Patient-reported outcomes are used in clinical practice and trials. We studied a large clinical practice to determine the minimally important difference (MID) estimates for (1) the Health Assessment Questionnaire-Damage Index (HAQ-DI): improvement and worsening using patient global assessment anchor; and (2) pain using a patient-reported pain anchor.
Patients with rheumatoid arthritis (RA; N = 225) had clinic visits at 2 timepoints within 1 year, completed the HAQ-DI and pain visual analog scale (VAS; 0-100 mm), and answered the question, "How would you describe your overall status/overall pain since the last visit?", as much worsened, somewhat worsened, the same, somewhat improved, or much improved. If rated as somewhat improved or worsened, they were defined as the minimally changed subgroups.
Eighty-three percent were women, mean age 60 years, with disease duration 11.7 +/- 10.7 years. The baseline HAQ-DI was 0.97 +/- SD 0.76, and at followup 1.0 +/- 0.77 (mean change +0.03 +/- 0.40). The baseline pain VAS was 42.3 +/- 28.8, and at followup 38.5 +/- 27.9 (mean change -2.8 +/- 25.9). The mean (SD) HAQ-DI change score was -0.09 (0.42) for somewhat improved and 0.15 (0.33) for somewhat worsened. The HAQ-DI change for somewhat/much better was -0.20 +/- 0.52, and for somewhat/much worse +0.21 +/- 0.33. For pain, somewhat improved changed by -11.9 mm on the VAS, and somewhat worsened by 6.8 mm. Estimates for HAQ-DI and pain were larger than the for no-change group, 0.03 (0.32) and -3.2 (20.9).
The MID for HAQ-DI in clinical practice is smaller than it is in trials. This may have implications for observational studies and clinical care.