Assessing fatigue in ankylosing spondylitis: the importance of frequency and severity

Objective. The aim of this study was to explore the value of assessing fatigue frequency and its relationship with fatigue severity in a UK cohort of AS patients. Methods. Single items from the Evaluation of AS Quality of Life and BASDAI were used to measure fatigue frequency and severity, respectively. Items were included in a questionnaire containing AS-specific and generic measures, completed by participants in a postal survey at baseline and 6 months. Respondents were categorized at baseline into four groups according to fatigue frequency and severity and compared on other measures of health status. Results. Of baseline responders who experienced fatigue (n = 451, 74%), 75% reported it to be frequent and severe, 15% frequent not severe and 10% severe not frequent. There was no difference between groups on gender, age or years with AS. Patients reporting frequent and severe fatigue had worse scores than other groups across all other health status measures. Patients reporting only frequent fatigue had similar scores to those reporting only severe fatigue, but worse than those without fatigue. Eighty-one per cent of non-fatigued patients and 79% of those with frequent and severe fatigue at baseline did not change their level of fatigue at 6 months. However, 80% of patients with frequent or severe fatigue at baseline changed, mainly to no fatigue (43%) or both frequent and severe fatigue (30%). Conclusion. Routinely assessing both the frequency and severity of fatigue is important in understanding the impact of fatigue and its change over time. Not assessing frequency could result in the failure to identify patients with significant fatigue. However, the multidimensional nature of fatigue should be further explored.