Are elevated plasma fibrinogen associated with lung function? An 8-year follow-up of the ELSA study.

BACKGROUND: Fibrinogen is an important biomarker of inflammation, but findings from longitudinal studies that correlated fibrinogen with lung function in older adults are inconsistent. AIM: To investigate the relationship between fibrinogen plasma levels and lung function impairment later in life.
METHODS: Longitudinal analysis of 2,150 participants of the English Longitudinal Study of Ageing (ELSA) aged 50 years and older. Associations between changes in plasma fibrinogen between waves 2 (2004-05) and 4 (2008-09) and lung function in wave 6 (2012-13) were performed using multiple linear regression adjusted by potential confounders.
RESULTS: Regarding the fibrinogen profile, 18.5% of the participants presented higher levels in both waves. In the adjusted models, the maintenance of high fibrinogen levels was associated with a significant reduction of lung function only for men. FEV1 showed a reduction of 0.17L, FVC of 0.22L, and the percentages predicted were 5.16% for FEV1 and 6.21% for FVC compared to those that maintained normal levels of fibrinogen. DISCUSSION: To the best of our knowledge, this was the first study investigating the relationship between changes in fibrinogen levels over a long follow-up period and lung function in older adults without pre-existing chronic diseases. ELSA has information on critical demographic and clinical parameters, which allowed to adjust for potential confounding factors.
CONCLUSION: It was found that the persistence of high levels of plasma fibrinogen in older English men, but not women, is associated with lung function decline. Therefore, plasma fibrinogen showed to be an important biomarker of pulmonary dysfunction in this population.

1. Introduction

Fibrinogen is a soluble protein involved in the blood clotting mechanism [1], and it is linked to pathological processes of lung diseases [2,3]. Inflammatory stimuli induce the lung’s alveolar epithelial cells to synthesize and secrete fibrinogen, demonstrating the importance of this biomarker in the inflammatory process [4,5]. In healthy lungs, low levels of fibrinogen are expected [6]. Experimental studies in animal models suggest that fibrinogen may be an essential effector molecule in airway disease [7,8]. Fibrinogen in the lungs can inactivate pulmonary surfactant, which causes increased surface-tension relationships, promote the expression of molecules that induces airway fibrosis, and activate plasminogen activator inhibitor type-1, leading to excess fibrin deposition in the airways and airway narrowing [9].

Fibrinogen plasma levels have been associated with reduced lung function in cross-sectional [10,11] and few longitudinal [12-14] analyses. Longitudinal studies were performed in the younger population [12,14] or individuals with pre-existing underlying diseases [13]. Other studies report that people with respiratory diseases had significantly higher levels of fibrinogen compared to controls [15-22]. However, the relationship between lung function and changes in systemic fibrinogen levels over time in community-dwelling older adults is not clear. Therefore, this study aimed to investigate the relationship between fibrinogen plasma levels and lung function changes later in life. We used eight years of data from a nationally representative sample of older English men and women. We hypothesized that there is a relationship between the change in plasma fibrinogen levels and lung function reduction overtime in an aging cohort.

2. Methods

2.1 Study population

Longitudinal study using data collected from the English Longitudinal Study of Ageing (ELSA), an ongoing panel study of community-dwelling men and women living in England aged 50 years and older that commenced in 2002. The ELSA sample was drawn from participants that had previously participated in the Health Survey for England. After baseline, follow-up interviews occur biannually and health examinations every four years. A detailed description of the study can be found elsewhere [23]. The National Research Ethics Service (London Multicentre Research Ethics Committee, MREC/01/2/91) approved the ELSA study. All participants gave written informed consent. Participants who had fibrinogen data from waves 2 and 4, and lung function tests at wave 6 were included in the study. The data collection process and methodology are fully described elsewhere [23]. This study followed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting guideline.

