Interstitial lung diseases (ILD) are a heterogeneous group of diffuse parenchymal lung disorders which, despite presenting diverse etiologies, share several clinical features.1,2 Although comprising various degrees of inflammation and/or fibrosis, a percentage of ILD patients can develop a progressive self-sustaining fibrosis, namely those with idiopathic nonspecific interstitial pneumonia, unclassifiable idiopathic interstitial pneumonia, connective tissue disease-associated ILDs, hypersensitivity pneumonitis, sarcoidosis and ILDs related to other occupational exposures (e.g. silicosis, asbestosis).3 This progressive-fibrosing phenotype may lead to worsening of symptoms (mainly exertional dyspnea, dry cough and fatigue), progressive impairment in gas exchange and lung function decline, reduced exercise capacity, muscle dysfunction and reduced quality of life.4,5 Some of these patients can also present a median survival rate below 5 years after diagnosis and generally exhibit high associated socioeconomic costs, high dependency on others to perform daily living activities and a massive burden on healthcare systems.6-9

In patients with ILD, reduced exercise capacity is associated with poor health-related quality of life and increased hospital admissions.10,11 Finding valid, feasible and standardized tests which enable comparison with standard values and provide agreement between healthcare professionals is, thus, a priority in order to provide the best quality of care to this population.11 The gold standard to evaluate exercise capacity is cardiopulmonary exercise testing (CPET).12 However, CPET is not easily available in clinical practice as it requires expensive equipment, the presence of specialized human resources and is time-consuming.13 To overcome these limitations, field tests such as the 6-minute walk test (6MWT), the incremental shuttle walk test (ISWT) and, more recently, the Chester Step Test (CST) have been used to assess exercise capacity in patients with chronic respiratory diseases.13-15 Field tests are more affordable and simpler to apply than CPET and are better related to patients’ demands during activities of daily living.13,16 Particularly, the CST requires less space than the other field tests, which allows it to be easily applied in different settings, including inpatient, outpatient and home-based settings.16

The CST is an externally paced, incremental and multistage test, designed to assess exercise capacity in healthy individuals.15,16 Recently, it has been validated to assess exercise capacity in patients with chronic obstructive pulmonary disease (COPD).15 However, the physiological mechanisms of exercise limitation in patients with COPD differ significantly from those in patients with ILD, in whom exercise intolerance is mostly due to impaired gas exchange and circulation limitation.10,17 Thus, it is imperative to test the measurement properties of the CST in ILD to assure that the selection of this instrument for research and clinical practice is evidence-based and its results can be reliably interpreted.18

The authors hypothesize that the number of steps in the CST will present: (1) excellent intra and inter-rater reliability; (2) significant, positive, and high correlation with the distance covered in the 6MWT (6MWD). The main purpose of this study was to assess the reliability (relative and absolute) and construct validity of the CST in patients with ILD.

Seventy-eight patients with ILD were screened to be included in the study. Sixty-six were eligible to participate and twelve were excluded for the following reasons: decline to participate (n=4), drop out for no reason given (n=1), presence of a significant cardiovascular disease (ischemic cardiomyopathy associated with myocardial infarction, n=4), presence of a significant musculoskeletal disease (severe gonarthrosis, n=2), presence of cognitive impairment (Alzheimer's disease, n=1). Sixty-six participants were included for the construct validity study and fifty-three for the reliability study, since thirteen individuals did not attend the second assessment session due to participants unavailability. A flow diagram of recruitment is provided in Fig. 1.

Fig. 1.

Flow diagram of participants with interstitial lung disease included in the study.

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Eligible participants were on average 65.5±12.9 years old, slightly overweight (body mass index= 28.9±5.2kg/m2) and 48.5% were male (n=32). Most prevalent types of ILD were chronic hypersensitivity pneumonitis (n=29, 43.9%), followed by idiopathic pulmonary fibrosis (n=16, 24.2%) and sarcoidosis (n=6, 9.1%). Participants presented a mean forced expiratory volume in one second (FEV1) of 81.9±19.9% of predicted and mean forced vital capacity (FVC) of 79.4±18.8% of predicted. Nineteen participants (28.8%) presented mild ILD (DLCO >60% predicted), 18 participants (27.3%) moderate ILD (40%≤DLCO≤60% predicted) and 19 participants (28.8%) severe ILD (DLCO<40% predicted). We were unable to access DLCO values for 10 patients (15.1%). Thirty-one participants used long-term oxygen therapy (47%) and five participants used non-invasive ventilation during sleep (7.6%). The mean number of steps taken in CST-1 and CST-2 were 77.7±50.2, and 82.4±55.7, respectively. The main reason for CST termination was the inability to maintain the required step cadence. The mean 6MWD was 399.4±128.2 meters (83.1±26.4% of predicted). A detailed sample characterization is summarized in Table 1.

