Fatigue in patients with ankylosing spondylitis: A comparison with the general population and associations with clinical and self-reported measures.

OBJECTIVE: To investigate 1) levels of fatigue in patients with ankylosing spondylitis (AS) compared with the general population; 2) the relationships between fatigue and demographic, self-reported, and clinical measures; and 3) the performance of both a generic and a disease-specific measure of fatigue.
METHODS: Patients with AS (n = 152) were compared with people from the general population (n = 2,323). Fatigue was assessed by the Short Form 36 (SF-36) vitality scale and the fatigue item of the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI). Other measures of self-reported health included BASDAI for disease activity, Bath Ankylosing Spondylitis Functional Index for functional abilities, and the SF-36 for mental health. Clinical measures comprised Bath Ankylosing Spondylitis Metrology Index for joint mobility and erythrocyte sedimentation rate and C-reactive protein as inflammatory markers. The explanatory power of demographic, self-reported, and clinical measures was examined in a block regression model.
RESULTS: The mean +/- SD SF-36 vitality score was 43 +/- 24 in the patients and 60 +/- 21 in the general population (P < 0.001). The SF-36 vitality and the BASDAI fatigue scores were consistently associated with measures of mental health and disease activity. Clinical measures did not show explanatory power. A cutoff at 70 mm on the BASDAI fatigue item implied specificity of 0.77 and sensitivity of 0.82.
CONCLUSION: Self-reported measures of disease activity and mental health contributed significantly to explain fatigue, whereas clinical measures of inflammation and joint mobility did not. The BASDAI fatigue item reached acceptable sensitivity and specificity with a cutoff at 70 mm when using the low vitality scores of SF-36 as an external indicator.