Assessing the responsiveness of functional scales to clinical change: an analogy to diagnostic test performance.

One characteristic of newer health or functional status scales which has received little attention is their responsiveness over time to clinical change. In part, this is because methods for assessing this characteristic are crude and not well standardized. We suggest that scales be viewed as "diagnostic tests" for discriminating between improved and unimproved patients. With this perspective, one may construct receiver operating characteristic (ROC) curves describing a scale's ability to detect improvement (or failure to improve) using some external criterion. This method is illustrated using data from a study of acute low back pain, comparing the Sickness Impact Profile, its major subscales, and a shorter, more disease-specific scale. The results demonstrate an advantage of the ROC approach over simple pre- and post-treatment score comparisons in assessing scale responsiveness. They also suggest some advantage for a brief disease-specific scale over the lengthier "generic" SIP.