| Construct validity |
| 1. Does the end-point measure physical activity? |
Scores for “amount of physical activity” and “difficulty during physical activity” showed good internal consistency and construct validity across sex, age, COPD severity, countries and languages [10] EMA supports C-PPAC & D-PPAC as end-points to measure physical activity in COPD [11]
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Good indicator of day-to-day activity in healthy subjects; however, pure step count cannot indicate relative effort required to complete steps in subjects with respiratory diseases Subject to seasonal variation and potentially skewed by occupation [12]
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Historically the most used field test to assess functional capacity Surrogate for physical activity prior to introduction of activity monitors Limited functional capacity indicates muscle depletion caused by physical inactivity [19] The test is self-paced and therefore subject to motivational effects
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| 2. Correlation to dyspnoea | Pooled data showed “difficulty during physical activity” scores correlated moderately to strongly with dyspnoea [10] |
21 days of fixed dose combination LABA/LAMA therapy reduced lung hyperinflation as measured by inspiratory capacity. This was accompanied by a significant increase in step count [16] mMRC score was weakly associated with daily step count of patients [12]
| 8 weeks of dual bronchodilator therapy elicited a reduction in dyspnoea intensity experienced during the 6MWD [17] |
| 3. Correlation to exercise capacity | Pooled data showed “amount” scores from both D-PPAC and C-PPAC moderately correlated with exercise capacity. Difficulty scores showed moderate-to-strong correlations with exercise capacity [10] |
| Inherently an end-point used to indicate exercise capacity and therefore an exact correlation to exercise capacity |
| 4. Correlation to HRQoL | Pooled data showed “difficulty” scores had moderate-to-strong correlations with HRQoL [10] |
A 4-month pedometer-based exercise programme, which improved step count, improved SGRQ by the minimum clinically important difference [14] SGRQ was not found to be associated with daily step count [12]
| Significant negative correlation between 6MWD and HRQoL, as measured by SGRQ symptoms domain, SGRQ impact domain and the SGRQ total score [18] |
| Content validity |
| 1. Does the end-point capture every aspect of physical activity? | The wide distribution of scores for all domains supports the use of these instruments to capture the diversity of amount and difficulty experienced during physical activity by patients with COPD. Qualitative and quantitative data from development and validation studies of both instruments support the hypothesis that amount and difficulty are two different dimensions of physical activity experience [10] |
Poor indicator of vigorous activity (crucial for long-term health) Poorly reflects patient experience, cannot indicate any pain experienced during essential mobility
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Exercise capacity comprises only one of the important dimensions which determine physical activity Behaviours and environmental factors play huge roles in the amount and frequency of physical activity performed by people; exercise capacity does not directly translate to physical activity
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| 2. Reflects respiratory disease state |
All D-PPAC and C-PPAC scores differentiated across severity of COPD [10] Instruments detected negative impacts on physical activity in patients who experienced exacerbations in the follow-up period [10] “Amount”, “difficulty” and total scores derived from D-PPAC and C-PPAC vary fittingly to patients with a range of clinical characteristics [10] The wide distribution of scores for all domains supports the use of these instruments to capture the diversity of amount and difficulty experienced during physical activity by patients with COPD [10]
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Average step count decreased with increasing GOLD stage [12] Completing an additionally 1000 steps at a low intensity corresponds to a 20% reduction in the risk of hospitalisations [20] An improvement of daily step count by 780 (as facilitated by a 4-month pedometer-based programme) was associated with significant improvements in health status of patients [14] Decline in average step count by 393 seen annually in patients with COPD monitored over 3 years, independent of COPD severity at baseline [19]
| 6MWD is inversely correlated with severity of COPD [18] |
| 3. Responsiveness to pharmacological intervention | ACTIVATE and PHYSACTO studies showed improvements in D-PPAC difficulty score following bronchodilator treatment [10] |
21 days of fixed dose combination LABA/LAMA therapy improved step count in moderate-to-severe COPD patients by an average of 358 steps [16] Short-term LABA therapy improved daily step count by an average of 1616 steps [15] Short term dual bronchodilator therapy improved step count by approximately 10% [13]
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