Measuring the functional status of patients with low back pain.

For trials of back pain therapy and for many clinical purposes, the outcome of greatest interest may be patient functioning. Physical measurements of muscle strength and range of motion are only weakly correlated with actual patient behavior or symptoms, so the behavior and symptoms must be measured directly. Commonly used physician ratings on ordinal scales are often highly subjective and fail to distinguish particular activities which may improve from those which do not. This paper briefly considers the pressures for improving functional status measurement, describes five criteria for assessing and comparing functional questionnaires, and selectively reviews some newer instruments using these criteria. These newer instruments offer standardization of measurement, comprehensiveness, and generally good reproducibility and validity. Unfortunately, little is known about the responsiveness of these measures to small but clinically important changes. There is a need in this field to define the optimal balance between brevity on the one hand and comprehensiveness on the other, to assess and improve instrument responsiveness, and to compare instruments "head-to-head" to allow rational selection for particular applications.