Interviewer‐assisted administration of patient‐reported outcome measures with intentional interviewer influence consistently produces more favourable scores than self‐administration after total knee arthroplasty: A prospective within‐subject comparative study

Abstract Purpose Patient‐reported outcome measures (PROMs) are key instruments for evaluating outcomes after total knee arthroplasty (TKA), but differences across studies suggest that methodological factors, including interviewer bias, may affect their validity. The purpose of this prospective within‐subject study was to determine whether intentionally influenced interviewer‐assisted administration produces more favourable Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), Forgotten Joint Score (FJS) and Oxford Knee Score (OKS) scores than self‐administration after TKA. Methods Three months after TKA, consecutive eligible patients completed the WOMAC, FJS and OKS twice: first independently at home and later in an intentionally influenced interviewer‐assisted format during routine follow‐up. During interviewer‐assisted administration, questions were clarified when needed, and when patients were unsure between two response options, the more favourable answer was suggested. Paired t tests and standardized paired effect sizes ( dz ) were calculated. Results Sixty patients with complete paired assessments were included (41 women and 19 men; mean age 63.7 ± 8.2 years). Interviewer‐assisted administration yielded mean differences of 35.9 ± 17.7 points for the FJS ( p < 0.001; dz = 2.03), 6.8 ± 4.7 points for the OKS ( p < 0.001; dz = 1.44) and 8.4 ± 10.9 points for the WOMAC ( p < 0.001; dz = 0.77), consistently indicating more favourable scores. Conclusions PROMs after TKA are strongly influenced by the mode of administration. Interviewer‐assisted administration involving intentional influence resulted in systematically more favourable scores across all questionnaires. For meaningful comparison of PROM‐based outcomes, the mode of administration and any interviewer guidance should be standardized or at least clearly reported. Level of Evidence Level III.