Objectives 1. To identify factors that influence fatigue in AS. 2. To assess whether fatigue is associated with quality of life in AS.

Methods A total number of 812 AS patients were included. Fatigue was measured with a single-item question elicited from the Bath AS Disease Activity Index (BASDAI) and with the Multidimensional Fatigue Inventory (MFI). The patients also completed questionnaires on functional ability (BASFI), global well-being (BAS-G), and quality of life (SF-36). In addition, patients were asked to identify factors that influence fatigue.

Patients were dichotomously divided into a F+ group if the BASDAI fatigue score was >5.0, labelled as experiencing fatigue as a major symptom, and a F- group in case the fatigue score was <5.0 (fatigue a minor symptom). Logistic regression analysis (dependent variable F+/F-) was used to determine which factors were associated with fatigue. Multiple regression analysis was used to investigate whether fatigue contributes in explaining quality of life (dependent variable dimensions from the SF-36; independent variable BASDAI fatigue question). In both analyses adjustments were made for age, sex, disease duration, and the presence of co-morbidity.

Results 53% of the patients were categorised to the F+ group. They scored significantly worse compared to the F- group with respect to each dimension of the MFI and to all other questionnaires studied (all p < 0.001). Fatigue was, in the opinion of the patients, highly influenced by pain (70% of the patients) and stiffness (54%). The hierarchical order of factors perceived to be associated with fatigue was grossly similar in the F+ and F- groups, but pain, poor sleep and work were significantly more often considered important by patients from the F+ group. Logistic regression analysis showed that scores on BASDAI (without fatigue question), BASFI and BAS-G were independently associated with fatigue (R2: 0.52). Multiple regression analysis showed that fatigue was significantly associated with all separate dimensions of the SF-36 studied (p < 0.001).

Conclusion Fatigue is a major symptom in AS and is associated with disease activity, functional ability, and global well-being. Fatigue independently contributes negatively to different dimensions of quality of life.