squared; HGS, handgrip strength; GS, gait speed.

Table 4

Linear regression analyses of indices by phase angle.

	Univariate		Multivariate
	Standardized β (SE)	P-value	Standardized β (SE)	P-value
Dependent variable: TMA/Ht2
Age	–0.23 (0.06)	0.036	–0.11 (0.06)	0.278
Sex (ref: men)	–0.35 (1.47)	0.001	–0.21 (1.38)	0.038
Diabetes mellitus	–0.00 (1.57)	0.983	0.06 (1.34)	0.508
Phase angle	0.52 (0.67)	<0.001	0.44 (0.72)	<0.001
Dependent variable: handgrip strength
Age	–0.31 (0.07)	0.004	–0.19 (0.05)	0.025
Sex (ref: men)	–0.56 (1.36)	<0.001	–0.48 (1.26)	<0.001
Diabetes mellitus	–0.20 (1.61)	0.075	–0.16 (1.22)	0.051
Phase angle	0.49 (0.71)	<0.001	0.27 (0.65)	0.004
Dependent variable: gait speed
Age	–0.38 (0.00)	<0.001	–0.27 (0.00)	0.007
Sex (ref: men)	–0.28 (0.04)	0.010	–0.18 (0.04)	0.073
Diabetes mellitus	–0.18 (0.04)	0.114	–0.11 (0.04)	0.259
Phase angle	0.46 (0.02)	<0.001	0.32 (0.02)	0.003

Multivariate analysis was performed using age, sex, presence of diabetes mellitus, and phase angle.

Abbreviations: SE, standard error; TMA/Ht2, thigh muscle area per height squared.

Association between phase angle and frailty, low GS, or HRQoL

The number of patients with frailty in the low, middle, and high tertiles was 12 (44.4%), 8 (28.6%), and 4 (14.3%), respectively (P = 0.048). The number of patients with low GS in the low, middle, and high tertiles was 13 (48.1%), 10 (35.7%), and 3 (10.7%), respectively (P = 0.009). The proportion of patients with frailty or low GS decreased as the phase angle tertile increased. The AUROCs of the phase angle for frailty and low GS were 0.68 (95% confidence interval [CI], 0.57–0.78; P = 0.010) and 0.75 (95% CI, 0.64–0.84; P < 0.001), respectively. The sensitivity and specificity for predicting frailty were 83.3% (95% CI, 62.6–95.3) and 62.7% (95% CI, 49.1–75.0), respectively. The sensitivity and specificity for predicting low GS were 88.5% (95% CI, 69.8–97.6) and 56.1% (95% CI, 42.4–69.3), respectively. In addition, the phase angle had a positive association with PCS and inverse association with BDI or BAI (S1 Table). Statistical significance was not reached in the association between the phase angle and MCS or KDCS.

The numbers of patients with severe limitation in performing vigorous physical activity were 17 (63.0%) in the low tertile, 14 (50%) in the middle tertile, and 11 (39.3%) in the high tertile (P = 0.081). The numbers of patients with severe limitation in performing moderate physical activity were 4 (14.8%) in the low tertile, 2 (7.1%) in the middle tertile, and 0 in the high tertile (P = 0.035). The mean follow-up duration was 596 ± 338 days. The patient survival rate in the low, middle, and high tertiles was 92.3%, 94.7%, and 100%, respectively (Fig 2A, P = 0.067). The hospitalization-free survival rate in the low, middle, and high tertiles was 38.5%, 70.9%, and 66.4%, respectively (Fig 2B, P = 0.001). Patient survival was significantly better in the high tertile than in the low tertile (P = 0.165 for low vs middle tertiles, P = 0.046 for low vs high tertiles, and P = 0.330 for middle vs high tertiles). Hospitalization free survival was significantly poorest in the low tertile (P = 0.003 for low vs middle tertiles, P = 0.006 for low vs high tertiles, and P = 0.540 for middle vs high tertiles). The number of deaths in the low, middle, and high tertiles was 5, 1, and 0 cases, respectively. The causes of deaths in the low tertile were cardiovascular disease (2 cases), infection (1 case), gastrointestinal disease (1 case), and suicide (1 case), respectively. One death in the middle tertile was caused by accident.

Fig 2

Kaplan-Meier curves for patient survival (A) and hospitalization-free survival (B).

