Test-retest reliability of disease activity core set measures and indices in rheumatoid arthritis

Aim: To examine the test-retest reliability of the rheumatoid arthritis (RA) core disease activity measures and derived composite indices. Methods: A total of 28 stable patients with RA had 2 complete assessments within 1 week, which included the 7 RA core disease activity measures and derived disease activity indices (28-joint Disease Activity Score (DAS28), Simplified Disease Activity Index (SDAI), Clinical Disease Activity Index (CDAI), RA Disease Activity Index (RADAI) and Routine Assessment of Patient Index Data (RAPID3)). The intraclass correlations (ICC), the smallest detectable difference (SDD) and minimal detectable change as percentage of the maximum score (MDC%) were estimated as measures of test-retest reliability. Results: Correlations for the disease activity indices were high. SDDs (MDC%) to detect a true improvement or deterioration with 95% confidence were: DAS28 1.32 ( 14.4%), SDAI 8.26 (9.6%), CDAI 8.05 (10.6%), RAPID3 1.48 (14.8%) and RADAI 1.49 (14.9%). Thus, SDDs were rather high, and the MDC% values were of a similar magnitude of 10% to 15% for all seven core data set measures. Conclusions: SDDs of the DAS28, SDAI and CDAI were close to limits to detect important improvement. Clinicians should be aware of measurement error. Nonetheless, RA core data set measures and indices obtained from a health professional, laboratory and patient self-report had similar reliability.