Quantitative in vitro assay for human IgM rheumatoid factors (autoantibodies of the IgM class against human IgG) in serum or plasma. This method calculates concentration of antibodies in the serum samples using point-to-point plotting of the extinction values measured for the 3 calibrators (standards) against the corresponding units (linear/linear). Samples greater than the first calibrator are marked as “> Standard1”.