Subgroup analyses according to age, sex, and DM

We have divided the patients into two age groups according to a median age of 57 years. For patients aged < 57 years, most variables except serum albumin and BMI showed a significant association with the phase angle (S2 Table). For patients aged ≥ 57 years, statistical significance was not reached for variables except TMA/Ht2, which showed a modest association. However, the trends were similar to those in patients aged < 57 years. On subgroup analyses according to sex or the presence of DM, the overall associations were greater in men or patients without DM than in women or patients with DM (S3 and S4 Tables).

Discussion

In our study, no significant differences were observed in the serum albumin level and BMI according to tertiles of the phase angle. However, the phase angle tertiles were associated with TMA/Ht2 as an accurate parameter for predicting muscle mass and HGS as an indicator of muscle strength. The phase angle was also associated with physical performance measurements, including GS, SPPB, 5STS, 6-MWT, and TUG. It was associated with PCS, BDI, and BAI. Subgroup analyses according to sex, age, and DM showed similar trends to those of the total cohort. Furthermore, the hospitalization-free survival rate and patient survival rate were favorable in patients with high values for the phase angle. The number of patients with severe limitation in physical activity increased as the tertile of phase angle decreased.

Previous studies have evaluated the association between the phase angle and nutritional status in patients with chronic kidney disease. Oliveira et al. enrolled 58 HD patients and showed that phase angle is associated with serum albumin, SGA score, and fat-free mass from BIA on univariate analysis alone [25]. Tan et al. showed the association between the phage angle and serum albumin, prealbumin, fat-free mass from BIA, or anthropometric measurements in 173 HD patients [26]. Beberashvili et al. performed an observational study using a relatively large sample and revealed that the phase angle was associated with the HGS, malnutrition-inflammation score, and HRQoL scales and that the phase angle was associated with cardiovascular events or mortality based on malnutrition-inflammation score [27].

Although our findings are consistent with those of previous studies that have shown the association between the phase angle and nutritional markers, muscle mass, and clinical outcomes, only a few studies have reported accurate and comprehensive measurements. First, our study evaluated TMA/Ht2 as an indicator of muscle mass. Previous studies on the association between the phase angle and muscle mass evaluated muscle mass using dual energy X-ray absorptiometry (DEXA) or BIA. However, these two measurements are not accurate in patients with unstable volume status, such as dialysis patients. DEXA measures lean mass, which is calculated as total body mass minus bone and fat mass [28]. In the general population, lean mass from DEXA is highly correlated with real lean mass or muscle mass. However, DEXA overestimates the real lean mass in patients with a hypervolemic status, such as those undergoing dialysis. BIA measures impedance from the body, which is used to calculate muscle mass with a regression equation derived from the general population. Although some validation was performed in previous studies, BIA-derived muscle mass may be inherently biased. We evaluated TMA/Ht2 using CT, which is a relatively accurate method for predicting muscle mass. Statistical significance was also reached in the association between the phase angle and muscle mass.

Our study evaluated muscle function including muscle strength and physical performance. We especially evaluated various measurements for physical performance. Physical performance tests can be influenced by the subjective status, and we used various measurements for accurate judgment. Evaluation of various physical performance measures, including GS, SPPB, 5STS, STS30, TUG, and 6-MWT, can be useful to attenuate the influence of the subjective status. Furthermore, our study evaluated HRQoL and mood status using the KDQoL-SFTM 1.3, BDI, and BAI scales. The physical component of the qulity-of-life scales was positively associated with the phage angle. Depression or anxiety mood increased as the phase angle decreased. We eventually evaluated the patient survival and hospitalization-free survival rates. The hospitalization-free survival rate was lower in the low tertile than in the other tertiles. Patient survival was lower in the low tertile than in the high tertile.

In our study, serum albumin and BMI, as classic nutritional indices, were not associated with the phase angle. Although these two indicators are well-known nutritional indicators, they also have drawbacks. BMI does not differentiate muscle mass from other components such as fat or bone. A normal serum albumin level does not necessarily reveal a normal nutritional status and vice versa. Serum albumin level is decreased by dilution caused by volume status and conditions with decreased albumin synthesis, such as liver diseases or inflammation. On the contrary, a mild catabolic status may be associated with normal serum albumin levels through metabolic adaptation in the hepatic synthesis of albumin [29].

