Reliability and validity of 4-metre gait speed in COPD

Reliability

The 4MGS showed excellent test-retest and interobserver reliability, with ICC values of 0.97 (95% CI 0.95–0.98) and 0.99 (95% CI 0.98–0.99), respectively. Measurement variability was low, with SEM 1.4% for interobserver and 1.5% for test-retest reliability. Similarly, no systematic variability was demonstrated with narrow limits of agreement between occasions (0.135 to -0.125) or observers (0.047 to -0.067) (fig. 1). Mean difference between test occasions was 0.005 m·s⁻¹ and between observers was -0.01 m·s⁻¹.

• Download figure
• Open in new tab
• Download powerpoint

Figure 1–

Bland–Altman plots for a) test-retest and b) interobserver reliability.

Cross-sectional measurement of 4MGS

Baseline characteristics are shown in table 1. Sixty-seven, thirty-three and four patients used a walking aid only, ambulatory oxygen only or both, respectively. There was a strong positive correlation between 4MGS and ISW (table 2 and fig. 2a). Inspection of figure 2a shows a curvilinear relationship (second-order polynomial) between 4MGS and ISW (goodness of fit: R² = 0.59) with the curve flatter at low ISW values, suggesting a floor effect to the ISW. 4MGS was also negatively correlated with age, MRC dyspnoea scale and SGRQ (activity and impact domains, total score in fig. 2b). There was a weak but statistically significant relationship with forced expiratory volume in 1 s (FEV₁) % pred (ρ = 0.10; p = 0.01) (fig. 2c) and body mass index (BMI) (ρ = -0.09; p = 0.03).

• Download figure
• Open in new tab
• Download powerpoint

Figure 2–

The relationship between 4-m gait speed (4MGS) and a) incremental shuttle walk (ISW) distance; b) St George's Respiratory Questionnaire (SGRQ) total score; c) forced expiratory volume in 1 s (FEV₁) % predicted.

Clinical characteristics related to slow gait speed

193 patients had a slow 4MGS and 393 had preserved 4MGS. Participants with slow 4MGS were slightly older and had higher BMI, but there were no differences in sex distribution or FEV₁ % pred. However, large deficits in exercise capacity and health status were observed in the slow 4MGS group in ISW distance and SGRQ total score (table 3).

Critique of the method

Although gait speed has been reported over a variety of distances, most investigators currently believe that 4 m is optimal for a field test [18], balancing a distance short enough to be feasible in most clinical settings, but long enough to ensure measurement accuracy. Test-retest reliability for 4MGS is better than that reported over an 8-foot course [19] and as reliable as over 6 m [20]. A 4-m course has also been shown to be feasible in 90% of households [21], allowing the possibility of using the 4MGS as a tool in community- or home-based studies. Consistent with this, in our study, we observed very high ICCs for both test-retest and interobserver reliability. Our results were perhaps towards the better end of those previously reported [8, 19, 20], and this may reflect differences in patient populations. It is important to note that reliable 4MGS measurements can be made even by nonprofessional staff [6] and good reliability has been shown in frail patients or those with cognitive dysfunction [19, 20].

We asked patients to walk at their usual or comfortable speed in line with most previous reports. Although some investigators have modified the test by assessing maximal walking speed [8, 18], much of the data on the prognostic value of gait speed have focused on usual speed, and there is no evidence to indicate that measurement of maximal walking speed is any more reliable than usual speed [8, 22]. However, we suspect that usual gait speed would be close to maximal speed in symptomatic severe COPD patients

Another consideration was the choice of gait speed cut-off. Several cut-off points for prediction of adverse health outcomes have been previously proposed. These have partly depended on the length of track, settings, the population being studied and the outcome of interest. For the 4MGS, “slow” is typically defined by expert consensus groups as a walking speed <0.8 m·s⁻¹ [1, 17]. Recently, in a pooled analysis of nine cohort studies of older adults (totalling 34 485 individuals), years of remaining life for each sex and age increased as gait speed increased, with a gait speed of 0.8 m·s⁻¹ at the median life expectancy for most ages [4]. Future longitudinal studies are required to validate the predictive value of this 4MGS cut-off in COPD populations, although, given the large differences in health outcomes demonstrated in our study, this cut-off appeared to be discriminatory.

Significance of the findings

The 4MGS as a single item tool has not been previously reported in COPD. Roig et al. [23] recently reported self-selected gait speed in 21 COPD patients over a 5-m course, demonstrating a 20% reduction in speed compared with age-matched controls. Ilgin et al. [24] described significant associations between gait speed and lung function parameters and health-related quality of life in COPD, although gait speed was calculated from 6-min walk distance rather than from a short course. Andersson et al. [25] showed high test-retest reliability when measuring walking speed in 49 COPD patients, although the 30-m course may limit generalisation of the findings. Evans et al. [26] demonstrated the validity of self-paced walking and responsiveness to pulmonary rehabilitation in 37 COPD patients. Interestingly, although the test was performed in an enclosed, temperature-controlled 250-m flat corridor, the repeatability was similar to that of 4MGS observed in our study. In addition to the considerably larger sample size, our data adds to the current literature by demonstrating that 4MGS correlates significantly to health outcome measures commonly used in COPD.

