Background
The Dynamic weight-bearing Assessment of Pain (DAP) for knee osteoarthritis (OA) is a simple test designed to assess pain during performance of a specified activity in patients with knee OA [
1,
2]. It is a simple performance test requiring repeated deep knee bends from a standing position for 30 s, with subsequent rating of pain on a 0–10 numeric rating scale (NRS), where 0 indicates ‘no pain’ and 10 ‘worst pain imaginable.’ The DAP follows current quality standards [
3] to scientifically qualify a common clinical practice: asking patients about their pain during performance of a test. An elaborated description of the test has been published previously [
2].
Application of the DAP is in line with The Osteoarthritis Research Society International (OARSI) suggestion for clinical trials on knee OA;
"Pain can be assessed after specific activities (e.g.
, a walk test). If a pain assessment occurs after an activity then the study team should ensure consistency throughout the trial with the type and duration of activity as well as the timing of the pain assessment after the activity" [
4]. This consistency is best obtained through a standardized test such as the DAP.
The DAP was developed based on input both from patients (with knee OA) and health care professionals. Through focus groups, ‘weight-bearing deep knee bends’ was identified as a clinically important pain-provoking activity that would translate to a feasible instrument for ‘pain during activity’ by adding patient reports of pain [
1]. The first version of the DAP had two scores: the pain rating, and the knee bend count. A study on reproducibility showed excellent properties of the pain score in a population of patients with knee OA (Intra-rater ICC = 0.93 [95 % CI 0.83 to 0.97], corresponding to a Standard Error of Measurement (SEM = of 0.70, and a Smallest Detectable Change (SDC) of 1.95; inter-rater ICC 0.91, [95 % CI 0.78 to 0.96], SEM 0.86, SDC 2.39), whereas the knee bend count did not reach acceptable level [
2]. Consequently, the knee bend score was omitted and thus the second version of the DAP now consists of a single pain score. Simultaneously we clarified the construct assessed with the DAP to be ‘pain during activity’ instead of ‘the interaction between pain and function.’ The level of reliability is similar to that of pain ratings subsequent to other performance tests in patients with hip pain [
5] and knee OA [
6]. However, the validity, responsiveness, or interpretability of combined performance tests and pain ratings have not been evaluated.
We anticipate that the DAP assesses pain during performance of a specific weight-bearing activity in patients with knee OA — a construct that adds a new perspective to existing instruments. Our objectives were to evaluate the construct (divergent and convergent) validity, responsiveness, and interpretability of the DAP instrument.
Discussion
These results suggest that the DAP has adequate properties for measuring a construct of pain during activity. This construct is not covered by widely used instruments for knee OA: the questionnaire KOOS and the performance test 6MWT, as confirmed by divergent validity. The correlation coefficient of 0.54 between the DAP and the 6MWTpain suggests that the constructs assessed with these two instruments are somewhat similar, although not fully convergent. The responsiveness of the DAP is adequate, shown by the different median DAP change scores in the group that reported change in pain by TRANS-Q and the group that reported no change, respectively. An improvement of 1.5 or more DAP points can be interpreted as clinically relevant, relating nicely to previous findings where the minimal clinically important difference in chronic musculoskeletal pain equals a reduction of 15.0 % on the 0–10 NRS [
21]. However, as the smallest detectable change of the DAP was previously estimated to be 1.95 for intra-rater and 2.39 for inter-rater [
2], the more conservatively estimated MIC of 2.4 (corresponding to the upper confidence limit of the mean DAP change in the group experiencing no change) should be used to inform clinical decisions for individual patients.