2.2 Variables

2.2.1 Fibrinogen

Blood samples were taken from willing ELSA core members, except those who had clotting or bleeding disorders, had ever had fits or convulsions, were not willing to give their consent in writing, or were currently on anticoagulant drugs. Respondents over 80 years, those known to be diabetic and on treatment, those with a clotting or bleeding disorder or on anticoagulant drugs, those who had ever had fits, those who seemed frail, or respondents whose health was a cause for concern for the nurse were not asked to fast. Fibrinogen analysis was carried out by the Department of Haematology at the Royal Victoria Infirmary, The Newcastle Upon Tyne NHS Foundation Trust. The coagulation analyser MDA 180 (Organon Teknika, Durham, USA) was used with a modification of the Clauss thrombin clotting method by one citrate blue tube [23]. The measurement of fibrinogen was done in the ACL TOP CTS analyser. The Fibrinogen assay on the IL TOP is a clotting assay. Analyser prepares a 1 in 10 dilution of plasma in factor diluent, by aspirating 17μl of plasma and mixing it with 153μl of factor diluent. 100μl of the diluted plasma was dispensed into a reaction cuvette and incubated for 60 seconds at 37°C. 50μl of Fibrinogen-C XL reagent was added, and the reaction monitored at 405nm for up to 120 seconds. The analyser monitors the change in light transmission caused by the conversion of soluble fibrinogen in plasma to cross-linked insoluble fibrin, and the clotting time threshold is determined to be 37% of the total change. The clotting time is directly related to the concentration of fibrinogen in the plasma. This time is converted to concentration in g/L by the automatic use of a calibration curve. For the analyses, we created a variable called fibrinogen profile which considered plasma fibrinogen levels between waves 2 and 4 (normal levels in waves 2 and 4, normal in waves 2 and altered in wave 4, altered in wave 2 and normal in waves 4; altered in waves 2 and 4). Upper tertile values of fibrinogen were considered altered (≥3.8 g/L for wave 2 and ≥3.9 g/L for wave 4).

2.2.2 Lung function

A spirometer (NDD Easy On-PC) was used to evaluate forced expiratory volume in the first second (FEV1) and forced vital capacity (FVC). At least three acceptable measurements were obtained, and the largest FEV1 and FVC values were considered. Respondents were excluded if they had had abdominal or chest surgery in the preceding three weeks, had been admitted to hospital with a heart complaint in the previous six weeks, had had eye surgery in the prior four weeks, had a tracheotomy, or were pregnant. Further, the tests were not done if the ambient temperature was less than 15°C or more than 35°C, as this affects the accuracy of the readings. FEV1 and FVC were expressed in absolute values (in liters), and percentage of predicted [24] were used for the analyses. All the lung function values refer to wave 6.

2.2.3 Covariates

Sex (female, male), household wealth (in quintiles), physical activity level (sedentary, mild, moderate, high), smoking status (non-smoker, former smoker, current smoker), physical activity (sedentary, mild, moderate, high), presence of one or more chronic diseases (osteoarthritis, cancer, osteoporosis, dementia, depressive symptoms, Alzheimer’s and Parkinson’s diseases), presence of one or more cardiocirculatory diseases (high blood pressure, angina, heart attack, congestive heart failure, heart murmur, and abnormal heart rhythm) and the presence of lung diseases (chronic obstructive pulmonary disease and asthma).

2.3 Statistical analysis

The variables of the study were plasma fibrinogen levels between waves 2 and 4 (normal levels in waves 2 and 4, normal in wave 2 and changed in wave 4, changed in wave 2 and normal in wave 4; changed in waves 2 and 4). The continuous variables were age (years), body mass index (kg/m²), height (m), FEV1 (litres and percentage of predicted) and FVC (litters and percentage of predicted) [23].

Categorical variables were analysed with frequencies and their respective 95% confidence intervals (CI). Continuous variables were described as mean and standard deviation. Associations between fibrinogen profile and FEV1 (outcome) were performed using multiple linear regression adjusted by age, wealth, physical exercise, current smoking status, chronic diseases, lung diseases, body mass index, and height. Crude and adjusted regression coefficients and 95%CI were estimated. Cardiocirculatory diseases were not included in the adjusted model because of the collinearity with the fibrinogen levels. Analyses were sex-stratified. The following post-estimation tests were performed: Shapiro-Wilk and Shapiro-Francia test for normality and Breuschof-Pagan and Cook-Weisberg test for heteroskedasticity. We also performed the command margins to calculate the predictions of the marginal effects of the variable fibrinogen. This information is presented in plots. These procedures were repeated with absolute values of FVC and percentage of predicted FEV1 and FVC. Analyses were performed in Stata SE version 14.0.