We performed two analyses using phase angle as continuous or categorical variables. Analyses using phase angle as a continuous variable may be useful to identify the association with quantitative variables. Analyses using tertiles by phase angle may be useful to identify differences in qualitative variables according to groups. In addition, categorization of continuous variables would be statistically useful to evaluate the association with hard outcomes, such as survival analysis. Therefore, we analyzed the association between outcomes using both phage angle as continuous variable and categorized groups according to phase angle. Our results showed that phase angle as a continuous variable was correlated with muscle mass, strength, physical performance, and HRQoL scales, as cross-sectional data. However, analyses using tertiles by phase angle showed that patient survival was significantly better in the low tertile than in the high tertile, and hospitalization free survival was significantly poorest in the low tertile. Although the Kaplan-Meier curve may show best hospitalization free survival in the middle tertile and best patient survival in the high tertile, there was no significant difference in two survivals between the middle and high tertiles. These findings reveal that it would be more important to identify whether phase angle is low value than to differentiate high values in patients without low phase angle.

Previous studies evaluated the association between phase angle and hard clinical outcomes, such as mortality or hospitalization, in chronic kidney disease patients. Bansal et al. analyzed non-dialysis chronic kidney disease patients and showed that patients with <5.59° defined as lowest quartile had greater mortality compared to those with ≥ 5.59° [30]. Two previous studies enrolled 760 or 48 peritoneal dialysis (PD) patients and showed the association of low phase angle with mortality [31, 32]. A prospective study enrolled 250 maintenance HD patients and showed an association between tertile of phase angle and mortality or hospitalization [27]. A study from Spain enrolled 164 dialysis patients (127 on HD and 37 on PD patients) and showed similar results [33]. In addition, a recent study enrolled 116 HD patients and divided patients into four groups according to quartiles of phase angle [34]. Their study using cross-sectional data revealed that the lowest quartile of phase angle is associated with greater risk of protein energy wasting, frailty, and cardiovascular risk score in HD patients. Markaki et al. showed an association between phase angle and depression in HD patients [35]. Although the association between phase angle and each indicator, such as malnutrition, hospitalization, frailty, depression, or mortality, is already established in dialysis patients, there were few studies for HD patients with comprehensive data including muscle mass measurements using CT, strength, HRQoL scales, various physical performance tests, frailty, depression, mortality, and hospitalization.

Differences in dry weight, achieved weight immediately after HD session, and body weight at BIA measurements may influence our results. Our study did not include data for ultrafiltration volume at HD session before BIA measurements. However, all patients achieved dry weight immediately after HD session, and our data includes the body weight at BIA measurements (on the day after the HD session). No significant differences were observed in dry weight and body weight at BIA measurements among the three tertiles. In addition, the difference between dry weight and body weight at BIA measurements was relatively small. These findings reveal that fluid status among the three tertiles was similar and relatively stable.

Our study had inherent limitations, including the use of data from a single center and the small number of analyzed patients. We believe that the lack of statistical significance in some physical performance tests or in the patient survival rate may be associated with the small number of patients. Second, in our study, participants in the high tertile were approximately 10 years younger than those in the other tertiles. To overcome this difference, we performed subgroup or multivariate analyses, but the effect of age was not completely overcome. Analyses using groups with similar age may be different. Considering the association of high phase angle with high muscle mass, strength, or physical performance, it may be an inevitable that patients with high phase angle are younger than those with low or middle phase angle, and this confounding bias, which is commonly observed in non-randomized studies or studies with a small sample size, can influence our results. Subgroup analyses divided according to a small interval of age or a propensity matching study can be useful to resolve this problem, but a study using a larger sample size is warranted. Third, phase angle value was obtained from a single measurement; however, an averaged value from repeated measurements would be more accurate. However, previous studies showed that intraclass correlation between multiple measurements was approximately 0.983~1.00 [36, 37]. Use of phase angle value from a single measurement can be a limitation of our study, but considering the high precision of the machine, the error from a single measurement may be attenuated. Fourth, in our study, muscle measurement was performed using CT. It is well known that the radiation dose in CT is greater than that in DEXA. Radiation dose by DEXA and CT was approximately 0.001 mSV for whole body and 1.0 mSV per single slice [38]. Although muscle mass measurement using CT would be more accurate than DEXA, routine use of CT should be avoided considering the high radiation by CT. Muscle mass measurement using CT may be useful for research purposes, wherease measurements using DEXA may be appropriate for the purpose of routine monitoring or screening. Despite these limitations, our study informs the association between phase angle and various clinical outcomes, including muscle mass, strength, physical performance, HRQoL scales, and further patient survival or hospitalization, in HD patients. Measurement of phase angle using BIA is cheap and safe, and it is easy to measure and interpret. Although the usefulness of phase angle for screening or diagnostic purposes was limited by our study design, phase angle may be an option to predict various clinical outcomes associated with poor muscle status in HD patients. To overcome the limitations of our study, such as the study design, small sample size, relatively short-term follow-up duration, or small number of death events, and identify clear a cut-off value for low phase angle or definite association with outcomes, further longitudinal studies using a large sample size and longer follow-up duration are needed.

