Rubella antibodies detected by several commercial immunoassays in hemagglutination inhibition-negative sera.

Although a very good correlation was found between the level of rubella antibodies measured by a standard hemagglutination inhibition (HI) test and by an enzyme-linked immunosorbent assay (ELISA) procedure (Cordia R), an appreciable proportion (31%) of ELISA-positive specimens were encountered among HI-negative sera. The reverse was rarely seen. Many of the HI-negative, ELISA-positive sera were also found to be positive for rubella antibodies by one or more other assay methods, including an immunofluorescence assay (IFA) procedure (FIAX), passive hemagglutination (PHA) (Rubacell and PHAST), latex agglutination (Rubascan), and a second ELISA procedure (Rubelisa). The specificity of all of the ELISA-positive HI-negative specimens was substantiated by absorption experiments. In these tests, the ELISA reactivities were blocked by rubella antigens, but not by a variety of tissue culture control antigens or by influenza virus grown on the same cell line. The findings indicate that many of the newer methods available for rubella antibody detection are more sensitive than HI for detecting low levels of rubella antibodies. Until more clinical information is available concerning the protective nature of these low levels of antibody, caution should be exercised in assessing the significance of these results.