170 Questionable measurement properties of the most used outcome questionnaire for children with ACL injury: Pedi-IKDC – a national study

Introduction Pedi-IKDC is the pediatric version of the International Knee Documentation Committee subjective outcome score (IKDC). It consists of two subscales, symptoms and sports, but all raw scores are aggregated to one sum score. It is primary outcome in two large scale initiatives for the treatment of children with ACL deficiency: the European ‘Paediatric ACL Monitoring Initiative’ (PAMI), and the North American ‘Pediatric ACL: Understanding Treatment Options’ (PLUTO). However, Pedi-IKDC has not been subjected to validity assessment with optimal methods: modern test theory (MTT) statistical models. Materials and Methods Data were collected prospectively before surgery and at 1-year follow-up from a nationwide, Danish cohort of 535 children with ACL injury, treated with epiphyseal sparing reconstruction at either Aarhus or Bispebjerg University Hospitals. We evaluated the fit to a confirmatory factor analysis (CFA) model and confirmed results by Rasch analysis for each subscale and for the aggregated score. Results Neither of the two subscales nor the aggregated score of Pedi-IKDC showed acceptable fit to the CFA model. Rasch analysis confirmed this. It was possible to adjust the subscales, achieving a much better fit to the CFA model for the symptoms scale, but only a slightly better fit for the sports scale. Reliability could not be reported due to inadequate model fit. Conclusion Pedi-IKDC has inadequate measurement properties for children with ACL-injury. Validity of previously collected data can be improved by modification of the scoring. As Pedi-IKDC also has questionable content validity, data obtained by Pedi-IKDC should be interpreted with great caution.