The validation of the sit-to-stand test for COPD patients

To the Editor:

We read with interest the publication by Crook et al. [1] on the validation and responsive properties of the 1-min sit-to-stand (STS) test in patients with chronic obstructive pulmonary disease (COPD) undergoing pulmonary rehabilitation. The authors performed a comprehensive evaluation of the minimal clinical meaningful difference of the 1-min STS test. In our own dataset of patients from a multicentre study, this STS test exhibited similar level of reliability, intra-subject repeatability [2], and responsiveness to pulmonary rehabilitation with an estimated minimal important difference (MID) of three repetitions [3]. In their study, Crook et al. [1] emphasised the change in STS repetitions, which is better related to change in subjective outcomes (feeling thermometer notably), rather than with physical capacity outcomes such as the 6-min walk distance (6MWD). It is, after all, largely accepted that health-related quality of life (HRQoL) tools are the most sensitive in pulmonary rehabilitation, given the multimodal and patient-tailored interventions addressed in order to optimise benefits, not only focused on exercise training, but also on change in education and behaviour [4].

However, in the STAND-UP group of their study [1], the absence of correlation between the change in STS repetitions and the 6MWD on one side, and between the change in STS repetitions and the quadriceps maximal voluntary contraction (QMVC) on the other side remains puzzling (table 3 of the article). We were wondering if this could be attributed to specific component of the STAND-UP pulmonary rehabilitation programme.

Given the correlation observed between the changes in STS repetitions and QMVC (r=0.24; table 3 of the article), one should expect that raw value of change in STS repetitions and 6MWD be correlated in STAND-UP. This result differs substantially from the result obtained in the RIMTCORE group of their study [1], and our data obtained from a prospective multicentre study including a group of 48 COPD patients [3] with slightly less severe disease in terms of forced expiratory volume in 1 s (FEV₁) (52.3±16.9% predicted versus 40.6±14.7%). Could this difference be attributed to the heterogeneity of the STAND-UP subjects, drastically reducing the raw correlations between STS repetitions and 6MWD?

In our data, the relationship between STS and QMVC confirms that muscle strength is an important determinant of 1-min STS performance. This test also integrates postural control, coordination and balance, making it particularly suitable for COPD patients, as observed for elderly people in terms of risk of fall [5]. In addition to the interesting results obtained by Crook et al. [1] we suggest that physiological and objective anchors be favoured while estimating the MID of these new simple field tests to better characterise their properties.

Relative changes in 1-min STS performance (i.e. change expressed as per cent of baseline) may also be a simple and practical approach for determining the meaningful response of a test (as we usually do to define the bronchodilator response with the change in FEV₁ and/or forced vital capacity). This seems appropriate with the STS test, which is less sensitive to baseline variability compared to the 6MWD. In our study, the magnitude of change after pulmonary rehabilitation did not correlate with the initial STS value, while this is the case with the 6MWD (r= −0.002, nonsignificant for STS, versus r= −0.42, p=0.006 for 6MWD) (figure 1). We would be interested to know if the authors observed similar results when comparing the changes of STS, 6MWD and feeling thermometer scale. In that case, this would reinforce the interest of STS test over the 6MWT, with the representation of the STS change as per cent of baseline value.

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FIGURE 1

Spearman correlation coefficient for the magnitude of change in 6-min walk distance (6MWD) and sit-to-stand (STS) test with their respective initial values.

Whether the STS test will be the future gold standard of home-based monitoring of physical capacity and HRQoL still remains to be confirmed, but it seems to present many advantages that put it in a good position.