Diagnosis for early stage knee osteoarthritis: probability stratification, internal and external validation; data from the CHECK and OAI cohorts

Objective: To internally and externally validate our diagnostic criteria of early stage knee osteoarthritis (OA) in the CHECK and OAI cohorts. Design: We applied two previously developed diagnostic models to all knees in CHECK and OAI cohorts to calculate probabilities of early stage knee OA at baseline. Knees were categorized into three groups based on probability: ‘no OA’ (probability ≤ 30%), ‘uncertain’ (probability between 30% and 70%) and ‘early stage OA’ (probability ≥ 70%). To validate the diagnosis, we obtained OA related outcome measures at 10-year follow-up in the CHECK cohort, and at 8-9-year follow-up in the OAI cohort. We compared outcome measures between ‘no OA’ and ‘early stage OA’ knees, and between ‘no OA’ and ‘uncertain’ knees using generalized estimating equations. Results: In CHECK (n = 1042 knees) both models showed ‘early stage OA’ knees presented with significant and clinically relevant higher WOMAC scores, higher Kellgren & Lawrence (KL) grade, and higher rates of joint space narrowing (JSN) progression after 10 years, compared to ‘no OA’ knees. In OAI (n = 2937 knees) both models showed ‘early stage OA’ knees presented with significant and clinically relevant higher WOMAC scores, higher KL grade, and higher rates of KL and JSN progression after 8-9 years, compared to ‘no OA’ knees. Smaller, but still significant differences between ‘uncertain’ and ‘no OA’ knees were observed in both cohorts. Conclusions: These results support internal and external validity of the two sets of diagnostic criteria for early stage knee OA.