Normative reference values for the two-minute walk test derived by meta-analysis.

[Purpose] This meta-analysis was undertaken to establish normative reference values for the two-minute walk test. [Methods] Three database searches and a hand search were conducted. Meta-analysis was used to consolidate two-minute walk test data stratified by gender and age group.
[Results] Data from four articles was consolidated. Normative two-minute walk test distances varied from 150.3 m (women, 70 to 79 years) to 217.9 m (men, 20 to 29 tears).
[Conclusion] Normative reference values derived using meta-analysis should provide a better standard than individual studies for interpreting the two-minute walk test performance of adults.

INTRODUCTION

Walk tests are widely used as a means of quantifying functional exercise capacity1). The duration of the tests varies, but the six minute duration has been recommended by the American Thoracic Society2). Their recommendation notwithstanding, many individuals have difficulty walking for such a long period of time. As a consequence, shorter walk tests, the two-minute walk test (2MWT) in particular, are being used increasingly.

Interpretation of 2MWT distances achieved by individuals and groups requires that normative reference values are available. Several studies have purported to provide reference values or equations for the 2MWT3,4,5). Other studies contain data that may be informative as to “normal” performance on the 2MWT6,7,8). Together, these studies have the potential to provide normative reference values that are more informative than data of any one study alone. The purpose of this study, therefore, was to use meta-analysis to determine normative reference values for the 2MWT. The study focused on adults.

METHODS

Three electronic databases (PubMed, CINAHL, and Scopus) were searched for studies reporting normative reference values for the 2MWT. The search was conducted on June 30, 2017 and involved the search string “’two minute walk test’ OR ‘2 minute walk test.’” Thereafter, a hand search was conducted using article reference lists and personal files.

Consideration for inclusion required that a study reported 2MWT distances for healthy adults. Based on previous research showing that 2MWT distance is influenced by gender and age3,4,5), studies were excluded if they did not stratify 2MWT distance by gender and decade of age or if 2MWT distance so stratified could not be obtained from the authors.

In addition to information on 2MWT distance, potentially relevant studies were scrutinized for information on the sample of adults tested, the walking course used, and instructions/feedback provided. The Comprehensive Meta-analysis (3.0) program was used to consolidate 2MWT distance data for each populated stratum (eg, 70 to 79 year old men) across studies. A random-effects model was used. Homogeneity was described by I2.

The quality of included studies was graded using a custom 6-item (10 point) checklist. Similar checklists have been used previously in descriptive meta-analyses9).

RESULTS

Figure 1 is a flowchart illustrating how 143 articles were culled from an initial 147 unique articles to yield 4 studies for inclusion in meta-analysis. Table 1 summarizes study specifics. The included studies contributed data from 40 to 3,075 participants who resided in the United States or India. All studies involved out-and-back courses with distances of 15.2 to 30.0 meters. The instructions indicated that participants should “cover as much ground as possible” or “walk as fast as you can.” Feedback was indicated in two studies; that is, “you are doing well” and 1 minute was left or remaining.

Fig. 1.

PRISMA flowchart illustrating selection of articles for meta-analysis.

Table 1.

Summary of four studies included in meta-analysis

Study	Sample	Course	Instructions/Feedback
Bohannon (2017)PhyStat 7	Community-dwelling adults(n=40) [United States]	Out-and-back(15.2 m)	“Cover as much ground as possible without running.”“You are doing well, you have 1 min remaining.”
Bohannon (2015)NIH Toolbox	Community-dwelling adults(n=1185) [United States]	Out-and-back(15.2 m)	“Walk as fast as you can.”“You are doing well, you have 1 min left.”
Priya & Verma(2015)	Healthy adults(n=300) [India]	Out-and-back(30.0 m)	Not stated
White et al. (2014)Health ABC	Well-functioning older adults(n=3,075) [United States]	Out-and-back(20.0 m)	“Cover as much ground as possible.”

Table 2 summarizes 2MWT distances by study and stratum. The 12 strata incorporated data from 2 to 4 studies and 94 to 1209 individuals. The 2MWT distance varied by study and stratum, but overall it was greatest for 20 to 29 year old men (217.9 meters) and least for 70 to 79 year old women (150.3 meters).

Table 2.

