Comparison of radial haemolysis and a commercial latex agglutination test (Rubalex) for the detection of rubella-specific antibody

Abstract

A total of 686 sera were tested by radial haemolysis (RH) and a commercial latex agglutination (LA) test (Rubalex) for rubella antibody. Of 624 sera which were either clearly positive (> 15 international units (iu) ml −1) or negative by RH, 616 (99%) gave a a concordant result by LA. One serum was negative by LA although having > 15 iu ml −1 rubella antibody by RH. Of five sera negative by RH but positive by LA, one contained specific IgM, suggesting recent primary rubella, and the others had rubella antibody detectable by enzyme linked immunosorbent assay (EIA) and/or haemagglutination inhibition (HI). The remaining two sera were equivocal by LA; one, with an HI titre of > 5120, being clearly positive at 1 100 dilution indicating a prozone effect. Eighteen sera which had low concentrations of rubella antibody (< 15 iu ml −1) by RH gave positive or equivocal results by LA. Forty-four sera were tested which had given equal zones on RH test and control plates. After absorption with sheep red blood cells, 31 were RH positive and eight RH negative; five still gave a zone of haemolysis on control plates. For the latter five the LA result agreed with the result obtained by testing by EIA. Two of the remaining 39 gave a discordant result with LA, RH > 15 iu ml −1 after absorption but LA negative; one was positive and one negative by EIA. Testing of a dilution series of the British Standard serum for rubella-specific antibody demonstrated that LA could detect rubella antibody down to a concentration of 5 iu ml −1. Thus LA is a sensitive and specific test for rubella antibody although occasional sera may give equivocal results and require testing by an alternative assay.