3. Results

The analytical sample included 2,150 participants from the ELSA cohort who had fibrinogen data in waves 2 and 4 and lung function data in wave 6. Fig 1 presents the flowchart with the selection process between waves 2 and 6. The follow-up period was 8 years.

Fig 1

Flowchart describing the sample selection from waves 2 (2004/5), 4 (2008/9) and 6 (2012/13), The English Longitudinal Study of Ageing, England, 2004 to 2013.

Table 1 shows sociodemographic and health variables frequency distribution (all sample and sex-stratified). The majority of participants were female (55.9%), belonging to the lowest wealth quintile (24.0%), with moderate physical activity level (51.9%), classified as ex-smokers (54.2%), with chronic diseases (56.6%) and no pulmonary diseases (80.2%). Regarding the fibrinogen profile, 55.4% of the participants remained within the normal levels in waves 2 and 4, and 18.5% showed higher levels in both waves. The main differences found between men and women were related to smoking, physical activity, and the presence of chronic diseases. Women had lower FEV1 and FVC values than men. The FEV1 and FVC percentage of predicted values were similar for men and women.

Table 1

Descriptive analyses of participants from ELSA who presented complete data for fibrinogen in wave 2 (2004–05) and 4 (2008–09) and lung function in wave 6 (2012–13), The English Longitudinal Study of Ageing, England, 2004 to 2013.

Variables		General		Male		Female
		n	% (95%CI)	n	% (95%CI)	n	% (95%CI)
Sex
	Male	948	44.1 (42.0;46.2)
	Female	1,202	55.9 (53.8;58.0)
Wealth
	Lowest quintile (poorest)	264	12.5 (11.1;13.9)	104	11.1 (9.3;13.3)	160	13,5 (11.6;15.5)
	2nd quintile	343	16.2 (14.7;17.8)	136	14.6 (12.5;17.0)	207	17.4 (15.4;19.7)
	3rd quintile	497	23.4 (21.7;25.3)	218	23.4 (20.8;26.2)	279	23.5 (21.2;26.0)
	4th quintile	507	23.9 (22.1;25.8)	239	25.6 (22.9;28.5)	268	22.6 (20.3;25.0)
	Highest quintile (richest)	510	24.0 (22.3;25.9)	236	25.3 (22.6;28.2)	274	23.1 (20.8;25.5)
Smoking status
	Non-smoker	795	37.0 (34.9;39.5)	265	27.9 (25.2;30.9)	530	44.1 (41.3;46.9)
	Former smoker	1,166	54.2 (52.1;56.3)	595	62.9 (59.7;65.9)	570	47.4 (44.6;50.2)
	Current smoker	189	8.8 (7.7;10.1)	87	9.2 (7.5;11.2)	102	8.5 (7.3;10.2)
Physical exercise
	Sedentary	70	3.3 (2.6;4.1)	28	3.0 (2.0;4.2)	42	3.5 (2.6;4.7)
	Low	296	13.8 (12.4;15.3)	93	9.8 (8.1;11.9)	203	16.9 (14.9;19.1)
	Moderate	1,116	51.9 (49.8;54.0)	484	51.1 (47.9;54.2)	632	52.6 (49.7;55.4)
	High	668	31.1 (29.1;33.1)	343	36.2 (33.2;39.3)	325	27.0 (24.6;29.6)
Chronic diseases
	No	930	43.4 (41.3;45.5)	509	53.8 (50.6;57.0)	421	35.1 (32.5;37.9)
	Yes	1,214	56.6 (54.5;58.8)	437	46.2 (43.0;49.4)	777	65.8 (62.1;67.5)
Lung diseases
	No	1,725	80.2 (78.5;81.9)	781	82.4 (78.8;84;7)	944	78.5 (76.1;80.8)
	Yes	425	19.8 (18.1;21.5)	167	17.6 (15.3;20.2)	258	21.5 (19.2;23.9)
Fibrinogen profile
	Normal	1,191	55.4 (53.3;57.5)	575	60.6 (57.5;63.7)	616	51.2 (48.4;54.1)
	Normal/changed	296	13.8 (12.4;15.3)	121	12.8 (10.8;15.0)	175	14.6 (12.7;16.7)
	Changed/normal	265	12.3 (11.0;13.8)	105	11.1 (9.2;13.2)	160	13.3 (11.5;15.4)
	Changed	398	18.5 (16.9;20.2)	147	15.5 (13.3;18.0)	251	20.9 (18.7;23.3)
		n	Mean (SD)	n	Mean (SD)	n	Mean (SD)
Age (years)		2,150	70.5 (7.1)	948	70.5 (7.1)	1,202	70.5 (7.1)
Height (cm)		2,143	165.1 (9.4)	941	172.8 (6.6)	1,202	159.2 (6.4)
Body mass index (kg/m²)		2,104	27.9 (4.6)	908	27.6 (3.6)	1,196	28.1 (5.3)
FEV1 (Liters)		2,103	2.3 (0.7)	928	2.7 (0.7)	1,175	1.9 (0.4)
FEV1 percentage of predicted		1,985	92.1 (16.6)	815	90.7 (15.6)	1,170	93.1 (17.2)
FVC (Liters)		2,129	3.2 (0.9)	940	3.9 (0.8)	1,189	2.7 (0.6)
FVC percentage of predicted		2,082	98.5 (15.7)	915	98.2 (15.7)	1,167	98.7 (15.7)