Summary of two-minute walk test distance stratified by gender and age from four studies

Group (n)	Bohannon (2017)Mean (SD) [n]	Bohannon, et al. (2015)Mean (SD) [n]	Priya & Verma (2015)Mean (SD) [n]	White et al. (2014)Mean (SD) [n]	Consolidated
					WeightedMean (SE) [n]	95% CI	I2
Men 20–29	219.6 (25.0) [10]	210.2 (28.8) [67]	225.4 (26.4) [31]		217.9 (5.4) [108]	207.2–228.6	0.00
Women 20–29	203.2 (15.3) [10]	180.7 (24.9) [171]	199.9 (24.4) [22]		194.1 (8.4) [203]	177.7–210.5	0.00
Men 30–39		201.4 (29.7) [74]	204.2 (28.3) [24]		202.1 (3.0) [98]	196.3–207.9	0.00
Women 30–39		181.8 (27.1) [215]	179.3 (26.0) [33]		181.4 (1.7) [248]	178.1–184.8	0.00
Men 40–49		191.1 (30.1) [85]	194.6 (27.9) [31]		192.1 (2.7) [116]	186.8–197.5	0.00
Women 40–49		183.3 (29.4) [151]	170.3 (18.3) [38]		180.7(10.4) [189]	160.3–201.0	0.00
Men 50–59		189.1 (28.7) [73]	190.9 (20.4) [25]		189.8 (2.6) [98]	184.7–194.9	0.00
Women 50–59		178.8 (22.6) [99]	158.8 (24.1) [31]		169.1 (10.0) [130]	149.6–188.7	0.00
Men 60–69	219.5 (28.7) [4]	177.3 (34.2) [60]	174.9 (19.4) [30]		183.0 (7.0) [94]	169.3–196.8	61.3
Women 60–69	184.5 (25.0) [6]	161.9 (29.7) [75]	151.7 (25.9) [18]		163.7 (6.9) [99]	150.0–177.3	36.5
Men 70–79	183.9 (27.2) [6]	164.4 (30.1) [59]	138.7 (28.2) [13]	167.7 (30.1) [1,131]	163.1 (5.3) [1,209]	152.7–173.5	55.1
Women 70–79	166.3 (29.6) [4]	145.4 (27.5) [56]	153.3 (16.2) [4]	150.7 (27.9) [1,124]	150.3 (1.3) [1,188]	147.7–152.9	0.00

Quality ratings (Table 3) ranged from 6 to 9 out of 10 possible points. Scores were limited by inadequate descriptions of the tested samples and of the 2MWT task.

Table 3.

Quality ratings of four studies included in the meta-analysis

Study	SampleDescribed (2)	Inclusion/Exclusion (2)	TaskDescribed (3)	MeasurementDescribed (1)	SummaryStatistics (1)	Reliability (1)	Total (10)
Bohannon (2017)	1	2	3	1	1	1	9
Bohannon (2015)	1	2	3	1	1	1	9
Priya & Verma (2015)	0	2	1	1	1	1	6
White et al. (2014)	2	2	2	1	1	1	9

*Items and scoring: 1) Sample adequately described: type, enrollment noted as consecutive or timeframe indicated, 2) Participant inclusion/exclusion criteria explicit, 3) Task described: instructions, feedback, course, 4) Measurement adequately described: criterion measure 5) Summary statistics (mean/median, SD/SE/range/ confidence inetrvals) provided, 6) Reliability of measures reported.

DISCUSSION

The intent of this study was to use meta-analysis to generate normative reference values for the 2MWT. The analysis was limited to adults whose data could be used to provide reference values for gender and age group strata. As the reference values derived in this study were based on multiple studies and relatively large total sample sizes, they should provide a better indication of normative performance than any one original study alone. The I2 values, which with 2 exceptions were all less than 40.0%, suggest that the data were homogeneous within strata and inter-study data consolidation was justified.

Like previous individual studies of the 2MWT, the present meta-analysis demonstrated that men walked further than women and younger age groups walker further than older age groups3,4,5). This is validating, as is the finding that normative walking distances derived in this meta-analysis are greater than the mean 2MWT distances reported for older adults residing in long-term care (77.4 m)10) and comparably aged adults with various pathologies or conditions: lower limb amputations (27.9 m)11), chronic stroke (58.5 m)12), late-onset sequelae of poliomyelitis (136.0 m)13), cardiac disease (138.0 m)14), and chronic obstructive pulmonary disease (150.0 m)15).

This study had several limitations, the principal being the limited number of studies contributing data to the analysis. This limitation was in part unavoidable; it stemmed from the small number of studies actually reporting 2MWT data. Nevertheless, it was aggravated by the inability to obtain necessary information from investigators who collected the required 2MWT data5, 7) but did not respond to requests for summary data stratified by gender and age group. Another limitation of this study was the lack of diversity in the nationality of participants in the studies contributing to it. A broader range of participant groups will be required if factors other than gender and age group are to be addressed.

For the time being, the normative reference values provided herein may prove useful for interpreting the 2MWT performance of adult men and women. However, the full value of the 2MWT will not be realized until normative reference values from multinational population based studies are completed.

Conflicts of interest

The author has no conflicts of interest to declare.