Using a ROC curve analysis of data from a large observational study of patients with rheumatoid arthritis in Japan, we estimated DAS28‐CRP threshold values corresponding to DAS28‐ESR remission, low disease activity and high disease activity to be 2.3, 2.7 and 4.1, respectively. As each of these threshold values is lower than the corresponding DAS28‐ESR threshold values, disease activity in patients with rheumatoid arthritis may be underestimated if the DAS28‐CRP is used, potentially diminishing or eliminating the capacity of therapeutic strategies designed to tightly control disease activity. Therefore, caution must be exercised when using the DAS28‐CRP.
Considering the strong linear relationship between DAS28‐ESR and DAS28‐CRP values (correlation coefficient 0.946), we suggest that the DAS28‐CRP can be used as an alternative to the DAS28‐ESR and would be useful in situations in which only CRP data are available. CRP measurements are widely used in the routine evaluation of several inflammatory diseases, further supporting the notion that utilisation of the DAS28‐CRP may be beneficial in clinical practice of rheumatology. However, because our cohort consists of Japanese patients with rheumatoid arthritis, multicultural comparisons are required to determine whether the DAS28‐CRP can be used as an alternative to the DAS28‐ESR in people of other ethnic groups.
From the residuals distribution and sensitivity/specificity derived from ROC curves, we found a relatively large fluctuation in the correlation of lower DAS28‐ESR and DAS28‐CRP values. This may have been caused by the difference in the distribution profile of DAS28‐ESR and DAS28‐CRP scores, as the DAS28‐CRP exhibited a slightly skewed distribution toward the left compared with the DAS28‐ESR. Consequently, the sensitivity and specificity of remission and low disease activity criteria were lower than those of high disease activity. Inconsistency of DAS28‐ESR remission criteria and of Pinals's criteria have been noted elsewhere,10
suggesting that the DAS28 system itself may have some inherent uncertainty when lower values are considered. When the proposed threshold values for the DAS28‐CRP were used in this study, the frequency of misclassification between the DAS28‐ESR and the DAS28‐CRP was 6.9% in patients with high disease activity and 11.2% in those in remission. Hence, when making clinical decisions based on remission or low disease activity criteria using the DAS28‐CRP in the place of the DAS28‐ESR, we should consider this divergence between these scoring systems.
In conclusion, we have evaluated the threshold values of the DAS28‐CRP that correspond to DAS28‐ESR values for remission, low disease activity and high disease activity criteria (2.3, 2.7 and 4.1, respectively) in Japanese patients with rheumatoid arthritis. Further study is required to determine the applicability of this scoring system in people of other ethnic groups and in those who fulfil remission or low disease activity (rather than high disease activity) criteria.