FEV1: Forced expiratory volume in the first second; FVC: Forced vital capacity.

95%CI: 95% confidence interval; SD: Standard deviation.

The associations between lung function parameters and fibrinogen categories are presented in Table 2. For men, in the crude analysis, the fibrinogen profile was associated with all pulmonary function measurements for all categories, except for FVC percentage of predicted in the changed/normal category. In the adjusted analysis, only the maintenance of high levels of fibrinogen was associated with a significant reduction of lung function measures. FEV1 showed a reduction of 0.17L, FVC of 0.22L, and the percentage of predicted was 5.16 percentage points for FEV1 and 6.21 percentage points for FVC compared to those that maintained normal levels of fibrinogen. Fig 2 shows the linear predictions of the mean values for each category for men.

Table 2

Regression of FEV1 (L), FVC (L), FEV1 percentage of predicted and FVC percentage of predicted, by gender, The English Longitudinal Study of Ageing, England, 2004 to 2013.

Lung function		n	Fibrinogen profile
			Normal/changed		Changed/normal		Changed
			Coefficient (95%CI)	p-value	Coefficient (95%CI)	p-value	Coefficient (95%CI)	p-value
Men
FEV1 (L)
	Crude	928	-0.21 (-0.33;-0.08)	0.002	-0.24 (-0.38;-0.10)	0.001	-0.42 (-0.54;-0.30)	<0.001
	Adjusted*	866	-0.06 (-0.17;0.04)	0.248	-0.05 (-0.16;0.06)	0.391	-0.17 (-0.27;-0.07)	0.001
Percentual predicted FEV1
	Crude	815	-3.40 (-6.66;-0.13)	0.041	-2.08 (-5.47;1.30)	0.228	-7.93 (-11.0;-4.88)	<0.001
	Adjusted*	758	-1.86 (-5.05;1.33)	0.253	-0.11 (-3.46;3.23)	0.948	-5.16 (-8.31;-2.00)	0.001
FVC (L)
	Crude	940	-0.25 (-0.41;-0.09)	0.002	-0.21 (-0.38;-0.04)	0.015	-0.50 (-0.65;-0.35)	<0.001
	Adjusted*	878	-0.07 (-0.20;0.05)	0.259	0.02 (-0.11;0.16)	0.753	-0.22 (-0.34;-0.09)	0.001
Percentual predicted FVC
	Crude	915	-3.93 (-7.06;-0.81)	0.014	-0.65 (-3.89;2.58)	0.691	-8.39 (-11.22;-5.56)	<0.001
	Adjusted*	855	-3.10 (-6.22;0.02)	0.052	1.12 (-2.14;4.39)	0.499	-6.21 (-9.20;-3.22)	<0.001
Women
FEV1 (L)
	Crude	1,175	-0.07 (-0.15;0.01)	0.068	-0.03 (-0.11;0.04)	0.380	-0.16 (-0.22;-0.09)	<0.001
	Adjusted*	1,151	-0.01 (-0.06;0.06)	0.934	0.02 (-0.04;0.09)	0.415	-0.02 (-0.08;0.03)	0.406
Percentual predicted FEV1
	Crude	1,170	-0.45 (-3.32;2.47)	0.764	0.30 (-2.73;3.33)	0.764	-2.96 (-5.45;-0.39)	0.024
	Adjusted*	1,147	0.93 (-1.82;3.71)	0.505	1.46 (-1.42;4.33)	0.321	-0.28 (-2.84;2.28)	0.832
FVC (L)
	Crude	1,189	-0.11 (-0.20;-0.01)	0.031	-0.08 (-0,18;0.02)	0.109	-0.22 (-0.31;-0.14)	<0.001
	Adjusted*	1,165	-0.01 (-0.09;0.06)	0.695	0.01 (-0.07;0.08)	0.856	-0.02 (-0.09;0.05)	0.563
Percentual predicted FVC
	Crude	1,167	-2.02 (-4.69;0.64)	0.137	-1.39 (-4.14;1.37)	0.323	-4.10 (-6.43;-1,76)	0.001
	Adjusted*	1,143	-0.37 (-2.93;2.20)	0.779	0.21 (-2.45;2,87)	0.875	-0.71 (-3.06;1.65)	0.554