The 4MGS is also noted to be a single component of the Short Physical Performance Battery (SPPB). Previous data on the SPPB in COPD have been described in a single cohort [27–29], demonstrating lower (worse) SPPB scores in COPD patients compared with age-matched healthy controls, and an association between increased lean-to-fat ratio and preserved lung function with higher SPPB scores. This cohort was restricted to 40–65 years of age and tended to have milder disease: 30% of patients were in the now-defunct GOLD stage 0 [27, 28]. Our patients were older and more typical of COPD patients seen in hospital practice. Furthermore, we demonstrated that just one component of the SPPB had a strong association with clinically relevant outcomes. The 4MGS is quicker to perform, and previous studies have shown that the 4MGS has similar associations and predictive abilities to the SPPB [6, 30].

The 4MGS has been frequently used in epidemiological studies of older adults due to its ease and speed of use. It has been shown to predict health status, functional decline, institutionalisation, onset of disability and mortality [1, 3, 18, 30]. Other investigators have used gait speed as a functional outcome measure [31], and expert working groups (including representatives from the US Food and Drug Agency) have identified the 4MGS as the most appropriate functional test for interventional studies in frail older adults and sarcopenia [9, 32]. The 4MGS has potential as an assessment tool in COPD. It is quick to perform, and cheap. Walking is familiar to the vast majority of adults, and the test is easy to understand; the 4MGS has been shown to be highly reliable even in people with cognitive dysfunction [20]. Furthermore, it requires little space, and the operator does not require any specialist training. These properties mean that the 4MGS could be adopted in almost any clinical environment (even at the bedside, or in primary care).

Our study demonstrated that 4MGS is reliable in COPD with excellent test-retest reliability. This is significant, particularly as current field tests of exercise capacity show a learning effect [11, 33] and some, like the 6-min walk test, require a 30-m course. Evidence of concurrent validity was demonstrated by showing significant associations with exercise capacity, health status and dyspnoea. By stratifying COPD patients according to 4MGS, we showed that the slow walking speed clinical phenotype exhibited significant deficits in exercise capacity, increased dyspnoea and poor health status in comparison to those with preserved walking speed. Interestingly, despite large differences in these outcome measures (which exceeded the commonly agreed minimum clinically important differences), there was no significant difference in FEV₁ % pred between the groups. Although 4MGS was shown to correlate negatively with age and BMI, it is unlikely that these were significant confounding factors for our results, given that there were still large differences in ISW % pred (which corrects for age and BMI). Furthermore, 4MGS was not simply a reflection of comorbidity burden; we showed no or very weak relationships between 4MGS and validated comorbidity indices. We propose that 4MGS, like exercise capacity, reflects global wellbeing and captures the multisystemic effects of disease severity in COPD rather than pulmonary impairment alone.

Limitations

The study population was selective in that most were GOLD stage II–IV and symptomatic hospital outpatients. Our results need to be corroborated in other COPD populations (e.g. hospitalised inpatients or milder asymptomatic patients) in different settings. As the current study was cross-sectional, we could not evaluate the prognostic value of 4MGS in COPD. Longitudinal studies are required to assess the ability of 4MGS to predict clinically useful end-points. Although 4MGS was shown to correlate significantly with ISW, it is important to stress that the tool is not a measure of peak exercise capacity. However, it is an indicator of functional impairment, particularly in frail older patients [9], and has been previously used as a functional outcome measure in interventional pharmacological trials [31]. The curvilinear relationship between ISW and 4MGS (fig. 2a) suggests that the 4MGS may have a particular role in the assessment of poorly functioning individuals (i.e. there was a wide range in 4MGS in patients with very low ISW values). Further studies are needed to test the responsiveness of 4MGS to intervention in COPD. 4MGS may also have a role in rapidly screening COPD patients for extrapulmonary manifestations prior to more complex objective measurement of the outcome of interest. We did not make measurements of muscle mass or function, but this seems likely to be a fruitful area of research in COPD, particularly as expert consensus groups in sarcopenia and cachexia already recommend 4MGS as a screening tool [17, 32].

In summary, our study confirms that 4MGS is feasible and reliable in COPD, and correlates with measures of exercise capacity and health status. This demonstrates the potential utility of 4MGS as a simple assessment tool of multisystem wellbeing in COPD.