Outcome measurement in functional somatic syndromes: SF-36 summary scores and some scales were not valid.

OBJECTIVE

This study aimed to test the validity of the 36-item Short-Form Health Survey (SF-36) scales and summaries in patients with severe functional somatic syndromes (FSS), such as fibromyalgia and irritable bowel syndrome.

STUDY DESIGN AND SETTING

One hundred twenty patients with severe FSS enrolled in a randomized controlled trial filled in the SF-36 questionnaire. We tested for data quality, central scaling assumptions, and agreement with the conceptual model.

RESULTS

Most SF-36 scales were found to be valid; however, three scales (role physical, role emotional, and general health) did not satisfy predefined criteria for construct validity, internal consistency, or targeting to the sample. The correlations between SF-36 scales differed considerably from those reported in the general population. As a consequence, the SF-36 summaries, physical component summary (PCS) and mental component summary (MCS), did not accurately reflect their underlying scales and were negatively correlated (r=-0.46, 95% CI [-0.60 to -0.31]).

CONCLUSION

Although the SF-36 is a valuable instrument to assess perceived health in patients with severe FSS, there are problems with some of the scales and with the scoring procedure of the summaries. The SF-36 PCS may, therefore, not accurately measure the physical health status of these patients. Alternative summary measures are needed.