In conclusion, the present study demonstrated that the phase angle is associated with muscle mass, strength, physical performance, HRQoL, and hospitalization-free survival in maintenance HD patients.

Correlation between phase angle and quality-of-life scales.

(DOCX)

Click here for additional data file.

Correlation between phase angle and various indices according to age.

(DOCX)

Click here for additional data file.

Correlation between phase angle and various indices according to sex.

(DOCX)

Click here for additional data file.

Correlation between phase angle and various indices according to presence of diabetes mellitus.

(DOCX)

Click here for additional data file.

26 Jul 2021

PONE-D-21-15274

Impedance-derived phase angle is associated with muscle mass, strength, quality of life, and clinical outcomes in maintenance hemodialysis patients

PLOS ONE

Dear Dr. Do,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

Please submit your revised manuscript by Sep 04 2021 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:

A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.

A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.

An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

If applicable, we recommend that you deposit your laboratory protocols in protocols.io to enhance the reproducibility of your results. Protocols.io assigns your protocol its own identifier (DOI) so that it can be cited independently in the future. For instructions see: http://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols. Additionally, PLOS ONE offers an option for publishing peer-reviewed Lab Protocol articles, which describe protocols hosted on protocols.io. Read more information on sharing protocols at https://plos.org/protocols?utm_medium=editorial-email&utm_source=authorletters&utm_campaign=protocols.

We look forward to receiving your revised manuscript.

Kind regards,

Vivekanand Jha

Academic Editor

PLOS ONE

Journal Requirements:

When submitting your revision, we need you to address these additional requirements.

1. Please ensure that your manuscript meets PLOS ONE's style requirements, including those for file naming. The PLOS ONE style templates can be found at

https://journals.plos.org/plosone/s/file?id=wjVg/PLOSOne_formatting_sample_main_body.pdf and

https://journals.plos.org/plosone/s/file?id=ba62/PLOSOne_formatting_sample_title_authors_affiliations.pdf

2. Please provide additional details regarding participant consent. In the ethics statement in the Methods and online submission information, please ensure that you have specified what type you obtained (for instance, written or verbal, and if verbal, how it was documented and witnessed). If your study included minors, state whether you obtained consent from parents or guardians. If the need for consent was waived by the ethics committee, please include this information

3. Thank you for including your ethics statement:  "This study was approved by the IRB of a tertiary medical center (No. 12-07).".

a. Please amend your current ethics statement to include the full name of the ethics committee/institutional review board(s) that approved your specific study.

b. Once you have amended this/these statement(s) in the Methods section of the manuscript, please add the same text to the “Ethics Statement” field of the submission form (via “Edit Submission”).

For additional information about PLOS ONE ethical requirements for human subjects research, please refer to http://journals.plos.org/plosone/s/submission-guidelines#loc-human-subjects-research.

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Yes

**********

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

**********

3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: No

Reviewer #2: No

**********

4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

**********

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: 1. The authors should specify how many hours after completion of dialysis was the BIA measurement done. The equilibration of fluid between various compartments after completion of HD is likely to affect measurement of phase angle in dialysis patients. Was this aspect standardized for all patients.

2. There is no mention of the fluid balance state of the study subjects. Had the patients achieved their dry weight on dialysis. Was there a difference in achieved vs dry weight between the tertiles of phase angle? This data must be included in the baseline characteristics.

3. What was the frequency of maintenance haemodialysis. Was it the same for all patients. The authors should specify.

4. Multiple measurements of phase angle over a period of time for a given patient is more likely to give accurate readings. The authors should specify whether in the current study it was a single reading or multiple readings were taken.

5. Phase angle is a continuous variable and should have been analysed accordingly. Why have the authors chosen to divide their patient population into tertiles for statistical analysis.

