Objective Thresholds of minimal clinically important improvement (MCII) are needed to plan and interpret clinical trials. We estimated MCIIs for the rheumatoid arthritis (RA) activity measures of patient global assessment, pain score, Health Assessment Questionnaire Disability Index (HAQ), Disease Activity Score-28 (DAS28), Simplified Disease Activity Index (SDAI), and Clinical Disease Activity Index (CDAI).

Methods In this prospective longitudinal study, we studied 250 patients who had active RA. Disease activity measures were collected before and either 1 month (for patients treated with prednisone) or 4 months (for patients treated with disease modifying medications or biologics) after treatment escalation. Patient judgments of improvement in arthritis status were related to prospectively assessed changes in the measures. MCIIs were changes that had a specificity of 0.80 for improvement based on receiver operating characteristic curve analysis. We used bootstrapping to provide estimates with predictive validity.

Results At baseline, the mean (±SD) DAS28-ESR (erythrocyte sedimentation rate) was 6.16±1.2 and mean SDAI was 38.6±14.8. Improvement in overall arthritis status was reported by 167 patients (66.8%). Patients were consistent in their ratings of improvement versus no change or worsening, with receiver operating characteristic curve areas ≥0.74. MCIIs with a specificity for improvement of 0.80 were: patient global assessment −18, pain score −20, HAQ −0.375, DAS28-ESR −1.2, DAS28-CRP (C-reactive protein) −1.0, SDAI −13, and CDAI −12.

Conclusions MCIIs for individual core set measures were larger than previous estimates. Reporting the proportion of patients who meet these MCII thresholds can improve the interpretation of clinical trials in RA.