Evaluation of IgM rheumatoid factors on rubella IgM antibody reactions in the direct immunofluoresecence and enzyme immunoassay tests
IgM rheumatoid factors (RF) have the potential to cause false-positive reactions in the indirect immunofluorescence (IIF) and enzyme immunoassay (EIA) tests for rubella IgM antibody. Sixty laboratories were selected from the Centers for Disease Control (CDC) Proficiency Testing Program to participate ina speci al study whi ch eval uated thi s potenti al.
Eight of ten serum pools were prepared by combining preassayed rubella IgG antibody positive sera with IgM RF positive sera. The activity of the pools were adjusted to desired concentrations by addi ng serum with a high or low rubella IgG antibody titer, serum positive for IgM RF, or serum nonreactive for both rubella antibody and IgM RF. The pools were determined to have no detectable rubella IgM antibody uSing hemagglutinationiii inhibition (HAl) in conjunction with ion exchange chromatography. Of the other two pool s, one was negat i ve for both rubella antibodies and RF, and the other was prepared from serum taken from a susceptible donor four weeks after vaccination with rubella vaccine and was positive in the IgM fraction for rubella antibody.
Thirty of the 60 laboratories were selected to quantitate rubella (lgM and/or IgG) antibody using IIF, EIA, or HAL The remaining 30 laboratories were selected to quantitate RF. False-positive reactions in IIF tests for rubella IgM antibody were found to occur in two basic conditions: 1. high concentrations of IgM RF with relatively low concentrations of rubella IgG antibody; and 2. hi,gh concentrations of rubella IgG antibody with relatively low concentrations of IgM RF. A more detailed study would be required to determine the optimal relative concentrations of rubella IgG antibody and IgM RF requi red to cause false-positive reactions in the rubella IIF test. There were no false-positive rubella IgM antibody results reported using the EIA test. However, additional investigation including a larger number of laboratories would be necessary to determine if IgM RF can cause false-positive reactions in the EIA test for rubella IgM antibody.