Comparative study of different enzyme immunoassays for measurement of IgM and IgA rheumatoid factors

Objective: To compare the value of various IgM and IgA rheumatoid factor (RF) tests for the diagnosis of rheumatoid arthritis (RA). Methods: Firstly, the latex test, one global assay (for IgM, IgA, and IgG RF), six IgM, and four IgA RF assays were compared in a particularly challenging situation—that is, with 67 patients with RA, many of whom were latex negative, and 91 non-RA controls, many of whom were latex positive. More detailed evaluation followed with three IgM RF tests (two commercially available kits and one assay developed in our laboratory) and two IgA RF tests (one commercially available and one from our laboratory) in two more representative samples of rheumatological patients (146 RA and 75 non-RA controls). Results: Diagnostic performance differed considerably between the assays. For IgM RF detection the highest sensitivity (88%) was obtained with the Diamedix kit (specificity 67%) and for IgA RF with the Inova kit (sensitivity 65%, specificity 88%). Combining one IgM and one IgA RF test improved diagnostic performance when both tests were in agreement, but at the cost of yielding 15–27% of discrepant results which did not help in ruling RA in or out. Mean concentration values differed significantly among IgM RF tests, and in most cases concentrations were not correlated. Conclusions: Available tests for IgM RF isotype vary in accuracy, and none is uniformly better than all the others. For IgA RF isotype, the Inova kit appears to be the best. Quantitative results cannot be compared across tests. Combination of one IgM and one IgA RF test may improve diagnostic accuracy.