J Clin Pathol 1981;34:1392-1395 Rubella-specific IgM detected by an antibody capture assay/ELISA technique

SUMMARY A sensitive method for detecting rubella IgM antibody, the first step of which is the attachment of the serum IgM to a solid phase, is described. Specific IgM antibody was found in all 52 people with acquired rubella in the early convalescent period, in all 38 infants with congenital rubella examined in the first seven months of life, and in 23 of 26 people immunised one to four months before testing. Twenty-four of 44 rheumatoid factor-positive sera, however, gave false-positive readings. In routine use the test was economical of both reagents and time. Detection of rubella IgM antibody is well established as a means of diagnosing recent infection in pregnant women or congenital infection in neonates. Most methods rely on a primary step to separate physically the IgM from the other serum immuno-globulins, for example, sucrose gradient centri-fugation or gel filtration. ' Unfortunately, such methods of separating immunoglobulins lead to some loss, thus reducing the sensitivity of the test. More recently, radioimmunoassay and enzyme linked immunosorbant assay (ELISA) tests in which rubella antigen is bound to a solid phase have been used,2 but when whole serum is added to a solid phase antigen, competition between antibodies of the various classes may cause false-negative results. In 1979 Duermeyer et a13 described a technique, the antibody capture assay, in which human IgM is selectively attached to the solid phase by anti-human IgM (anti-u chain) antibody. When antigen is then added it will bind to any of its antibody present in the IgM fraction of the serum being tested. Finally, the presence of the bound antigen is detected by antibody, labelled with an enzyme or radioisotope. We report here the antibody capture assay using an enzyme linked indicator system for detecting rubella IgM antibody. Material and method