95% CI: 95% Confidence intervals; FEV1: Forced expiratory volume in the first second; FVC: Forced vital capacity.

*Adjusted by age in exam, wealth in quintiles, physical exercise, current smoke habits, chronic diseases, lung diseases, height and body mass index.

Fig 2

Linear predictions for the FEV1 (L), FVC (L), FEV1 percent predicted and FVC percent predicted in men, The English Longitudinal Study of Ageing, England, 2004 to 2013.

For women, in the crude analysis, only the maintenance of the abnormal fibrinogen profile was associated with lung function measures. However, none of the categories were associated with lung function measures in women in the adjusted analysis (Table 2). Fig 3 shows the linear predictions of the mean values for each category for women.

Fig 3

Linear predictions for the FEV1 (L), FVC (L), FEV1 percentage of predicted and FVC percentage of predicted in women, The English Longitudinal Study of Ageing, England, 2004 to 2013.

4. Discussion

Our findings showed an inverse significant relationship between lung function and fibrinogen plasma levels in older English men, but not in women. Persistent high fibrinogen plasma levels in men were significantly associated with reduced lung function measures independently of age, wealth, physical activity, smoking, chronic diseases, lung diseases, body mass index and height.

To the best of our knowledge, this is the first study to investigate changes in fibrinogen levels over a long follow-up period and their relationship with lung function later in life in community-dwelling older adults without pre-existing lung and chronic diseases like chronic obstructive pulmonary disease, asthma, and cancer. Findings from longitudinal studies that correlated fibrinogen with lung function in other populations are inconsistent due to different methodological designs and statistical models used, making comparability of results difficult [12-14].

Some authors emphasized that systemic inflammation may be involved in the pathogenesis of chronic lung disorders and suggest that fibrinogen may accelerate lung function decline in young adults besides predispose them to airway diseases [12,14]. However, a study carried out in New Zealand, with adults aged 32 to 38 years, showed no significant association between fibrinogen and lung function in men at age 32. In women aged 32, FEV1 and FVC were inversely associated with fibrinogen. But in the longitudinal models, the authors did not find evidence that systemic inflammation predicted a decline in lung function [25]. Another study, using data from the Newcastle Thousand Families birth cohort at age 49–51 years, found significant inverse associations between both FEV1 and FVC and plasma fibrinogen concentration, even after adjustments for potential confounders, including early life indicators, total pack/years of cigarette smoked and contemporaneous BMI. However, their findings did not remain significant after adjustment for contemporary percent body fat [26].

ELSA findings demonstrate that lung function decline is independent of chronic and respiratory diseases [27]. ELSA has information on important demographic and clinical parameters, which allowed us to carefully adjust potential confounding factors in our study. The most important potential confounding factor was smoking, which in itself induces systemic inflammation and is a known risk factor for pulmonary function decrease [28]. It was reassuring that, even for non-smokers and individuals with cancer or COPD, there is a relationship between the maintenance of altered fibrinogen levels and the reduction in FEV1 and FVC only in men. Changes in fibrinogen levels i.e., altered to normal overtime were not associated with impaired lung function. For women, the changes in fibrinogen levels did not affect their lung function over time. So far, we cannot easily explain the sex difference in our results. It is necessary to assess whether this persists with advancing age in future ELSA cohort assessments. However, Hancox and colleagues showed that the association between inflammatory markers and lung function appears to be stronger in men than in women [25].

