Role of Rheumatoid Factor Isotypes and Anti-citrullinated Peptide Antibodies in the Differential Diagnosis of Non-selected Patients with Inflammatory Arthralgia.

Rheumatoid arthritis (RA) is a chronic inflammatory disease characterized by swelling, tenderness and destruction of synovial joints, leading to severe disability and premature mortality. The aim of the study was to determine the diagnostic accuracy of the 3 isotypes of rheumatoid factor (RF), anti-citrullinated peptide antibodies (ACPA) and the combination of both, for the diagnosis of rheumatoid arthritis (RA) in non-selected patients with inflammatory arthralgia.We include 129 patients with inflammatory Arthalgia from a third level reference Center of rheumatic diseases in Monterrey, México. Their samples were analyzed for RF isotypes (IgA, IgG, and IgM) by ELISA (EUROINMUN), using a cut-off of 20IU/ml, and for ACPA's 5IU/ml; a medical examination was performed to obtain the definitive diagnoses of the patients. Data analysis was carried out using ROC curves for the measurement of sensitivity, specificity, for diagnostic accuracy to verify if the use of 3 RF isotypes and ACPA had a better prediction for the diagnosis of RA than use only one isotype and the ACPA alone.The ROC showed a sensitivity and specificity of the different antibodies with different cut-off points, being the best for the IgM with 0.802 followed by ACPA, IgA and IgG with 0.771, 0.63, and 0.728 respectively without statistical difference, the sensitivity and specificity of the combination of the 4 antibodies were 81.4 and 73.66%.In non-selected patients with inflammatory arthralgia, the combination of ACPA and isotypes of RF did not demonstrate more sensibility and specificity than IgM isoform of rheumatoid factor measurement only. We recommend that in the clinical scenario of arthralgia, where the diagnoses are Lupus, Sjogren syndrome and Osteoarthritis, RF IgM isoform is used followed by ACPA.