The PROactive instruments to measure physical activity in patients with chronic obstructive pulmonary disease

Discussion

Our analyses resulted in two innovative reliable and valid instruments to fully evaluate the concept of physical activity in patients with COPD: the daily (D-PPAC) and the clinical visit (C-PPAC) versions of the PROactive instruments. The PPAC instruments are hybrid tools that combine PRO and activity monitors to assess physical activity. Interestingly, the C-PPAC turned out to have nearly twice as many items as the D-PPAC. We are not surprised by this fact, because some activities are not performed every day although they are covered in a 1-week period, therefore data about difficulties with such activities may not be captured with a 1-day-recall item but are captured with a 1-week-recall item.

The first important finding of the study confirms that the concept “physical activity” in patients with COPD has two domains, which we labelled “amount of physical activity” and “difficulty with physical activity”. The “amount” domain is covered by two PRO items (amount of walking outside and chores outside) and by two variables from the activity monitor (steps and VMU), in both PPAC instruments. The inclusion of other items or activity monitor outputs did not enrich the final score. The combination of PRO and activity monitor data yields a much more robust measure of “amount” than either of the components, a unique observation from our study. The “difficulty” domain contains only PRO items and includes items about avoiding or pacing activities and breathlessness during activities. Demonstrating that physical activity is structured in two domains concurs with our qualitative research of experience of physical activity where COPD patients universally described three themes: “amount of”, “symptoms during” and “adaptations to” [11]. Based on the analysis of this study, the themes “symptoms” and “adaptations” merged into a single domain (“difficulty”).

The second relevant finding is that the final PPAC instruments are hybrid tools combining PRO and activity monitors for the assessment of amount of physical activity in COPD patients. Our comprehensive analyses clearly showed that neither PRO nor activity monitors alone could discriminate well (PSI<0.4) within the latent patient-centred construct “amount of physical activity”, while the combination of both tools achieved good discrimination at all ranges of the scale. Restricting assessment to either a PRO or an activity monitor does not provide a complete measure of the amount of physical activity. It is possible that activity monitors do not totally capture activities of low intensity, or that relevant activity is not appropriately recalled by patients.

It could be argued that the combination of the PRO with the accelerometer may impair the use of the PPAC instruments. However, the use of activity monitors is becoming more frequent both in clinical practice [10] and in clinical trials [28–30]. In addition, research shows limitations in using only questionnaires for assessing physical activity [13]. The patient-centred development of the PPAC instruments provides comprehensive assessment of physical activity incorporating the patient viewpoint. Importantly, the PPAC instruments will not be limited to the two activity monitors reported in this paper: other activity monitors can be combined with the PRO items after appropriate validation is complete.

It could be argued that some of the excluded items should have been kept in the PPAC instruments due to their clinical relevance. For example, in the “amount” domain, no items about activities inside the house were kept. The item “chores inside the house” did not fit well with the rest of the items according to exploratory factor analysis and showed differential item functioning between males and females (online supplementary table E5). In addition, some male patients did not understand the item during interviews or cognitive debriefings because they considered this item to be more relevant to females. Two items about outside activities were included (“walking outside” and “chores outside”). Although superficially similar, these two items increased discrimination and covered different parts of the range of physical activity, as shown in PSI values and person-item maps. Interestingly, they completely overlapped with the range covered by “chores inside”. Finally, VMU, a variable from the activity monitors, showed the best discriminant ability at low amounts of activity (online supplementary fig. E1), suggesting that it conceptually covers what the item “chores inside” was not able to capture.

The item “difficulty climbing stairs” could seem to be important because climbing stairs represents a substantial effort, and was also identified by patients as a priority during interviews and focus groups [11]. However, Rasch analysis showed improper ordering of the item, suggested a poor fit with the other items, presented a floor effect and showed high correlation with other remaining items in the difficulty domain (online supplementary table E5). Both “chores inside the house” and “difficulty climbing stairs” are examples of items that are intuitively relevant which were removed based on corroborating arguments from different sources (statistics and qualitative studies).

