Anti-citrullinated peptide antibodies and rheumatoid factor isotypes in the diagnosis of rheumatoid arthritis: an assessment of combined tests.

ACPA (anti-citrullinated protein antibody) tests are today systematically added to clinical and radiological investigations when diagnosing rheumatoid arthritis (RA), and the inclusion of ACPA positivity in the new 2010 RA criteria underlines their importance. The aim of this study was to determine the sensitivity and specificity of different ACPA assays and IgA, IgG and IgM isotypes of rheumatoid factor (RF) in a cohort of patients with early RA in order to assess the value of combining the tests. The serum samples were obtained from 46 RA patients, 80 patients with systemic rheumatic disease, and 20 blood donors. ACPAs were measured using five different commercial kits. The receiver operating characteristic (ROC) curves of the anti-ACPA tests had area under the curve (AUC) values of 0.60-0.83. The diagnostic accuracy of the Bio-Rad multiplex flow immunoassay, a new technology for ACPA testing, was very similar to that of the other widely used commercial immunoassays. The EliA CCP-Phadia test was the most specific, and had the best positive likelihood ratio and positive predictive values, whereas the anti-CCP Inova 3.1 test was the most sensitive, and had the best negative likelihood ratio and negative predictive values. The best combination to use for early RA screening was an ACPA test together with IgM and IgA RF.