Data from 3,257 Japanese individuals showed that fibrinogen plasma levels were significantly associated with decreased FVC and FEV1 values in men with restrictive, obstructive, and mixed ventilatory dysfunctions. However, there was no association between fibrinogen plasma levels and the reduction of pulmonary function measurements in women [11]. These results are consistent with our findings.

The association between fibrinogen levels and lung function has been investigated in people with COPD and asthma, as well as smokers. Inflammatory markers are useful in COPD to identify the risk of exacerbations and mortality [29-32]. Besides that, circulating fibrinogen was significantly increased in subjects with heart disease, hypertension, and diabetes and coexistent COPD [33]. Hyun et al.[29] demonstrated that patients with COPD with high plasma fibrinogen concentrations and low 25-OH vitamin D presented lower pulmonary function, higher COPD severity index, and higher rate of severe exacerbations over 24 months.

In individuals with asthma and chronic cough, findings have shown that levels of plasma fibrinogen are higher than in those without chronic cough [34]. In smokers with symptoms but with preserved spirometry, fibrinogen levels were not associated with respiratory symptom burden, exacerbations, 6-minute walk distance, and FEV1 [35]. Previous studies have demonstrated that high fibrinogen levels increase, especially when lung disease is present, but our findings have shown that higher levels are involved in reduction of lung function even in older adults without respiratory dysfunctions, especially in men.

The present study has some strengths that should be considered. First, we used two well-validated measures i.e., fibrinogen and spirometry from a large population-based and nationally representative cohort of English adults aged 50 years and older. These measures were collected through well-standardized procedures. Both tests could provide valuable information about diagnosis and prognosis. Secondly, lung function evaluation was standardized with quality criteria [23]. Thirdly, we used the reference prediction equations for lung function, which have been validated for most countries [24]. This study also has some limitations that need to be recognized. Survival bias could be a potential limitation. Besides, the lack of cause-specific mortality data of individuals with lung impairment information during the follow-up period. Another potential limitation of this study is the unavailability of information about the hepatic function or chronic hepatic disease. The liver plays a key role in the blood coagulation process because it is the site of synthesis of all clotting factors and their inhibitors. Liver damage is commonly associated with impairment of haemostasis. The activation of blood coagulation process is associated with decreased lung function, and that systemic inflammation may contribute to this relation [36].

5. Conclusion

We found that persistent high levels of plasma fibrinogen in older English men, but not in women, are associated with a reduced lung function eight years later. Our findings suggest that plasma fibrinogen may be an important biomarker of pulmonary dysfunction.

17 Jun 2021

PONE-D-20-39275

Are elevated plasma fibrinogen associated with lung function? An 8-year follow-up of the ELSA study

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Reviewer #1: Yes

Reviewer #2: Partly

Reviewer #3: Partly

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

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

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

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Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

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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

Reviewer #3: Yes

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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: Overall Impressions

This is an important scientific work that brings out the correlation between the level of fibrinogen and lung functions in old men. From the data and conclusion of the authors, significantly high levels of plasma fibrinogen are observed in the old men with lung problem. Thus the implementation of this parameter as a biomarker or indicator in the diagnosis and/or prognostics of lung dysfunction could be envisaged after a thorough follow up or control of this work.

Corrections

Minor

Page 2, Line 28. ‘percentages of predicted’ should read ‘percentages predicted’

Line 33. Conclusion: ‘We found’ should read ‘It was found’

Line 37. Key words: ‘Respiratory Function Tests’ should read ‘Respiratory Functions’

Page 4, Line 88. ‘plasma is dispensed’ should read ‘plasma was dispensed’

Line 88. ‘C XL reagent is added’ should read ‘C XL reagent was added’

Page 5, Line 103. ‘FEV1 and FVC expressed’ should read ‘FEV1 and FVC were expressed’

Page 7, Line 150. Table 1 ………. ‘ELSA who presented complete data from for fibrinogen’

Major

None detected

Reviewer #2: Thank you for inviting me to review the manuscript entitled “Are elevated plasma fibrinogen associated with lung function? An 8-year follow-up of the ELSA study” It is a good piece of work however, in my opinion it lacks novelty.

