Responsiveness and interpretability of the pain subscale of the Knee and Hip Osteoarthritis Outcome Scale (KOOS and HOOS) in osteoarthritis patients according to COSMIN guidelines
Authors: Rienstra Wietske , Stevens Martin , Blikman Tim , Bulstra Sjoerd K. , van den Akker-Scheek Inge 
Published: November 2023

Background
The pain subscales of the Knee and Hip Osteoarthritis Outcome Scores (KOOS and HOOS) are among the most frequently applied, patient reported outcomes to assess pain in osteoarthritis patients and evaluation of the results after Total Knee Arthroplasty (TKA) and Total Hip Arthroplasty (THA). For the evaluation of change over time it is essential to know the responsiveness and interpretability of these measurement instruments. Aim of this study is to investigate responsiveness and interpretability of the KOOS and HOOS pain subscales in patients with knee or hip OA and patients after TKA and THA as recommended by COSMIN guidelines. COSMIN stands for COnsensus based Standards for the selection of health Measurement Instruments. COSMIN recommends methods for assessing responsiveness similar to those assessing validity, using extensive hypothesis testing to assess criterion validity and construct validity of the change score.

Design
This clinimetric study was conducted using data obtained from the Duloxetine in OsteoArthritis (DOA) trial. Primary knee or hip osteoarthritis patients were included. During the study, half of the participants received pre-operative targeted treatment with duloxetine, and all participants received TKA or THA. Patients filled out a set of patient-reported outcomes at several time points.

Methods
Using the criterion validity approach the change scores of the KOOS and HOOS pain subscales directly after duloxetine treatment but before TKA and THA were correlated to the Patient Global Improvement anchor-question (PGI-I). Receiver Operating Characteristic curves (ROC curves) were obtained. Using the construct validity approach, hypothesis testing was conducted investigating the correlation between change scores in the KOOS and HOOS pain subscale with change scores in other questionnaires six months after TKA and THA. For interpretability, an anchor-based approach was used to consider the Minimally Important Change (MIC) of the KOOS and HOOS pain subscale. We compared the outcomes after duloxetine treatment and six months after TKA and THA in order to investigate any response shift.

Results
Ninety-three participants (53 knee patients and 41 hip patients) were included. Mean change was 4.3 and 4.6 points after conservative treatment for knee and hip OA patients respectively and 31.7 and 48.8 points after TKA and THA respectively. The KOOS and HOOS pain subscales both showed a gradual increase in change scores over the different categories of improvement on the PGI-I, with an Area Under the Curve of 0.72 (95% CI 0.527–0.921) and 0.79 (95% CI 0.588–0.983) respectively. Of the predefined hypotheses, 69% were confirmed for both subscales. The MICs were between 12.2 to 37.9 for the KOOS pain subscale, and between 11.8 to 48.6 for the HOOS pain subscale, depending on whether the PGI-I was administered after conservative treatment, or six months after TKA and THA.

Conclusions
This study endorses the responsiveness of the KOOS and HOOS pain subscales in patients with knee or hip OA and patients after TKA and THA based on construct and criterion validity approaches. The KOOS pain subscale might be able to detect the MIC at an individual level after arthroplasty, but both the KOOS and HOOS pain subscales were not able to do so after conservative treatment. This study is the first to report a considerable response shift in MIC of the KOOS and HOOS pain subscales. This should be taken into consideration when evaluating MIC of the KOOS and HOOS pain subscale after conservative versus operative treatment. Future research should present more reference data regarding MIC scores after different treatments.