Rheumatoid factor in acute viral infections: interference with determination of IgM, IgG, and IgA antibodies in an enzyme immunoassay.

IgM rheumatoid factor (RF), an autoantibody to the Fc fragment of IgG, was determined by solid-phase enzyme immunoassay (EIA). RF levels were significantly higher in patients with rubella virus infection than in patients with infections due to influenza virus, cytomegalovirus, respiratory syncytial virus, parainfluenza virus, adenovirus, mumps virus, or herpes simplex virus. To evaluate the role of RF in EIA determinations of viral antibodies, IgM RF from IgM-IgG cryoglobulin or control IgM was added to patient sera before assay for viral antibodies. IgM RF inhibited virus-specific IgG and IgA reactions and gave nonspecific IgM reactions in EIA for antibodies to rubella and influenza viruses, but had little or no effect on antibodies to cytomegalovirus. The minimal effective amounts of RF were 100-500 ng/ml for inhibition of IgG, 300-1,000 ng/ml for IgA, and 25-500 ng/ml for IgM. The control IgM preparation gave no such effects. These studies reinforce the need to eliminate RF interference in solid-phase EIA.