No further gain can be achieved by calculating Disease Activity Score in 28 joints with high-sensitivity assay of C-reactive protein because of high intraindividual variability of C-reactive protein: A cross-sectional study and theoretical consideration

Disease Activity Score in 28 joints (DAS28) is commonly used to evaluate disease activity of rheumatoid arthritis (RA) and is a guide to treatment decision. The aim of this study was to evaluate the impact of lower reporting limit for C-reactive protein (CRP), with respect to intraindividual biological variability, on the calculation of DAS28 and subsequent patient classification. This study consists of 2 sections: a theoretical consideration discussing the performance of CRP in calculating DAS28 taking intraindividual biological variation and lower reporting limit for CRP into account and a cross-sectional study of RA patients applying our theoretical results. Therefore, we calculated DAS28 twice, with the actual CRP values and CRP = 9 mg/L, the latter to elucidate the positive effects of reducing the lower reporting limit of CRP from <10 to <3 mg/L. Lower-reporting limit of <10 mg/L leads to overestimate DAS28. However, reducing lower reporting limit for CRP to <3 mg/L results in optimizing DAS28 calculation. Further lowering of reporting limit for CRP to <3 mg/L does not increase the precision of DAS28 owing to the relatively large intraindividual biological variation. Five hundred twelve patients were included. There was a significant difference between recalculated and patients DAS28 (P < 0.001). One hundred nine patients had DAS28 deviation (compatible to remission to low: 66, low to moderate: 39. and moderate to high: 4). Owing to significant impact of intraindividual biologic variation on DAS28 and patient classification, special attention should be paid to calculate DAS28 when CRP values are within normal range. Furthermore, we conclude that results of different studies evaluating DAS28 and treatment response are not comparable if the reporting limits of CRP are unknown.