RA_CP is a double antibody sandwich enzyme-linked immunosorbent assay for the detection of rheumatoid arthritis related citrullinated proteins in human serum. It can specifically identify rheumatoid arthritis-specific citrullinated proteins in serum, independent of genetic background, with high sensitivity and high specificity. The use of RA_CP can improve the accuracy of clinical diagnosis of rheumatoid arthritis, reduce the incidence of clinical under diagnosis, and is of great significance for the early diagnosis of RA patients.