Physical impairment index: reliability, validity, and responsiveness in patients with acute low back pain.

STUDY DESIGN Cohort study of patients with acute low back pain undergoing physical therapy. OBJECTIVES Examine the reliability and validity of the Physical Impairment Index in a group of patients with acute low back pain and determine the responsiveness and minimum detectable change of the index and its component tests. SUMMARY OF THE BACKGROUND DATA The Physical Impairment Index was originally described as a reliable and valid means of assessing physical impairment in patients with low back pain. The psychometric properties of the index have not been reported in patients with acute low back pain, nor has its responsiveness been examined. METHODS Seventy-eight patients with acute (<3 weeks duration) low back pain participating in a clinical trial were assessed at baseline and after 4 weeks. Interrater reliability of the index was examined in a subgroup of 20 patients. Validity was examined through correlations with concurrent measures of pain, disability, and psychosocial variables. Changes in the index over 4 weeks were used to assess responsiveness and minimum detectable change. RESULTS Interrater reliability of the index was high (intraclass correlation coefficient = 0.89), and its validity was generally supported by the pattern of correlations. The index was more responsive to change than any of its component tests but was less responsive than the Oswestry disability score. The minimum detectable change on the index was approximately 1 point. CONCLUSIONS The Physical Impairment Index appears to be a reliable and valid measure of physical impairment for patients with acute low back pain and may be useful as an adjunct outcome measure for studies involving these patients. Further research on patients with chronic pain is needed before it can be advocated for outcomes research with this population.