Reviewer #3: In this study, the authors reported that a high level of plasma fibrinogen in older English men, but not women, is associated with a decline in lung function. In my opinion, the study is interesting however, I have some major concerns regarding the study design and the inclusion criteria implied in this study. I would like the authors to address the following concerns;

1- The major concern is the inclusion of subjects with pulmonary diseases which affect the lung function in these subjects and thus affect the reliability of the results. These subjects should be excluded to express only the relationship between fibrinogen level and lung function.

2- It is not clear is there any subjects in this study with cardiocirculatiry disorder?

3- The study design regarding the timeline of fibrinogen assessment and Lung function evaluation should be further described briefly. What is meant by wave 2, 4, and 6? Please provide a brief clear description of this part in methodology.

4- Methods: Line 80: “those with a clotting or bleeding disorder or on anticoagulant drugs, those who had ever had…. were not asked to fast.” These cases were stated to be excluded. In this statement they were asked not to fast before sampling? Did you include these cases?

5- In table 2: the significance should be defined with p-values.

6- The limitations: Selection bias should be addressed as a limitation in this study.

**********

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Reviewer #1: Yes: Dr LUNGA Paul KEILAH

Reviewer #2: No

Reviewer #3: Yes: Muhammad Tarek Abdel Ghafar

[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.]

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Submitted filename: Review comments.pdf

Click here for additional data file.

Submitted filename: review.docx

Click here for additional data file.

16 Oct 2021

Dear Emiliano Giardina and Reviewers,

We would appreciate the consideration of our Research Article entitled “Are elevated fibrinogen plasma levels associated with lung function? An 8-year follow-up of the ELSA study” for publication in PLOS One.

Thank you for the reviewers' considerations and suggestions. After corrections we present a new version of the article, and we hope to be in agreement for publication.

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

We appreciated this comment, and we review the style of the document.

2. In the methods section please provide details regarding how house hold wealth and physical exercise was stratified for analysis.

Thanks for the comment. We added at the references that describle how these variables are stratified.

Socioeconomic position was based on the National Statistics Socio-Economic Classification (NS-SEC) and quintiles of total wealth, defined from the sum of financial, physical (e.g. businesses, land) and housing wealth, minus debts and pension payments (Banks J, Batty G, Coughlin K, et al (2019) English Longitudinal Study of Ageing: Waves 0-8, 1998-2017. 29th Edition. UK Data Service. SN: 5050, London)

Participants were asked how often they took part in three different types of physical activity: vigorous, moderate and low intensity. The response options were more than once a week, once a week, one to three times a month and hardly ever/never. For the purposes of the analysis, we derived a summary index of physical activity by summing responses to the three physical activity questions after they had been dichotomised around the frequency cut-point of once a week or more often. The derived summary index categorised physical activity as follows: (1) physical inactivity; (2) low-intensity but not vigorous/moderate-intensity physical activity at least once a week; and (3) vigorous/moderate-intensity physical activity at least once a week. (Demakakos, P., Hamer, M., Stamatakis, E. et al. Low-intensity physical activity is associated with reduced risk of incident type 2 diabetes in older adults: evidence from the English Longitudinal Study of Ageing. Diabetologia 53, 1877–1885 (2010). https://doi.org/10.1007/s00125-010-1785-x)

3. Thank you for stating the following in the Acknowledgments Section of your manuscript:

"The English Longitudinal Study of Ageing was developed by a team of researchers based at the University College London, NatCen Social Research, the Institute for Fiscal Studies and the University of Manchester. The NatCen Social Research collected the data. The National Institute of Aging Grant R01AG017644 and a consortium of UK government departments coordinated by the Economic and Social Research Council provide the funding."

We note that you have provided funding information that is not currently declared in your Funding Statement. However, funding information should not appear in the Acknowledgments section or other areas of your manuscript. We will only publish funding information present in the Funding Statement section of the online submission form.

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to publish or prepare the manuscript."

Please include your amended statements within your cover letter; we will change the online submission form on your behalf.

Thanks for the corrections, all were executed in the manuscript.

