Diagnostic Performance of Tests for Anti-cyclic Citrullinated Peptides and Rheumatoid Factor in Rheumatoid Arthritis

Abstract

Background: To compare the use of anti-cyclic citrullinated peptide antibodies (anti-CCPs) and rheumatoid factor (RF) in the diagnosis of rheumatoid arthritis (RA). Methods: Serum concentrations of anti-CCPs and RF were measured in 244 cases, including 39 patients with RA and 205 patients with other rheumatic diseases (non-RA). The age, sex, initial presentation, RF, anti-CCP results and the final diagnoses were recorded and analyzed. The sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), positive likelihood ratio (LR+), and negative likelihood ratio (LR-) were calculated. Results: Among all 244 patients, 39 (15.2%) were diagnosed with RA and 205 (84.8%) were diagnosed with other rheumatic diseases (non-RA). Among the RA patients, 26 (67%) tested positive for RF, and 31 (79%) tested positive for anti-CCPs. RF was positive in 2 of 8 (25%) anti-CCP-negative patients with RA, and anti-CCP was positive in 7 of 13 (53.8%) RF-negative patients with RA. In the diagnosis of RA, the sensitivity, specificity, PPV, NPV, LR+ and LR- of the RF test were 67%, 79%, 37%, 93%, 3.11, and 0.42, respectively. The corresponding data for the anti-CCP test were 79%, 98%, 86%, 96%, 32.59, and 0.21, respectively. The presence of either anti-CCPs or RF increased the sensitivity to 84.6%, and when they both were present, the specificity increased to 98.1%. Conclusion: The RF and anti-CCP tests are complementary in the diagnosis of RA, and positivity in both tests improves the detection of RA. The results of the present study show that anti-CCPs had better sensitivity (79% vs. 67%) and better PPV (86% vs. 37%) than RF for the diagnosis of RA in our population. However, these two tests had similar NPVs (96% vs. 93%). In cases presenting with clinical symptoms or signs of RA but negative RF results, screening for anti-CCPs is still warranted.