Measuring fatigue in SSc: a comparison of the Short Form-36 Vitality subscale and Functional Assessment of Chronic Illness Therapy-Fatigue scale

Objective. Fatigue is a common and important problem in SSc. No studies, however, have compared the properties of fatigue measures in SSc. The objective of this study was to compare the performances of the Short Form-36 (SF-36) Vitality subscale and Functional Assessment of Chronic Illness Therapy-Fatigue (FACIT) in SSc. Methods. Cross-sectional, multi-centre study of Canadian Scleroderma Research Group Registry patients. The associations of the two instruments with other patient-reported outcome measures, as well as physician- and patient-rated disease variables were compared. Item response theory models were used to compare the degree to which items and the total scores of each measure effectively covered the full spectrum of fatigue levels. Results. There were 348 patients (297 women, 85%) in the study. The instruments correlated at r = 0.65 with each other. The FACIT tended to correlate slightly higher than the SF-36 Vitality subscale with physician- and patient-rated disease variables and patient-reported physical function and disability, whereas the SF-36 Vitality subscale correlated minimally higher with mental health measures. The FACIT had markedly better discrimination across the range of fatigue, particularly at average to high fatigue levels, whereas the SF-36 Vitality subscale discriminated well only among patients in the low to average range. Conclusion. The FACIT discriminates better than the SF-36 Vitality subscale at average to high ranges of fatigue, which is common in SSc, suggesting that it is preferred for measuring fatigue in SSc.