Both PPAC instruments demonstrated construct validity, showing ability to measure the construct that they intended to measure. As expected, “amount” scores exhibited weak correlations with health status, and moderate correlations with dyspnoea and exercise capacity, while “difficulty” scores showed moderate-to-strong correlations with health status, dyspnoea and exercise capacity. Additionally, all correlations were <0.9, demonstrating that the PPAC instruments provide different information than existing scales, i.e. that PPAC instruments indeed measure physical activity and not exercise capacity or health status [23]. Similarly, the instruments showed good known-groups validity when comparing across previously defined groups, detecting differences in physical activity (using variables not included in the PPAC scoring) between them. Finally, the discriminant validity showed that the difficulty and amount domains from the PPAC instruments were unrelated (predefined as correlation <0.3) to the constructs that were expected to be unrelated such as patient height. In comparison with previous studies that aimed to validate questionnaires to measure physical activity in patients with COPD, our instruments exhibited better correlations for convergent validity. For example, correlations of ∼0.37 were found between the several outputs from the Yale Physical Activity Survey and the 6-min walking distance [31], compared to correlations of ∼0.55 in our instruments.

Measuring the experience of physical activity is important for COPD management because physical activity is important from the patient's perspective [11] and thus for their daily life, and no valid instruments are currently available to capture it. In particular, because patients report that treatments (which need not be limited to pharmacological approaches) that improve physical activity are of value to them [11], the availability of PPAC tools which have been developed in line with current guidance will help future clinical trials aiming to include physical activity as an regulatory-accepted outcome [14].

A strength of this study is that the instruments were developed according to current best practice [14, 17], and their psychometric properties were rigorously tested using appropriate methodology and predefined and rigorous statistical procedures [32]. Both the D-PPAC and the C-PPAC item pools were developed through a systematic process of literature reviews [12, 13], input from experts and, as recommended [33–35], input from patients through qualitative research using one-to-one interviews, focus groups and cognitive debriefing interviews. Patients recruited were representative of a wide spectrum of COPD disease severity, comorbidities and different geographical and cultural backgrounds [11]. The information from this qualitative research was also used during the item reduction process when deciding to remove or keep an item. An advisory board provided advice to guide the whole process for the item reduction and validation of the tools. Moreover, the item reduction and initial validation reported here were also based on a heterogeneous sample of patients with COPD that covered a whole range of physical activity and other clinical determinants.

Another strength of the PPAC instruments is that they are based on a conceptual framework. A systematic review identified the absence of a conceptual framework of physical activity in all existing instruments to assess it in patients with COPD [13], which raises doubts about the validity of these instruments. We drafted an initial conceptual framework from extensive patient input and literature review [11], which was refined after item reduction and confirmed using appropriate statistical methods.

Our study has some limitations. Firstly, although the sample size was sufficient and the population was quite heterogeneous to achieve the objective of the study, further validation is needed in specific subgroups of patients with COPD. For example, the person item maps of the “difficulty” domain (factor 2) suggest that our instruments perhaps cover patients with greater difficulty in physical activity than the difficulty shown by patients in our sample, since the person distribution is skewed to the lower end of the scale. Secondly, the PPAC instruments have been validated only in a European population and using only three different languages. Further studies are ongoing to validate the PPAC instruments in patients with both milder and more severe COPD, from other countries and using validated translations of the instruments. In the same way, additional studies are required to confirm the best scoring system for the PPAC instruments, to assess responsiveness to change due to interventions and to define levels of response.

In conclusion, both the D-PPAC and the C-PPAC, combining a short list of PRO items and two activity monitor variables, provide a simple, valid and reliable measure of physical activity in COPD patients. These tools will be further evaluated in different clinical situations and for responsiveness to drug and nondrug interventions.