6. The patient population was significantly younger by nearly a decade in the group with highest tertile. Although, adjustment for age, sex and Diabetes was done on multivariate analysis this is a drawback of the study. If age was equally distributed across all the tertile of phase angle, the result may have been different.

7. Hospitalisation free survival was best in the middle tertile, whereas the patient survival was best in the high tertile. What is the author’s analysis/explanation.

8. What were the causes of mortality in low phase angle tertile.

Reviewer #2: I would like to make the following observations:

1) This study looks at the association of phase angle by BIA to a variety of measurements assessing muscle function, quality of life and more importantly clinical outcomes like hospitalisation free survival and patient survival. The emphasis on hard clinical outcomes is one of the strengths of this study

2) There is paucity of well designed studies on this particular subject and this study informs the readers on the possible clinical applications of phase angel by BIA.

2) There could be more clarity on the timeline of the tests used to assess the physical performance - gait speed, 30 second sit to stand test etc - where these performed on dialysis days (before or after HD) or on dialysis free days and was this uniformly followed for all patients in the study? Where all tests performed on a single day or over a few days? This information would be useful to interpret the results of these tests.

3) The study uses multislice CT to measure thigh muscle area to circumvent the drawbacks of using DEXA in hypervolemic patients. However, the radiation dose for CT evaluation is several times higher than DEXA. These concerns have not been addressed in the manuscript.

4) Apart from the studies quoted in the draft, a recent similar but less elaborate study has been published in HD patients the findings of which could be discussed. See here - "Saitoh M, Ogawa M, Kondo H, et al. Bioelectrical impedance analysis-derived phase angle as a determinant of protein-energy wasting and frailty in maintenance hemodialysis patients: retrospective cohort study. BMC Nephrol. 2020;21(1):438. Published 2020 Oct 19. doi:10.1186/s12882-020-02102-2"

**********

6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: Yes: Dr (Brig) A Jairam

Reviewer #2: Yes: Sukanya Govindan

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

25 Aug 2021

Reviewer #1:

1. The authors should specify how many hours after completion of dialysis was the BIA measurement done. The equilibration of fluid between various compartments after completion of HD is likely to affect measurement of phase angle in dialysis patients. Was this aspect standardized for all patients.

Answer: Thank you for your comments. In our study, all patients underwent three HD sessions per week. In addition, all measurements, including BIA, muscle mass, strength, physical performance, and health-related quality of life scales, were performed on the day after the midweek HD session. Therefore, all patients had a stable fluid status between compartments. We have added these comments in the Methods section.

2. There is no mention of the fluid balance state of the study subjects. Had the patients achieved their dry weight on dialysis. Was there a difference in achieved vs dry weight between the tertiles of phase angle? This data must be included in the baseline characteristics.

Answer: Thank you for your comments. Our study did not include data for ultrafiltration volume at the HD session before BIA measurements. However, all patients achieved dry weight immediately after the HD session. Dry or achieved weight immediately after the HD session in the low, middle, and high tertiles was 58.6 ± 10.7, 62.2 ± 8.4, and 64.9 ± 13.3 kg, respectively (P = 0.107). Body weight at BIA measurements in the low, middle, and high tertiles was 58.9 ± 11.2, 62.4 ± 8.8, and 65.2 ± 13.9 kg, respectively (P = 0.128). Difference between dry weight and weight at BIA measurements in the low, middle, and high tertiles was 0.3 ± 0.9, 0.2 ± 1.0, and 0.3 ± 0.9 kg, respectively (P = 0.912). No significant differences were observed in dry weight and body weight at BIA measurements among the three tertiles. In addition, the difference between dry weight and body weight at BIA measurements were relatively small. These findings reveal that the fluid status among the three tertiles was similar and relatively stable. We have added these comments in the Results and Discussion sections.

3. What was the frequency of maintenance haemodialysis. Was it the same for all patients. The authors should specify.

Answer: Thank you for your comments. In our study, all patients underwent three HD sessions per week. We have added this comment in the Methods section.

4. Multiple measurements of phase angle over a period of time for a given patient is more likely to give accurate readings. The authors should specify whether in the current study it was a single reading or multiple readings were taken.

Answer: Thank you for your comments. We agree with the reviewer’s comments. In our study, PhA value was obtained from a single measurement, but an averaged value from repeated measurements would be more accurate. However, previous studies showed that intraclass correlation between multiple measurements was approximately 0.983~1.00 [1,2]. Use of PhA value from a single measurement can be a limitation of our study, but considering the high precision of the machine, the error from a single measurement may be attenuated. We have added these comments in the Discussion section.

