Reliability and validity of the International Knee Documentation Committee (IKDC) Subjective Knee Form

Patient-oriented questionnaires are important measures of clinical outcomes in medical practice but require systematic testing of reliability and validity. The International Knee Documentation Committee (IKDC) Subjective Knee Form is a patient-oriented questionnaire that assesses symptoms and function in daily living activities. The purpose of this study was to validate the IKDC Subjective Knee Form in a large patient population with various knee disorders.

One thousand five hundred and thirty-four knee patients seen at a sports medicine clinic at a large medical center completed the IKDC Subjective Knee Form. Factor structure was determined by exploratory factor analysis with promax rotation, and internal consistencies of the identified subscales were calculated with Cronbach's alpha. Concurrent validity was assessed by correlating the IKDC Subjective Knee Form dimensions to the summary scales of the SF-12. Finally, item characteristics were analysed using graded response item response theory (G-IRT).

Exploratory factor analyses yielded a two-factor solution, and the dimensions were termed as follows: symptom and knee articulation (SKA), and activity level (AL). Confirmatory factor analysis confirmed the selection of 15 items within the study. Both SKA and AL demonstrated good internal consistency (0.87 for SKA; 0.88 for AL). Both SKA and AL demonstrated statistically significant correlations to the SF-12 total score, more substantively to the physical component summary scale than to the mental component summary scale of the SF-12.

G-IRT analyses revealed that nearly all questionnaire items demonstrated clear response patterns, with higher levels of the latent trait corresponding to more adaptive clinical endpoints concerning pain, symptoms, function, and sport activity.

The IKDC is a reliable and valid instrument worthy of consideration for use in a broad patient population.