Psychometric properties of the 12-item Knee injury and Osteoarthritis Outcome Score (KOOS-12) Spanish version for people with knee osteoarthritis

Abstract
To evaluate the validity, reliability, and responsiveness to change of the 12-item Knee injury and Osteoarthritis Outcome Score (KOOS) Spanish version questionnaire. This study was based on a questionnaire validation design. A cross-sectional survey of 199 patients with knee osteoarthritis (KOA) and ten healthy controls was studied to evaluate the validity and reliability of KOOS-12. One hundred and sixteen patients were assessed for test-retest reliability, and 38 patients were included for a responsiveness assessment. Structural validity was assessed by the confirmatory factor analysis (CFA). Item response theory-based methods were used to determine the performance of the items. Internal consistency reliability was appropriate for all scales (Cronbach’s alpha = 0.85–0.94). The intra-class correlation coefficient of KOOS-12 scales ranged from 0.60 to 0.71. The CFA and generalized partial credit model showed that KOOS-12 scales presented a good overall model fit. No differential item functioning was found. Convergent validity was demonstrated by strong correlations (Spearman’s rho ≥ 0.70) with KOOS, International Knee Documentation Committee subjective knee evaluation form (IKDC), and Knee Intermittent and Constant Osteoarthritis Pain (ICOAP). Known-groups validity showed that KOOS-12 well discriminated subgroups of patients (radiographic severity and nutritional status). Standardized response means for KOOS-12 scales were ≥ 0.75. Changes in KOOS-12 scales had a moderate to strong correlation (Pearson’s r ≥ 0.40) with the changes in the KOOS, ICOAP, and IKDC scales. The KOOS-12 Spanish version is a valid, reliable, and responsiveness to change questionnaire to measure patients’ opinions about their knee and associated problems in Mexican subjects with KOA.