[1] Gibson AL, Holmes JC, Desautels RL, Edmonds LB, Nuudi L. Ability of new octapolar bioimpedance spectroscopy analyzers to predict 4-component-model percentage body fat in Hispanic, black, and white adults. Am J Clin Nutr. 2008;87:332-338.

[2] Schubert MM, Seay RF, Spain KK, Clarke HE, Taylor JK. Reliability and validity of various laboratory methods of body composition assessment in young adults. Clin Physiol Funct Imaging. 2019;39:150-159.

5. Phase angle is a continuous variable and should have been analysed accordingly. Why have the authors chosen to divide their patient population into tertiles for statistical analysis.

Answer: Thank you for your comments. Analyses using PhA as a continuous variable may be useful to identify the association with quantitative variables. Analyses using tertiles by PhA may be useful to identify differences in qualitative variables according to groups. In addition, categorization of a continuous variable would be statistically useful to evaluate the association with hard outcomes, such as survival analysis. Therefore, we analyzed the association between outcomes with both PhA as continuous variable and categorized groups according to PhA. Our results showed that PhA as a continuous variable was correlated with muscle mass, strength, physical performance, and quality of life scales, as cross-sectional data. However, analyses using tertiles by PhA showed that patient survival was significantly better in the low tertile than in the high tertile, and hospitalization-free survival was significantly poorest in the low tertile. These findings reveal that for patient or hospitalization-free survival, it would be more important to identify whether PhA is a low value than to differentiate high values in patients without low PhA. However, our study had a small sample size, relatively short-term follow-up duration, and small number of death events. To identify a clear cut-off value for low PhA or definite association with outcomes, further studies with a large sample size and longer follow-up duration are needed. We have added these comments in the Discussion section.

6. The patient population was significantly younger by nearly a decade in the group with highest tertile. Although, adjustment for age, sex and Diabetes was done on multivariate analysis this is a drawback of the study. If age was equally distributed across all the tertile of phase angle, the result may have been different.

Answer: Thank you for your comments. We completely agree with the reviewer’s comments. In our study, participants in the high tertile were approximately 10 years younger than those in the other tertiles. To overcome this difference, we performed subgroup or multivariate analyses, but the effect of age could not be completely overcome. As the reviewer pointed out, an analysis using groups with similar age may be different. Considering the association of high PhA with high muscle mass, strength, or physical performance, it may be inevitable that patients with high PhA are younger than those with low or middle PhA, and this confounding bias, which is commonly observed in non-randomized studies or studies with a small sample size, can influence our results. Subgroup analyses according to small age interval or propensity matching study can be useful to resolve this problem, but a study with a larger sample size is warranted. We have added these comments in the Discussion section.

7. Hospitalisation free survival was best in the middle tertile, whereas the patient survival was best in the high tertile. What is the author’s analysis/explanation.

Answer: Thank you for your comments. We performed comparison between two groups. Patient survival was significantly better in the low tertile than in the high tertile (P = 0.165 for low vs middle tertiles, P = 0.046 for low vs high tertiles, and P = 0.330 for middle vs high tertiles). Hospitalization-free survival was significantly poorest in the low tertile (P = 0.003 for low vs middle tertiles, P = 0.006 for low vs high tertiles, and P = 0.540 for middle vs high tertiles). Although the Kaplan-Meier curve may show best hospitalization-free survival in the middle tertile and best patient survival in the high tertile, there was no significant difference in the two survivals between the middle and high tertiles. These findings reveal that it would be more important to identify whether PhA is a low value than differenting high values in patients without low PhA. We have added these comments in the Results and Discussion sections.

8. What were the causes of mortality in low phase angle tertile.

Answer: Thank you for your comments. The number of deaths in the low, middle, and high tertiles was 5, 1, and 0 cases, respectively. The causes of deaths in the low tertile were cardiovascular disease (2 cases), infection (1 case), gastrointestinal disease (1 case), and suicide (1 case), respectively. One death in the middle tertile was caused by accident. We have added these comments in the Results section.