Ideally, evaluating the construct validity of the DAP should involve a comparison with a gold standard — another instrument that is proven to reliably assess pain during activity. As no such gold standard exists, we focused on the divergent aspect of construct validity, hypothesizing that the instruments focusing solely on pain (KOOSpain) or function (KOOSfunction, 6MWT) would not correlate highly (prespecified as r < 0.7) with the DAP. Furthermore we added a pain rating to an existing performance test (6MWTpain), expecting this addition to reflect a similar construct of pain during activity. A high correlation (prespecified as r > 0.7) with the 6MWTpain would thus support convergent validity. Although our assumption of divergent validity held true, the DAP and the 6MWTpain had lower correlations than expected, not supporting convergent validity. This finding may be explained by the differences between the DAP and 6MWTpain. Importantly, the 6MWTpain is not a standardized instrument and is not designed to incorporate a pain score. Further, the activities of knee bends versus 6 min of walking may not be equally pain provoking in a population of patients with knee OA. There are differences in the activities – knee bending and walking. During the performance of the two tests there were different responses from the patients; walking seemed to be more painful in the beginning, but then fading out, while the pain increased with more knee bends. This supports that the DAP assess a construct of weight-bearing activity that is relatively more pain provoking than 6 min of walking. The DAP is a simple test with a short duration and a focus on the pain intensity during performance of a specified activity. In the 6MWTpain, the distance walked is the main score, whereas the pain score can be considered a ‘nice-to-know’ add-on. Even though the exact construct assessed by the DAP cannot be described based on these results, it is suggested that the DAP assess a different construct than other commonly used instruments, but has the closest relation to the 6MWpain.
The correlation among change scores of the DAP and the 6MWTpain was low, potentially diluting the DAP responsiveness either due to poor qualities of the DAP or inappropriate selection of comparable instruments (lack of gold standard). However, the correspondence to patient-reported change in pain (TRANS-Q) may be seen as an even stronger indicator of actual change than other instruments show. Thus, we interpret the responsiveness of the DAP as adequate.
With a median DAP score of 4 (range 0–9) at baseline, most of the participants had relatively low pain levels, as also reflected by the other instruments. Nine participants reported to have no pain (DAP score 0 at baseline). No participants experienced worsening of their pain as assessed by TRANS-Q, and only four participants reported ‘no change’. This cause a limitation to the study, as only the MIC for improvement can be established, not for deterioration. Another limitation is constituted from our choice of anchor. Had the anchor been more specific, e.g. asking about pain during a weight-bearing activity, or more general, e.g. asking about wellbeing, the MIC may have been different. Likewise the TRANS-Q only gives information about the size of experienced change, not whether it is important. Thus, the choice of which change that is considered important was in this case not based on patient-reports. However, the final MIC of 2.4 is a conservative estimate, in this population corresponding to ‘a good deal better’ or more, which presumably reflects an important improvement.
It is a limitation to the DAP that not everyone experience pain during the test. This may reflect day-to-day variability of knee pain in patients with knee OA, or that pain occurs in various situations for the individual patient (at rest, walking, standing, kneeling etc.). Another possible explanation for the low pain level could be that the study population was eligible for pharmacological treatment and rehabilitation, thus not very severely affected. On the other hand, all participants did have both symptomatic and radiographic signs of knee OA, pointing at advanced knee OA. Including more severely affected patients (e.g., those eligible for surgery) could influence the results. However, DAP scores of 0 is not considered a ceiling effect because when there is no pain, there is no need for further discrimination on this parameter [
3]. The DAP did not show signs of a floor effect either, as no participants reported ‘worst pain imaginable’ (DAP score 10). Whether this absence of floor and ceiling effects would hold true in a more severely affected population has not been established.
So far, the DAP has been tested by physiotherapists in a research setting only in a population of people with knee OA and mild to moderate symptoms, who mostly benefitted from the exercise intervention in a clinical trial. Testing beyond these limitations is ongoing.
Abbreviations
6MWT, the 6-min-walk-test; 6MWTpain, the 6-min-walk-test with subsequent pain rating; BMI, body mass index; COSMIN, the COnsensus-based Standards for the selection of healthMeasurement INstruments; DAP, the dynamic weight-bearing assessment of pain; ICC, intraclass correlation coefficient; KL, Kellgren Lawrence; KOOS, the knee injury and osteoarthritis outcome score; MIC, minimal important change; NRS, numeric rating scale; OA, osteoarthritis; OARSI, The Osteoarthritis Research Society International; QoL, quality of life; ROC, receiver operating characteristic; SDC, smallest detectable change; SEM, standard error of measurement; TRANS-Q, transition questionnaire
Acknowledgements
We would like to thank the participants in this study. The Parker Institute is grateful for the support from the Oak Foundation.