What is the ability of anti-cyclic citrullinated peptide antibodies determination in synovial fluid in discriminating rheumatoid arthritis from non-rheumatoid arthritis patients? A Tunisian cross-sectional study.

Anti-cyclic citrullinated peptide antibodies (ACPA) seem to be produced locally at the site of joints inflammation in the first stage of rheumatoid arthritis (RA). A strong correlation between serum ACPA and ACPA in the synovial fluid (SF-ACPA) is now suggested. A case-control study was conducted to evaluate the usefulness of ACPA determination in SF of patients with RA. A total of 53 patients with a knee-joint effusion (26 RA, 18 peripheral spondyloarthropathies (SPA), and 9 osteoarthritis (OA)) were included in our study. SF samples were obtained by performing therapeutic arthrosynthesis. IgG serum ACPA and SF-ACPA levels were determined by the enzyme-linked immunosorbent assay (ELISA). We have also determined IgG levels in serum and SF by nephelometry. Higher levels of IgG ACPA antibodies in SF (p = 0.045) and serum (p = 0.045) were found in patients with RA with respect to SPA and OA patients. The Spearman correlation analysis showed a significant and positive correlation between ACPA in serum and SF (rho = 0.516; p = 0.007) not only in the RA group but also in patients with SPA. Serum ACPA discriminated RA from non-RA at a cut-off value of 2.7 U/ml (sensitivity, 69%; specificity, 78%; and area under the curve (AUC), 0.72), whereas SF-ACPA discriminated RA from non-RA at a higher cut-off value of 4.95 U/ml (sensitivity, 73%; specificity, 61%; and AUC, 0.71). Our study suggests that the determination of SF-ACPA give complement information to serum ACPA in patients with RA.