ANTIBODIES AGAINST CITRULLINATED PROTEINS IN PATIENTS WITH EARLY RHEUMATOID ARTHRITIS: RELEVANCE OF LIKELIHOOD RATIOS

Background: Antibodies against citrullinated proteins (ACPA) and rheumatoid factor (RF) are part of the classification criteria for the diagnosis of rheumatoid arthritis (RA) and are useful in the diagnosis and prognosis of RA. Objective: To measure serum ACPA and RF levels in patients with early RA (up to 12 months)—including very early RA (<3 months)—who had not received treatment with disease-modifying antirheumatic drugs. Ninety patients with other autoimmune rheumatic diseases served as controls. Materials and Methods: There were 269 patients (226 women and 43 men) with a median age of 50 years seen at our hospital between 2012 and 2022. ACPA levels were measured using chemiluminescence (cut-off >17 U/mL) and RF levels were primarily measured using latex agglutination tests (cut-off ≥160). Sensitivity, specificity, likelihood ratios (LRs) and CI95% were calculated. Results: 215 (80%) patients tested positive for ACPA; specificity was 93% and the positive LR was 11.9 (excellent clinical utility). The sensitivity of the ACPA+RF combination was 43%, with 93% specificity and an LR of 6.3 (good clinical utility). RF sensitivity and specificity were 62% and 67%, respectively, with a positive LR of 1.8 (poor clinical utility). Among the 90 controls, 6 (7%) tested positive for both ACPA and RF ("rupus"), and 33 tested positive for RF. Conclusion: ACPA levels in early RA demonstrated high sensitivity, specificity, and LR >10. The combination of ACPA and RF offers good clinical utility compared to testing for RF alone.