Measuring functional outcomes in therapeutic trials for chronic disease

For clinical trials of many chronic diseases, the outcome of greatest interest may be patient function. Unfortunately, most scales for measuring functional status are crude and rarely consider important psychosocial impacts of disease. This paper briefly considers the pressures for improving functional status measurement, proposes six criteria for assessing functional or "health status" scales, and selectively reviews representative instruments using these criteria. Older functional classifications and many scales used for quality-of-care assessment are narrowly focussed on physical function. Their reliability, validity, and sensitivity to clinical changes are generally unknown. Traditional scales of "Activities of Daily Living" are similarly focussed on physical function, and are most appropriate for severely disabled inpatients. A new generation of "health status" instruments offers wider applicability, more comprehensiveness, and feasibility for clinical applications. Their validity and reliability are generally quite good. None of the instruments reviewed, however, has yet demonstrated convincing success as a "transition" variable (sensitivity to small but clinically important changes). Based on this analysis, future investigation should seek to (1) define the optimal balance between brevity on the one hand, and comprehensiveness and reliability on the other, (2) describe the sensitivity of scales to clinically important changes, and (3) directly compare existing instruments to aid selection by investigators who are considering their use in clinical trials.