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OBJECTIVE: To examine the influence of components of the Disease Activity Score 28 (DAS28) [tender joint count (TJC), swollen joint count (SJC), patient's general health (GH), and erythrocyte sedimentation rate (ESR)] on the total DAS28 score, and overlapping of the 4 individual components in rheumatoid arthritis (RA) patients with low, moderate, or high disease activity. METHODS: The effect of each component was studied in the FIN-RACo trial patients at 6 months and in a "theoretical model," where each component of the DAS28 ranged as follows: TJC and SJC from 0 to 28, GH from 0 to 100, and ESR from 1 to 100, while the other 3 components were 0 (ESR1). Overlapping of the components was studied in the FIN-RACo trial patients at 6 months with low (DAS28 < or = 3.2), moderate (DAS28 > 3.2 and < or = 5.1), and high (DAS28 > 5.1) disease activity. The higher limit for overlapping was defined as the highest SJC in the low disease activity group, and the lower limit as the lowest SJC in the high disease activity group; the percentage of patients who fall between these limits represent overlapping in SJC. Overlapping was calculated similarly concerning TJC, ESR, and GH. RESULTS: ESR had the greatest effect on DAS28, followed by TJC, GH, and SJC, while in the "theoretical model" TJC had the greatest effect on the DAS28, followed by ESR, SJC, and GH. At 6 months, overlapping was present in 54%, 45%, 49%, and 31% of patients in SJC, TJC, GH, and ESR, respectively. CONCLUSION: In real-life patients, ESR had the greatest effect of the 4 components of DAS28 on the total DAS28 score. The values of the individual components of DAS28 overlap considerably among the 3 disease activity groups.