Discrepancies in categorizing rheumatoid arthritis patients by DAS-28(ESR) and DAS-28(CRP): can they be reduced?

OBJECTIVES: The 28-joint disease activity score (DAS-28) guides the use of biologics in RA. The aims of this study were to investigate agreement between the ESR- and CRP-based DAS-28 definitions, and to examine how this agreement may be improved.
METHODS: Data were obtained from registers of early (n = 520) and established RA (n = 364) patients. Agreement over disease activity levels (remission, low, moderate and high) at baseline and 6 months, and EULAR responder status at 6 months, were assessed in the early cohort. Two alternative DAS-28(CRP) definitions, obtained through linear regression analyses at baseline in the early RA patients, were validated with 6-month data from both the cohorts.
RESULTS: In early RA patients, despite a high percentage of exact agreement over DAS-28 categories (88.2%), 38 (30.4%) of 125 patients with 'moderate' DAS-28(CRP) at baseline had 'high' DAS-28(ESR). This agreement was improved by modifying the DAS-28(CRP) definition, and by incorporating age and gender: e.g. in early RA patients with moderate original DAS-28(CRP), 30.4% had 'high' DAS-28(ESR), whereas 3.2% had 'low' DAS-28(ESR); following DAS-28(CRP) transformation both proportions were 6.6%. Incorporating age and gender did not improve agreement over EULAR response states.
CONCLUSION: The DAS-28(ESR) and DAS-28(CRP) definitions differ substantially in classifying RA patients as having moderate or high disease activity, with the ESR definition resulting in a higher proportion of high DAS-28 especially in women. Our results suggest that modifying the DAS-28(CRP) definition may improve agreement with DAS-28(ESR). There are important implications for meta-analyses and for therapy driven by DAS scores.