Analysis of outcome measures for persons with patellofemoral pain: which are reliable and valid?

To examine the test-retest reliability, validity, and responsiveness of several outcome measures in the treatment of patellofemoral pain.

Evaluation of the clinimetric properties of individual outcome measures for patellofemoral pain treatment, using data collected from a previously published randomized controlled trial (RCT).

General community and private practice.

The data from 71 persons enrolled in an RCT of a conservative intervention for patellofemoral pain were used to evaluate the measures' validity and responsiveness. A subset of this cohort (n=20) was used to assess reliability.

Not applicable.

Three 10-cm visual analog scales (VASs) for usual pain (VAS-U), worst pain (VAS-W), and pain on 6 aggravating activities (walking, running, squatting, sitting, ascending and descending stairs) (VAS-activity); the Functional Index Questionnaire (FIQ); the Anterior Knee Pain Scale (AKPS); and the global rating of change.

The test-retest reliability ranged from poor (intraclass correlation coefficient [ICC]=.49) to good (ICC=.83), and the measures correlated moderately with each other (r range,.56-.72). Median change scores differed significantly between improved and unimproved persons for all measures. The effect sizes for VAS-U (.79), VAS-W (.88), and the AKPS (.98) were large, indicating greater responsiveness than the FIQ (.37) and VAS-activity (.66). Similarly, the AKPS and VAS-W were the most efficient measures for detecting a treatment effect when compared with a reference measure (VAS-U, which was assigned a value of 1). The minimal difference that patients or clinicians consider clinically important for the AKPS is 10 (out of 100) points and for the VAS it is 2cm (out of 10cm).

The AKPS and VAS for usual or worst pain are reliable, valid, and responsive and are therefore recommended for future clinical trials or clinical practice in assessing treatment outcome in persons with patellofemoral pain.