Reviewer #2: I would like to make the following observations:

1) This study looks at the association of phase angle by BIA to a variety of measurements assessing muscle function, quality of life and more importantly clinical outcomes like hospitalisation free survival and patient survival. The emphasis on hard clinical outcomes is one of the strengths of this study

Answer: Thank you for your comments. We performed additional analyses for patient or hospitalization-free survival, and compared between the two groups. Patient survival was significantly better in low tertile than in high tertile (P = 0.165 for low vs middle tertiles, P = 0.046 for low vs high tertiles, and P = 0.330 for middle vs high tertiles). Hospitalization-free survival was significantly poorest in the low tertile (P = 0.003 for low vs middle tertiles, P = 0.006 for low vs high tertiles, and P = 0.540 for middle vs high tertiles). Although the Kaplan-Meier curve may show best hospitalization-free survival in the middle tertile and best patient survival in the high tertile, there was no significant difference in the two survivals between the middle and high tertiles. These findings reveal that it would be more important to identify whether PhA is low value than to differentiate high values in patients without low PhA.

Previous studies evaluated the association between PhA and hard clinical outcomes, such as mortality or hospitalization, in chronic kidney disease patients. Bansal et al. analyzed non-dialysis chronic kidney disease patients and showed that patients with <5.59°, defined as the lowest quartile, had greater mortality compared to those with ≥ 5.59° [1]. Two previous studies enrolled 760 or 48 PD patients and showed the association of low PhA with mortality [2,3]. A prospective study enrolled 250 maintenance HD patients and showed an association between tertile of PhA and mortality or hospitalization [4]. A study from Spain enrolled 164 dialysis patients (127 on HD and 37 on PD patients) and also showed similar results [5]. In addition, a recent study enrolled 116 HD patients and divided patients into 4 groups according to quartiles of PhA [6]. Their study using cross-sectional data revealed that the lowest quartile of PhA was associated with greater risk of protein energy wasting, frailty, and cardiovascular risk score in HD patients. Markaki et al. showed an association between PhA and depression in HD patients [7]. Although the association between PhA and each indicator, such as malnutrition, hospitalization, frailty, depression, or mortality, is established in dialysis patients, there were few studies on HD patients with comprehensive data including muscle mass measurements using CT, strength, quality of life scales, various physical performance tests, frailty, depression, mortality, and hospitalization. We have added these comments in the Results and Discussion sections.

[1] Bansal N, Zelnick LR, Himmelfarb J, Chertow GM. Bioelectrical Impedance Analysis Measures and Clinical Outcomes in CKD. Am J Kidney Dis. 2018 Nov;72(5):662-672.

[2] Huang R, Wu M, Wu H, Ye H, Peng Y, Yi C, Yu X, Yang X. Lower Phase Angle Measured by Bioelectrical Impedance Analysis Is a Marker for Increased Mortality in Incident Continuous Ambulatory Peritoneal Dialysis Patients. J Ren Nutr. 2020 Mar;30(2):119-125.

[3] Mushnick R, Fein PA, Mittman N, Goel N, Chattopadhyay J, Avram MM. Relationship of bioelectrical impedance parameters to nutrition and survival in peritoneal dialysis patients. Kidney Int Suppl. 2003 Nov;(87):S53-6.

[4] Beberashvili I, Azar A, Sinuani I, Shapiro G, Feldman L, Stav K, Sandbank J, Averbukh Z. Bioimpedance phase angle predicts muscle function, quality of life and clinical outcome in maintenance hemodialysis patients. Eur J Clin Nutr. 2014 Jun;68(6):683-9.

[5] Abad S, Sotomayor G, Vega A, Pérez de José A, Verdalles U, Jofré R, López-Gómez JM. The phase angle of the electrical impedance is a predictor of long-term survival in dialysis patients. Nefrologia. 2011;31(6):670-6.

[6] Saitoh M, Ogawa M, Kondo H, Suga K, Takahashi T, Itoh H, Tabata Y. Bioelectrical impedance analysis-derived phase angle as a determinant of protein-energy wasting and frailty in maintenance hemodialysis patients: retrospective cohort study. BMC Nephrol. 2020 Oct 19;21(1):438.

[7] Markaki AG, Charonitaki A, Psylinakis E, Dimitropoulakis P, Spyridaki A. Nutritional status in hemodialysis patients is inversely related to depression and introversion. Psychol Health Med. 2019 Dec;24(10):1213-1219.

2) There is paucity of well designed studies on this particular subject and this study informs the readers on the possible clinical applications of phase angel by BIA.

