Psychometric properties of the functional rating index in patients with low back pain

Main Problem: The purpose of this study was to validate the psychometric properties of the functional rating index (FRI), establish the instrument's minimum clinically important difference (MCID), and compare its psychometric properties with the Oswestry questionnaire. Methods: This was a cohort study of patients with low back pain (LBP) undergoing physical therapy. One thirty one patients with a primary complaint of LBP participating in a clinical trial were assessed at baseline and at a 1- and 4-week follow-up. Test-re-test reliability was examined using the intraclass correlation coefficient, and validity was examined by determining the association between the FRI and Oswestry, a concurrent measure of disability. Responsiveness was examined by calculating the standard error of the measure, minimum detectable change, area under a receiver operating characteristic curve, and minimum clinically important difference. Changes in clinical status at each follow-up period were compared to the average of the patient and therapist's perceived improvement using the 15-point global rating of change scale. Results: Test-retest reliability of the FRI was moderate, with an intraclass correlation coefficient equal to 0.63 (0.35, 0.80). Validity of the FRI was supported by a moderate correlation between the FRI and Oswestry (r=0.67, P < 0.001). Area under the curve for the FRI was 0.93 (0.89, 0.98), and the minimum clinically important difference was approximately nine points. Conclusions: The FRI is less reliable than the Oswestry but appears to have comparable validity and responsiveness. Before the FRI can be recommended for widespread use in patients with neck and low back pain, it should be further tested in patients with neck pain.