Disease activity measurement of rheumatoid arthritis: Comparison of the simplified disease activity index (SDAI) and the disease activity score including 28 joints (DAS28) in daily routine

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Abstract
Objective To assess the reliability and congruency of the Simplified Disease Activity Index (SDAI) compared with the Disease Activity Score including 28 joints (DAS28) in daily practice. Methods In 399 consecutive rheumatoid arthritis patients (307 women, 92 men), the SDAI and the DAS28 were calculated. Additionally, 115 of them were observed for 1 year and changes of both values were recorded. Joint assessments were performed by 4 experienced physicians. DAS28 and SDAI values and the respective changes were compared by correlation and regression analyses. Reliability assessment and factor analyses were performed. Disease activity categorizing was compared by the Wilcoxon's rank sum test. Results The median ± SD scores were 3.42 ± 1.45 for the DAS28 and 11.50 ± 11.50 for the SDAI. Spearman's rho was 0.897 (P < 0.0001). Score changes were also significantly correlated. Reliability testing and factor analysis revealed that both scores can be regarded as monocomponent. Categorizing patients according to the European League Against Rheumatism response criteria (EULARC) or the SDAI revealed statistically significant differences between the 2 scales (P < 0.0001). Conclusion SDAI values are considerably shifted to the left compared with DAS28 levels. Internal consistency and reliability of both scores are comparable. For the differences in disease activity categorizing due to the SDAI compared with the EULARC, a major limitation of the application of this newly developed disease activity score is given, unless these incongruencies can be cleared.