Context: The International Knee Documentation Committee (IKDC) Subjective Knee Evaluation is the most frequently used patient-reported measure of subjective knee function among individuals with ACL reconstruction (ACLR). Yet, limitations with traditional validation approaches leave it unclear whether the IKDC measures knee function as intended. Rasch analysis offers a robust validation approach, which may enhance clinical interpretation of the IKDC. Objective: 1) To assess the psychometric properties, 2) ability to classify health status, and 3) relationships between the IKDC and objective measures of strength and functional performance relative to a newly proposed reduced-item instrument. Design: Cross-sectional. Setting: Laboratory. Patients or other Participants: Seventy-seven individuals with primary, unilateral ACLR (21.9±7.8 years, 6.2±1.0 months post-surgery), and seventy-six age-matched controls. Main Outcome Measure(s): Rasch analysis was used to assess the psychometric properties of the IKDC. Receiver-operator-characteristic curves and logistic regression were used to assess the accuracy of classifying ACLR versus control participants. Correlations (Pearson and Spearman) were used to assess relationships between subjective knee function, quadriceps torque, and single-limb hop performance.
Results: Rasch analysis aided the development of a reduced 8-item instrument (IKDC-8), which yielded improved psychometric properties in misfit analysis, percent of variance accounted for by one dimension (IKDC-8=71.5%; IKDC=56.7%), and item reliability. The IKDC was an outstanding diagnostic tool and the IKDC-8 was excellent, correctly classifying 87.2% and 82.7% of cases, respectively. The Hanley-McNeil formula found that there was no significant difference in the areas under the respective ROC curves. Equivalent associations between subjective and objective knee function were observed regardless of instrument used. Conclusions: We observed evidence of enhanced reliability and validity for a parsimonious measure of subjective knee function. The proposed instrument reduces the number of items, increases score interpretability as measuring a single construct, and improves the rating scale functioning, while not significantly diminishing its ability to classify ACLR versus control participants or changing existing relationships with objective measures of recovery. We suggest the IKDC-8 may enhance clinical use by reducing administration time, improving the interpretation of the subjective knee function score, and clarifying functional ability.