Context: Among numerous knee-related patient-reported outcome measures, the International Knee Documentation Committee Subjective Knee Form (IKDC-SKF) has been used across a wide variety of knee pathologies. However, traditional validation procedures (classical test theory) and existing studies (estimating item parameters) have limitations in establishing the measurement properties of the IKDC-SKF. Rasch analysis reveals a strong validation approach to improve IKDC-SKF clinical interpretation with larger samples.
Objective: To assess psychometric properties, including differential item functioning, of the IKDC-SKF as a patient-reported measure of knee function.
Design: Cross-sectional study.
Setting: Secondary data. Data were extracted from the cloud-based orthopedic and sports medicine global registry Surgical Outcome System (Arthrex).
Patients: A total of 1725 individuals who underwent an arthroscopic knee procedure and completed all items on the IKDC-SKF.
Main outcome measure(s): Rasch analysis including model-data fit, rating scale's function, item-person map (distribution of item difficulty and person ability), and differential item functioning (sex and age groups) was used to evaluate the psychometric properties of the IKDC-SKF.
Results: Ten misfit items were found and removed. The 5-point Likert scale of the 9-item IKDC-SKF worked well. Item difficulty ranged from 0.58 to 0.81 logits, and person's knee function had ranged from -5.56 to 4.86 logits, with a wide distribution. The IKDC-SKF was found to function similarly for sex (male vs female) and age.
Conclusions: Rasch analysis identified a unidimensional structure retaining 9 of the original IKDC-SKF items; however, a more comprehensive inventory is necessary to assess a wider range of knee function and improve measurement validity.
Keywords: IKDC-SKF; Rasch analysis; psychometric evaluation.