Establishment of conversion equation from DAS28CRP to DAS28ESR and derivation of threshold of DAS28CRP

Abstract

Objective To establish the conversion equation from DAS28CRP to DAS28ESR and derive the corresponding thresholds of DAS28CRP using DAS28ESR as the standard. Methods Patients who received simultaneous measurement of DAS28ESR and DAS28CRP at Department of Rheumatology of Sun Yat-sen Memorial Hospital were recruited and randomly assigned into the training set （70%） and validation set （30%）. The Bisquare-weighted robust regression was applied to establish the conversion equation from DAS28CRP to DAS28ESR. The Wilcoxon signed-rank test was used to compare the difference between the converted values and actual values of DAS28ESR and the kappa values were used to assess the agreement in disease activity classification. Results A total of 955 patients were enrolled in the study. The conversion equation was： DAS28ESR1 = 1.06 × DAS28CRP + 0.4275. According to the conversion equation, the corresponding thresholds for DAS28CRP were 2.05, 2.61 and 4.40, respectively. After conversion, there was no statistical difference between the converted values and actual values of DAS28ESR （P > 0.05）. The kappa values were increased from 0.54 to 0.78. Conclusions The conversion equation from DAS28CRP to DAS28ESR is successfully established, which facilitates the comparison of findings among different studies. The derived DAS28CRP thresholds contribute to improving the agreement with DAS28ESR in assessing the disease activity.

Keywords

• |rheumatoid arthritis|disease activity|conversion equation|threshold|esr|crp