A sensitive radioimmunoassay for quantitation of IgM rheumatoid factor

A solid-phase radioimmunoassay capable of detecting nanogram quantities of human IgM rheumatoid factor (RF) in biologic fluids has been developed. Binding curves for monoclonal IgM RF and polyclonal IgM rheumatoid factors were similar under the conditions utilized for the assay. Human IgG did not interfere with the detection of IgM RF by this method. Small quantities (less than or equal to 0.2%) of nonspecific binding by nonRF IgM to the human IgG coated tubes utilized in the assay were corrected for by assaying samples in parallel bovine serum albumin coated control tubes. As expected, patients with seropositive rheumatoid arthritis (RA) had significantly higher concentrations of IgM RF than seronegative RA patients (mean +/- 1 SD = 652 +/- 553 microgram/ml versus 11.3 +/- 13.3 microgram/ml, P less than 0.001). In contrast, all normal control sera assayed to date contained less than 0.1 microgram/ml of IgM RF. The capacity of the assay to detect nanogram quantities of IgM RF should permit investigation of the cellular mechanisms underlying RF production.