Validity of the SF-36 in patients with morbid obesity

The Medical Outcome Study 36-Item Short Form Health Survey (SF-36) is one of the most commonly applied generic quality of life instruments. The construct validity of the instrument in patients with morbid obesity is not established.

A total of 475 morbidly obese patients (68% women) with a mean (standard deviation) age of 44.7 (11.8) years, weight of 123.5 (24.1) kg and BMI of 41.7 (6.3) kg/m(2), who had been referred to a rehabilitation center, completed the SF-36 form. Exploratory factor analyses were performed to examine the underlying component structure of the questionnaire. Confirmatory factor analyses were performed to assess model fit.

The analysis suggested a 6-component structure rather than the 8-component structure used in the original SF-36. The first component consisted of items from the physical functioning subscale, the role physical subscale and the general health subscale, and explained 31% of the variance. The 6 components explained 61% of the total variance. The items loaded as expected in a physical and mental component. The assessment of model fit confirmed these findings.

The 2 summary scales of the SF-36 have satisfactory validity in patients with morbid obesity. However, the validity of the 8 subscales is questionable, and the subscales should be interpreted with care.