Problems in using Health Survey questionnaires in older patients with physical disabilities - Can proxies be used to complete the SF-36?

Background.-The SF-36 Health Survey questionnaire has been proposed as a ge neric measure of health outcome. However, poor rates of return and high lev els of missing data have been found in elderly subjects and, even with face -to-face interview, reliability and validity may still be disappointing, pa rticularly in cognitively impaired patients. These patients may be the very patients whose quality of life is most affected by their illness and their exclusion will lead to biased evaluation of health status. A possible alte rnative to total exclusion is the use of a proxy to answer on the patient's behalf, but few studies of older people have systematically studied patien t-proxy agreement. Objective: To compare the agreement between patients, la y and professional proxies when assessing the health status of patients wit h the SF-36. Methods: The SF-36 was administered by interview to 164 cognit ively normal, elderly patients (Mini-mental State Examination 24 or more) r eferred for physical rehabilitation. The SF-36 was also completed by a pati ent-designated lay proxy (by post) and a professional proxy. Agreement betw een proxies and patients was measured by intraclass correlation coefficient s (ICCs), and a bias index. Results: Professional proxies were better able to predict the patients' responses than were the lay proxies. Criterion lev els of agreement (ICC 0.4 or over) were attained for four of the eight dime nsions of the SF-36 by professional proxies, but for only one dimension by lay proxies. In professional proxies, the magnitude of the bias was absent or slight (<0.2) for six of the eight dimensions of the SF-36 with a small (0.2-0.49) negative bias for the other two. Lay proxies showed a negative b ias (i.e. they reported poorer function than did the patients themselves) f or seven of the eight dimensions of the SF-36 (small in two and moderate (0 .5-0.79) in five). Conclusions: For group comparisons using the SF-36, prof essional proxies might be considered when patients cannot answer reliably f or themselves. However, in the present study, lay proxy performance on a po stal questionnaire showed a strong tendency to negative bias. Further resea rch is required to define the limitations and potentials of proxy completio n of health status questionnaires. Copyright (C) 2001 S. Karger AG, Basel.