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Physical Impairment Index: Reliability, Validity, and Responsiveness in Patients with Acute Low Back Pain

Fritz, Julie M., PhD, PT, ATC*; Piva, Sara R., MS, PT

doi: 10.1097/01.BRS.0000067270.50897.DB
Exercise Physiology and Physical Exam
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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.

From the *Department of Physical Therapy and

the †School of Health and Rehabilitation Sciences, University of Pittsburgh, Pittsburgh, Pennsylvania.

Supported by a grant from the Foundation for Physical Therapy.

Acknowledgement date: April 30, 2002.

First revision date: September 23, 2002.

Acceptance date: October 28, 2002.

The submitted manuscript does not contain information about medical devices or drugs.

Foundation funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

Address correspondence to Julie M. Fritz, PhD, PT, ATC, 6035 Forbes Tower, Pittsburgh, PA 15260, USA; E-mail: jfritz@pitt.edu

© 2003 Lippincott Williams & Wilkins, Inc.