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While the validity and reliability of many newer health status instruments have been reported, few data are available regarding the sensitivity of these instruments to clinically discernible changes in patient status. We studied this feature of the Sickness Impact Profile (SIP) in a group of patients with rheumatoid arthritis, comparing it with more traditional measures of functional status (the American Rheumatism Association (ARA) functional classification and a patient self-rating scale). Four different approaches were devised to measure "sensitivity to clinical change." These involved comparisons of functional score changes with clinical changes in patient status which were independently agreed upon by both clinician and patient, and also comparisons with several clinical disease severity indicators. When applied to groups of patients, the SIP and the patient self-rating scale were modestly superior to the ARA scale, but neither the SIP nor the self-rating scale was clearly superior to the other. For considering individual patients, all of the scales were relatively insensitive, and predictive accuracy for clinically estimated change was low. New strategies for assessing sensitivity to small changes should be developed and applied to health status and functional scales. Attention to this characteristic should allow refinement of existing scales and may enhance their clinical usefulness.