Equivalence of the acute phase reactants C-reactive protein, plasma viscosity, and Westergren erythrocyte sedimentation rate when used to calculate American College of Rheumatology 20% improvement criteria or the disease activity score in patients with early rheumatoid arthritis

Objective. In an additive cohort of patients with early rheumatoid arthriti s (RA), to determine the effect of substituting one acute phase reactant fo r another on the number of patients satisfying the American College of Rheu matology (ACR) 20% preliminary criteria for improvement, and on calculated Disease Activity Scores (DAS). Methods, A total of 251 patients with 6.4 months average disease duration h ad detailed clinical assessments at entry and 6, 12, and 24 months in a mul ticenter prospective longterm observational study. Matched erythrocyte sedi mentation rate (ESR), C-reactive protein (CRP), and plasma viscosity (PV) a ssays were done at 366 time points. Disease modifying antirheumatic drugs w ere not started until after the baseline evaluation. Results. After 6, 12, and 24 months, 50%, 53%, and 57% of patients were res ponders, as defined by the ACR 20% improvement criteria. The difference in response rates when ESR, CRP, or PV was used as the acute phase reactant ra nged from 0.4% at 12 months to 3% at 24 months. Percentile distributions of the 366 matched CRP, ESR, and PV values were used to prepare a nomogram th at can be used to calculate the other acute phase reactant values if the va lue of one is known. When the nomogram was used to impute ESR values from o bserved PV or CRP values, average DAS scores calculated with the actual ESR values were not different from average DAS scores calculated from the impu ted ESR values, Conclusion. ESR, CRP, and PV are equally useful in calculating ACR 20% resp onse rates in patients with active early RA. A nomogram can be used to impu te ESR values from CRP or PV values; use of the imputed ESR values is as ac curate as use of the actual ESR values to calculate average DAS.