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Combining two assays for rheumatoid factor (RF) may improve the diagnosis of rheumatoid arthritis (RA), according to the first study to measure performance of commercial test kits for IgM or IgA RF.
Three IgM kits and two IgA kits with superior sensitivity, specificity, and agreement in an initial screening failed to outperform each other when tested further.
By combining one each of the three IgM tests with the best IgA test in tandem, diagnostic performance improved for test results that agreed but was not useful for differing results, which occurred at a level of 15–27%. Each combination satisfied only one criterion: overall balance, confirmation of RA, or minimising discrepant results.
Twelve assays were evaluated initially for their sensitivity, specificity, and percentage likelihood agreement in measuring RF in 62 patients with RA and 91 healthy controls. The conditions were challenging: many patients had a negative latex test result and many controls had a positive result. The latex test was compared with a global ELISA (for IgA, IgM, IgG RF) and six IgM and four IgA assays. Two IgM, and two IgA assays were devised by the researchers. The best performers were tested further in 146 patients with a more typical RF profile and 75 controls. Three were IgM tests (two commercial), two were IgA tests (one commercial), and one was a global ELISA.
Numerous assays, based on different primary antibodies, are available for testing for RF, but published results reporting 30–90% of RA patients positive for RF illustrate their inaccuracy.
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