CD40LG and rheumatoid arthritis: Several studies reported the presence of IgA-RF in approximately 50 % of RA patients compared to 60–70 % for IgM-RF and consistently showed higher specificity and positive likelihood ratios (PLR) of IgA-RF determined by ELISA as compared to the IgM isotype determined by nephelometry (largely measuring IgM) or IgM specific ELISA.