Evaluating the diagnostic accuracy of a screening tool for low physical activity in independently ambulating adults with knee osteoarthritis: A prospective cohort study.

Background and purpose: Patients with knee osteoarthritis (OA) can benefit from resistance training exercises, range of motion and flexibility maintenance, and low-load aerobic exercises, as per the relevant clinical guidelines. However, certain patients might be unable to progress to higher physical levels despite such physical therapy programs. This study aimed to evaluate the diagnostic accuracy of a screening tool for determining physical activity levels in individuals with OA undergoing standard physical therapy regularly, using likelihood ratios and predictive values.

Method: This prospective observational study included 135 patients undergoing standard physical therapy for OA from six medical facilities. The primary outcome was low physical activity or moderate to high physical activity levels based on 1-month Self-Rating Frenchay activities index scores. Backward elimination was used to perform binomial logistic regression analysis after identifying the independent variables in a univariate logistic regression analysis. Among the independent variables adopted in the logistic regression model, receiver operating characteristic analysis using Youden's index was performed for quantitative variables, which were converted to binary values at the cut-off points. Subsequently, the clinical prediction rule (CPR) was derived.

Results: According to the binomial logistic regression analysis, age, knee flexion muscle strength, and visual analog scale (VAS) were risk factors for low physical activity, and the CPR was derived from these variables. The pre-test probability of the low physical activity group was 37.0% (50 out of 135 participants). For a total CPR score of three points (one point for each item: age ≤69 years, knee flexion muscle strength ≤0.36 Nm/kg, and VAS ≥33 mm), the positive likelihood ratio was 13.60 and the post-test probability increased to 88.9%.

Discussion: The CPR identified patients who might not benefit from the standard physical therapy program. This screening tool could improve patient management, allowing for more tailored approaches in physical therapy programs.