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OBJECTIVE: To explore whether commonly used osteoarthritis (OA) health outcome instruments (and items) are measuring single or multiple health outcomes using the International Classification of Functioning, Disability and Health (ICF) definitions. METHODS: Ten expert judges allocated 342 items from 13 instruments to one or more ICF construct, i.e., Impairment (I), Activity Limitation (A), and Participation Restriction (P). One-sample t tests were used to classify each item as measuring uniquely I, A, or P or some combination (i.e., IA, IP, AP, or IAP). RESULTS: Overall, 12 of the 13 instruments had items that measured a combination of outcome domains (i.e., IA, IP, AP, or IAP). Only the American Knee Society Score (AKS) had all items uniquely measuring either I or A. The instrument with the best representation of items for Impairment was the AKS, for Activity Limitation the WOMAC and Lequesne knee index, and for Participation Restriction the Disease Repercussion Profile. CONCLUSION: All the existing OA outcome instruments, except one, had some items that were assessing more than one health outcome. Use of these instruments may either mask true treatment effects or make an effect difficult to attribute if the content is unclear. We determined which instruments were the best for measuring each health outcome. To improve the assessment of health outcomes in OA, new instruments that uniquely measure the 3 ICF constructs should be developed and all 3 should be included in relevant studies.