Dynamic weight-bearing assessment of pain in knee osteoarthritis: construct validity, responsiveness, and interpretability in a research setting

BACKGROUND: The Osteoarthritis Research Society International (OARSI) has suggested to asses pain after specific activities consistently in clinical trials on knee OA. The Dynamic weight-bearing Assessment of Pain (DAP) assesses pain during activity (30 s of performing repeated deep knee-bends from a standing position). The purpose of this study is to evaluate the construct validity, responsiveness, and interpretability of the DAP for knee osteoarthritis (OA). METHODS: One-hundred participants with knee OA were tested twice each with the DAP, the Knee injury and Osteoarthritis Outcome Score (KOOS), six-minute-walk-test (6MWT), and 6-min-walk-test with subsequent pain rating (6MWTpain), and once with a transition questionnaire (TRANS-Q) for the patient-reported change in pain after 12 weeks of exercise. Construct validity (baseline-scores) and responsiveness (change-scores) were estimated by Spearman Correlation Coefficients. We hypothesized that no correlations would be excellent (<0.7) (divergent validity), except for the 6MWTpain (convergent validity). The TRANS-Q was used for interpreting the DAP change-scores in terms of responsiveness and Minimal Important Change (MIC). RESULTS: Divergent validity with the KOOS subscales (r = −0.31 to–0.45) and the 6MWT (r = −0.25) was supported. Convergent validity with the 6MWTpain was not supported (r = 0.54). The DAP change-scores corresponded to patient-reported change in pain (TRANS-Q), while correlations with change-scores on the other instruments were <0.35. The MIC was 2.4 DAP points. CONCLUSIONS: The DAP possesses divergent validity compared to other instruments for knee OA, supporting the potential for this new way of assessing pain directly during activity. Importantly, the DAP change-scores correspond to patient-reported changes in pain, showing responsiveness. A change of 2.4 or more can be interpreted as clinically relevant. The DAP is a promising alternative to using ‘pain on walking’ as a clinical trial inclusion criterion/outcome.