The Value of Rheumatoid Arthritis Citrullinated Peptides (RACP) System in the Diagnosis of Rheumatoid Arthritis: A Retrospective Analysis of 2632 Patients

ABSTRACT Objectives and Aim This study aimed to evaluate the diagnostic accuracy and prognostic value of the Rheumatoid Arthritis Citrullinated Peptides (RACP) assay in a real‐world cohort, assessing its performance both independently and in combination with established biomarkers such as anti‐cyclic citrullinated peptide (anti‐CCP), rheumatoid factor (RF), and anti‐keratin antibodies (AKA). Methods A single‐center retrospective analysis included 2632 patients who underwent serological testing between September 2022 and September 2023. Of these, 644 patients met ACR/EULAR classification criteria for rheumatoid arthritis (RA), while 1988 patients served as non‐RA controls, including an osteoarthritis (OA) subgroup. RACP, anti‐CCP, RF, and AKA levels were measured by enzyme‐linked immunosorbent assay (ELISA). Diagnostic sensitivity, specificity, and combination strategies were assessed using chi‐square and t ‐tests. Results RACP alone demonstrated a sensitivity of 76.86% and a specificity of 89.94%, closely comparable to anti‐CCP (70.87% sensitivity and 89.92% specificity). Area under the curve (AUC) values were 0.805 for RACP, 0.783 for RF, 0.799 for anti‐CCP, and 0.623 for AKA. Combining RACP with RF improved sensitivity (85.36%), while dual positivity (RACP + AKA) optimized specificity (96.61%). In the OA subgroup, biomarker positivity remained minimal, confirming strong discriminatory capacity. Patients positive for RACP or anti‐CCP exhibited significantly higher tender joint counts, correlating biomarker positivity with clinical disease severity. Conclusion RACP is a robust biomarker for RA diagnosis, offering sensitivity and specificity comparable to or exceeding traditional assays. Combinations with RF or anti‐CCP enhance diagnostic accuracy, supporting its utility in early RA detection, differential diagnosis from OA, and potentially reflecting disease activity. Prospective studies are warranted to confirm these findings.