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Data Sharing

The English Longitudinal Study of Ageing data are available to the scientific community from the UK Data Service for researchers who meet the criteria for access to confidential data, under conditions of the End User License http://ukdataservice.ac.uk/media/455131/cd137-enduserlicence.pdf. The data can be accessed from: https://beta.ukdataservice.ac.uk/datacatalogue/series/series?id=200011#!/access-data. Contact with the UK Data Service regarding access to the English Longitudinal Study of Ageing can be made through the website https://www.ukdataservice.ac.uk/about-us/contact, by phone +44 (0)1206 872143 or by email at help@ukdataservice.ac.uk.

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Thanks for the comments. The author Ione Schneider has two accounts in PLOS. One is related to username ione.schneider and the other one, to the username ione.jayce. We need to merge these two accounts to be able to validate with Ione’s ORCID (0000-0001-6339-7832).

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The paragraph on ethics statement was removed from the end and is only described in the methods section.

Reviewer #1:

Corrections

Page 2, Line 28. ‘percentages of predicted’ should read ‘percentages predicted’

Line 33. Conclusion: ‘We found’ should read ‘It was found’

Line 37. Key words: ‘Respiratory Function Tests’ should read ‘Respiratory Functions’

Page 4, Line 88. ‘plasma is dispensed’ should read ‘plasma was dispensed’

Line 88. ‘C XL reagent is added’ should read ‘C XL reagent was added’

Page 5, Line 103. ‘FEV1 and FVC expressed’ should read ‘FEV1 and FVC were expressed’

Page 7, Line 150. Table 1 ………. ‘ELSA who presented complete data from for fibrinogen’

Thanks for the corrections, all were executed in the manuscript.

Reviewer #2:

Thank you for inviting me to review the manuscript entitled “Are elevated plasma fibrinogen associated with lung function? An 8-year follow-up of the ELSA study” It is a good piece of work however, in my opinion it lacks novelty.

Thanks for the comments. This is the first time that the relation between lung function and fibrinogen is studied in the English population.

Reviewer #3:

In this study, the authors reported that a high level of plasma fibrinogen in older English men, but not women, is associated with a decline in lung function. In my opinion, the study is interesting however, I have some major concerns regarding the study design and the inclusion criteria implied in this study. I would like the authors to address the following concerns;

1- The major concern is the inclusion of subjects with pulmonary diseases which affect the lung function in these subjects and thus affect the reliability of the results. These subjects should be excluded to express only the relationship between fibrinogen level and lung function.

The questionnaire used contained information about self-reported lung diseases (COPD and asthma). Thus, this self-reported information was used as adjustment variables in the analyses and interactions and multicollinearity between variables were tested, which were not detected.

2- It is not clear is there any subjects in this study with cardiocirculatiry disorder?

The disease high blood pressure, angina, heart attack, congestive heart failure, heart murmur, and abnormal heart rhythm were classified in the presence of one or more. However, they were excluded due to multicollinearity.

3- The study design regarding the timeline of fibrinogen assessment and Lung function evaluation should be further described briefly. What is meant by wave 2, 4, and 6? Please provide a brief clear description of this part in methodology.

Thanks for the comments. We clarified the information about the timeline of fibrinogen and lung function between the waves.

4- Methods: Line 80: “those with a clotting or bleeding disorder or on anticoagulant drugs, those who had ever had…. were not asked to fast.” These cases were stated to be excluded. In this statement they were asked not to fast before sampling? Did you include these cases?

The members with described problems are not eligible to the blood sample. This way, these cases are not included in the analysis.

5- In table 2: the significance should be defined with p-values.

This information was updated at the table.

6- The limitations: Selection bias should be addressed as a limitation in this study.

Thanks for the comments. We addressed the selection bias to the limitations.

Thank you for your time and consideration.

Yours sincerely

Dr Ione Jayce Ceola Schneider

Affiliate Academic

University College London, Department

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

21 Oct 2021

Are elevated plasma fibrinogen associated with lung function? An 8-year follow-up of the ELSA study

PONE-D-20-39275R1

Dear Dr. Schneider,

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.

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Kind regards,

Emiliano Giardina

Academic Editor

PLOS ONE

26 Oct 2021

PONE-D-20-39275R1

Are elevated plasma fibrinogen associated with lung function? An 8-year follow-up of the ELSA study

Dear Dr. Schneider:

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.

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on behalf of

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Academic Editor

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