Answer: Thank you for your comments. Our study informs the association between PhA and various clinical outcomes, including muscle mass, strength, physical performance, quality of life scales, and further patient survival or hospitalization, in PD patients. Measurement of PhA using bioimpedance analysis is cheap and safe, and it is easy to measure and interpret. Although the usefulness of PhA for screening or diagnostic purposes was limited by our study design, PhA may be an option to predict various clinical outcomes associated with poor muscle status in PD patients. We have added these comments in the Discussion section.

2) There could be more clarity on the timeline of the tests used to assess the physical performance - gait speed, 30 second sit to stand test etc - where these performed on dialysis days (before or after HD) or on dialysis free days and was this uniformly followed for all patients in the study? Where all tests performed on a single day or over a few days? This information would be useful to interpret the results of these tests.

Answer: Thank you for your comments. In our study, all patients underwent three HD sessions per week. In our study, all measurements, including BIA, muscle mass, strength, physical performance, and health-related quality of life scales, were performed on the day after the midweek HD session. Therefore, all measurements were performed regardless of fluid status or influence of HD session. We have added these comments in the Methods section.

3) The study uses multislice CT to measure thigh muscle area to circumvent the drawbacks of using DEXA in hypervolemic patients. However, the radiation dose for CT evaluation is several times higher than DEXA. These concerns have not been addressed in the manuscript.

Answer: Thank you for your comments. We agree with the reviewer’s comments. In our study, muscle measurement was performed using CT. It is well known that the radiation dose in CT is greater than that in DEXA. The radiation dose by DEXA and CT was approximately 0.001 mSV for whole body and 1.0 mSV per single slice [1]. Although muscle mass measurement using CT would be more accurate than with DEXA, routine use of CT should be avoided considering the high radiation by CT. Muscle mass measurement using CT may be useful for research purposes, whereas measurements using DEXA may be appropriate for the purpose of routine monitoring or screening. We have added these comments in the Discussion section.

[1] Lee K, Shin Y, Huh J, Sung YS, Lee IS, Yoon KH, et al. Recent Issues on Body Composition Imaging for Sarcopenia Evaluation. Korean J Radiol. 2019;20:205-217.

4) Apart from the studies quoted in the draft, a recent similar but less elaborate study has been published in HD patients the findings of which could be discussed. See here - "Saitoh M, Ogawa M, Kondo H, et al. Bioelectrical impedance analysis-derived phase angle as a determinant of protein-energy wasting and frailty in maintenance hemodialysis patients: retrospective cohort study. BMC Nephrol. 2020;21(1):438. Published 2020 Oct 19. doi:10.1186/s12882-020-02102-2"

Answer: Thank you for your comments. We have added the reference and comments as per the reviewer’s suggestion. Detailed comments were expressed in response to the previous request.

Submitted filename: Answers for reviewers comments-complete.doc

Click here for additional data file.

24 Nov 2021

Impedance-derived phase angle is associated with muscle mass, strength, quality of life, and clinical outcomes in maintenance hemodialysis patients

PONE-D-21-15274R1

Dear Dr. Young Do

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

An invoice for payment will follow shortly after the formal acceptance. To ensure an efficient process, please log into Editorial Manager at http://www.editorialmanager.com/pone/, click the 'Update My Information' link at the top of the page, and double check that your user information is up-to-date. If you have any billing related questions, please contact our Author Billing department directly at authorbilling@plos.org.

If your institution or institutions have a press office, please notify them about your upcoming paper to help maximize its impact. If they’ll be preparing press materials, please inform our press team as soon as possible -- no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact onepress@plos.org.

Kind regards,

Pasqual Barretti, Ph.D., MD

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

After criterious analysis of the manuscript as well its sumbission history, my option is "accept"

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

Reviewer #2: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: No

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: I thank the authors. They have addressed all the comments and queries satisfactorily. They have made the necessary changes in the manuscript.

Reviewer #2: (No Response)

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: Yes: Anantharam Jairam

Reviewer #2: Yes: Sukanya Govindan

22 Dec 2021

PONE-D-21-15274R1

Impedance-derived phase angle is associated with muscle mass, strength, quality of life, and clinical outcomes in maintenance hemodialysis patients

Dear Dr. Do:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

If we can help with anything else, please email us at plosone@plos.org.

Thank you for submitting your work to PLOS ONE and supporting open access.

Kind regards,

PLOS ONE Editorial Office Staff

on behalf of

Prof. Pasqual Barretti

Academic Editor

PLOS ONE