The Minimally Important Difference for the Fatigue Visual Analog Scale in Patients with Rheumatoid Arthritis Followed in an Academic Clinical Practice

Abstract

To estimate the minimally important difference (MID) for a fatigue visual analog scale (VAS) using patient-reported anchors (fatigue, pain, and overall health). Patients with rheumatoid arthritis (RA; n = 307) had 2 clinic visits at a median of 5.9 months apart. They completed a fatigue VAS (0-10 scale) and the retrospective anchor items, "How would you describe your overall fatigue/pain/overall health since the last visit?" with response options: Much worsened, Somewhat worsened, Same, Somewhat better, or Much better. The fatigue anchor was used for primary analysis and the pain/overall health anchors for sensitivity analyses. The minimally changed group was defined by those reporting they were somewhat better or somewhat worsened. The mean [standard deviation (SD)] age was 59.4 (13.2) years, disease duration was 14.1 (11.5) years, and 83% of patients were women. The baseline mean (SD) Health Assessment Questionnaire-Disability Index score was 0.84 (0.75). The baseline fatigue VAS score was 4.2 (2.9) and at followup was 4.3 (2.8) [mean change of -0.07 (2.5); p = not significant]. The fatigue change score (0-10 scale) for Somewhat better and Somewhat worsened for the fatigue anchor averaged -1.12 and 1.26, respectively. Using the pain anchor, the fatigue change score for Somewhat better and Somewhat worsened averaged -0.87 and 1.13; and using the global anchor, the fatigue change score for Somewhat better and Somewhat worsened averaged -0.82 and 1.17, respectively. Effect size estimates using 3 anchors were small for the Somewhat better (range 0.27-0.39) and Somewhat worsened (0.40-0.44) groups, but larger than for the no-change group (0.03-0.08). The MID for fatigue VAS is between -0.82 for -1.12 for improvement and is 1.13 to 1.26 for worsening on a 0-10 scale in a large RA clinical practice, and is similar to that seen in RA clinical trials. This information can aid in interpreting fatigue VAS in day-to-day care in clinical practice.