Reliability

The CST demonstrated excellent relative reliability, for both intra-rater reliability (ICC1,1=0.95; 95%CI 0.91-0.97) and inter-rater reliability (ICC2,1=1.0; 95%CI 0.99-1.0). Regarding absolute reliability, SEM and MDC95 values were 11.8 steps (SEM%=14.7%) and 32.6 steps (MDC95%=40.7%), respectively. The Bland-Altman plot was created and a mean difference of -4.72 steps was observed with the LoA95 ranging from -37.28 and 27.84 steps (Fig. 2).

Fig. 2.

Bland and Altman plot of the difference between number of steps in the Chester Step Test-1 (CST-1) and CST-2 against the mean of the number of steps in test -1 and test -2 in patients with interstitial lung disease (n=66). The dashed horizontal line represents the mean difference, and the solid horizontal lines represent the 95% upper and lower limits of agreement.

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Validity

The correlation between the number of steps of the best CST and the 6MWD was significant, positive, and strong (rs=0.85, p=0.001) (Fig. 3).

Fig. 3.

Correlation between the number of steps of the best Chester Step Test (CST) and the 6-minute walk distance (6MWD) in patients with interstitial lung disease (n=66).

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Excellent intra-rater and inter-rater reliability were found for the CST. A similar study, which focused on patients with COPD and assessed the reliability of the CST also showed excellent relative reliability (ICC of 0.99; 95%CI 0.97-0.99)(15). This finding indicates that CST provides consistent results and excellent agreement between healthcare professionals, which allows for comparisons of patients’ results even when CST is applied by different raters on different occasions, i.e., pre and post PR program.11,33 Moreover, it also suggests that only minimal training is required for healthcare professionals to apply the CST.

Our findings suggest that it is necessary to improve above 32.6 steps to assume that the change observed was above the measurement error.18 Although this cut-off is informative, whether that change is clinically meaningful remains unknown. Moreover, our MDC95% was above the 30% acceptable limit.31 It is likely that this finding was influenced by the high heterogeneity of symptoms and exercise capacity observed in our sample of patients within disease subgroups (mild DLCO 28.8%; moderate DLCO 27.3% and severe DLCO 28.8%), thus increasing the MDC95 value.1,34 Therefore, future studies determining the minimal clinically important differences of the CST in patients with ILD, ideally using homogeneous samples, and using both anchor (i.e., mean change, the receiver operating characteristic curves and linear regression analysis) and distribution-based methods (i.e., 0.5 times the SD, SEM, 1.96 times the SEM; MDC95 and ES)14 are needed.

In the present study, no evidence of systematic bias was observed. In patients with COPD, the CST showed smaller and narrower values of mean difference and LoA95 (mean difference of -1.1 steps with LoA95 ranging from -20.2 to 17.9 steps).15 It is likely that the high heterogeneity observed in our sample of patients within disease subgroups may have contributed to the wider range of the LoA95, especially those patients that achieved higher stages on the CST.

The CST has adequate construct validity as shown by the strong correlation between the number of steps and the 6MWD. Similar results were found for the ISWT in the same population (r=0.76, p<0.0001), while a lower correlation was observed in patients with COPD (r=0.60, p=0.001).13,15 In addition to the adequate validity, the CST presents some advantages over the ISWT and the 6MWT, such as the apparent absence of a learning effect and requiring minimal space for its application.35 As reported previously, the 6MWT requires a 30 meters length corridor, and the ISWT a 10 meters corridor, which is often difficult to find in home and clinical settings.35,36

The CST may also be useful in other contexts and populations. During the COVID-19 pandemic, the need to transfer patients to rehabilitation facilities or discharge them to home increased exponentially and so did the demand for home-based and/or remote rehabilitation.37,38 Worldwide, healthcare professionals and researchers were looking for validated, easy and practical to implement exercise tests that allowed assessing and prescribing exercise in non-clinical settings.39 Step tests emerged as validated measures, however up until now only the 3-minute step test had been used in such settings.39 To the best of the authors´ knowledge, the current study is the first to show that the CTS is safe, reliable, and valid to be conducted in a community setting. Futures studies should be conducted to assess its applicability in other populations, such as patients post-COVID-19.

Assessment of exercise capacity is of paramount importance as exercise capacity in patients with ILD has been associated with a more reserved prognosis and a poorer health-related quality of life.11,17,1 Clinicians and researchers now have at their disposal a field test that is valid and reliable to assess exercise capacity in patients with ILD even in limited space settings.

This study has some limitations that need to be acknowledged. First, we only assessed construct and not criterion validity (i.e., correlation with the gold standard – CPET). Second, we did not evaluate the effects of the disease severity and variability on the MDC95, since our sample size was too small to perform subgroup analysis. Third, our study only included patients with clinical stable ILD and participants with any other significant impairment or disease were excluded, thus our results cannot be generalized to all patients. Modified versions of the CST may be needed either to assess patients only with mild ILD or to safely assess patients with more severe impairments.

The CST is a reliable and valid test to evaluate exercise capacity in patients with clinical stable ILD. Due to its characteristics, the CST may constitute an appropriate alternative to the 6MWT and the ISWT in limited space environments.