SF-36 summary scores: are physical and mental health truly distinct?

OBJECTIVES: Data from 536 primary care patients initiating antidepressant treatment were used to examine the performance of the Medical Outcomes Study SF-36 Physical Component and Mental Component summary scores.
METHODS: The SF-36 was administered at baseline and readministered after 1 month and 3 months (n = 482).
RESULTS: At the baseline assessment, patients reported modest impairment on the Physical Function, Role--Physical, Bodily Pain, and General Health Perceptions subscales (0.10 to 0.68 standard deviations below national norms), but the Physical Component summary (mean = 51) indicated no impairment based on a population norm of 50. During 3 months of follow-up evaluation, the Physical Function, Role--Physical, Bodily Pain, and General Health Perceptions subscales all showed moderate and statistically significant improvement (range from 0.28 to 0.49 standard deviation units), whereas the Physical Component summary score was essentially unchanged (mean = 50 at both 1-month and 3-month assessments). This pattern of results is a natural consequence of the assumptions and methods used to calculate these summary scores-orthogonal factor rotation and negative scoring coefficients.
CONCLUSIONS: These findings suggest caution in the interpretation of Mental Component Summary and Physical Component Summary scores when the condition or treatment of interest has strong effects on scales with negative scoring coefficients (Mental Health and Role--Emotional for the Physical Component; Physical Function, Role--Physical, and Bodily Pain for the Mental Component).