Diagnostic Accuracy of the Anti-Citrulline Antibody Assay for Rheumatoid Arthritis

Abstract

Background: Rheumatoid arthritis (RA) is the most common autoimmune rheumatic disease, but specific and practicable tests for its diagnosis are lacking. We evaluated the diagnostic accuracy of a new commercial ELISA in detecting anti-cyclic citrullinated peptide (CCP) antibodies for the diagnosis of RA. Methods: Anti-CCP antibodies were determined in 330 serum samples: 98 from RA patients and 232 from controls, including patients with connective tissue diseases, other rheumatic diseases, viral infections, Lyme disease, autoimmune thyroiditis, cancer, and monoclonal gammopathy, and sex- and age-matched healthy subjects. Intra- and interassay CVs were 5–13% and 9–17%, respectively. Rheumatoid factor (RF) was also assayed in every sample, and results were compared to anti-CCP for sensitivity and specificity. Results: At a cutoff value of 50 units, sensitivity was 41% (confidence interval, 31–50%) and specificity was 97.8% (95–100%). Anti-CCP-positive RA patients had a mean antibody concentration of 1100 units (range, 57–3419 units), and anti-CCP-negative RA patients and controls had mean values of 7.6 and 6.8 units, respectively (range, 1–39 units). The area under the ROC curve was 0.71 (95% confidence interval, 0.63–0.78). RF had a higher sensitivity (62%) and a lower specificity (84%) than anti-CCP. When the two antibodies were used together, specificity was 99.6%. Conclusion: Anti-CCP antibody testing may be useful if performed concomitantly with RF assay to diagnose patients with suspected early RA.