Problems in using health survey questionnaires in older patients with physical disabilities. The reliability and validity of the SF-36 and the effect of cognitive impairment

Reliability and validity of the SF‐36 Health Survey Questionnaire was assessed in older rehabilitation patients, comparing cognitively impaired with cognitively normal subjects. The SF‐36 was administered by face‐to‐face interview to 314 patients (58–93 years) in the day hospital and rehabilitation wards of a department of medicine for the elderly. Reliability was measured using Cronbach’s alpha (for internal consistency) on the main sample and intraclass correlation coefficients on a test–retest sample; correlations with functional independence measure (FIM) were examined to assess validity. In 203 cognitively normal patients (Mini‐Mental State Examination ≥24), Cronbach’s alpha scores on the eight dimensions of the SF‐36 ranged from 0.545 (social function) to 0.933 (bodily pain). The range for the 111 cognitively impaired patients was 0.413–0.861. Cronbach’s alpha values were significantly higher (i.e. reliability was better) in the cognitively normal group for bodily pain (P = 0.003), mental health (P = 0.03) and role emotional (P = 0.04). In test–retest studies on a further 67 patients, an intraclass correlation coefficient of 0.7 was attained for five out of eight dimensions in cognitively normal patients, and four out of eight dimensions in the cognitively impaired. Only the physical function dimension in the cognitively normal group attained the criterion level (r > 0.4) for construct validity when correlated with the FIM. In this group of older physically disabled patients, levels of reliability and validity previously reported for the SF‐36 in younger subjects were not attained, even on face‐to‐face testing. Patients with coexistent cognitive impairment performed worse than those who were cognitively normal.