To examine the determinants of the modest correlation between self-report and performance-related measures in patients with osteoarthritis of the hip or knee.
Measures included the Lower Extremity Functional Scale (LEFS), the self paced walk, timed up-and-go, and stair test. Each performance measure consisted of 3 domains: time, pain (visual analog scale), and exertion (Borg scale). Activity specificity was assessed by examining correlations between the LEFS with single activity and multiple activity time scores. Domain specificity was examined by comparing correlations between the LEFS and single and multiple domain scores. The impact of measurement error was considered.
Increasing the number of activity time scores had no effect. Forming a composite performance score based on time, pain, and exertion substantially increased the correlation from 0.44 (composite timed score) to 0.59 (pooled domain and activity score) (P = 0.009).
Performance scores based on time alone appear to inadequately represent the breadth of health concepts associated with functional status.