Assessing fibromyalgia-related fatigue: Content validity and psychometric performance of the Fatigue Visual Analogue Scale in adult patients with fibromyalgia

Objectives: To document 1) the content validity and 2) measure improvements in fatigue, using the Fatigue Visual Analogue Scale (VAS) assessment tool in patients with fibromyalgia. Methods: The relevance and comprehensiveness of the Fatigue VAS were tested through a qualitative analysis of 20 subjects' verbatim transcripts from semi-structured qualitative interviews. Data from two randomised, controlled trials in fibromyalgia (n=1121) were used to conduct correlation analyses with the Fatigue and Tiredness items from the Fibromyalgia Impact Questionnaire (FIQ) and the Short Form-36 Vitality scale. Known-groups and cross classification analyses were conducted to demonstrate the ability to measure improvement in fatigue using the Fatigue VAS. Results: All subjects spontaneously reported that fatigue was an important symptom to capture in fibromyalgia. The Fatigue VAS was well understood by most subjects (n=18/20). High correlations (Pearson r>0.75) and good agreement (k>0.66) were found between the Fatigue VAS and the FIQ tiredness items no. 16 and 17 and SF-36 ^™ Vitality scale. In both clinical trials there was a substantial separation of approximately 20 points on the mean change in the Fatigue VAS score between responders (>30% improvement in pain VAS) and non-responders. Conclusion: Previous studies have confirmed that fatigue is a major component of the fibromyalgia experience. This current study reports that fibromyalgia patients spontaneously rated fatigue as a highly significant feature of their illness, and supports the use of the Fatigue VAS as a valid questionnaire in fibromyalgia clinical trials.