COMPARISON OF RUBELLA IMMUNOASSAYS In the past, the recommended method for deter- mining immune status to rubella was the hemagglu- tination inhibition (HI) test. For the most part, HI has been replaced with less cumbersome methods, including passive hemagglutination, 1-5 latex agglu- tination, 1-13 fluorescenceimmunoassay, 1--4,6,14,15 enzyme immunoassay, 1,2,4,5,10, 1,15--18 and radial hemolysis in gel. 4’13 These methods have been shown to be as accurate as HI and in some cases more sensitive, specific, and reproducible, s’8’9’11 Recently, a fully automated IMx immunoassay analyzer was developed for the detection of IgG and IgM antibodies to rubella virus. 16,19 A fully automated system provides for a more objective result, decreases the technical time and cost re- quired to perform the assay, offers standardization of the test procedure, and lends itself to physician office testing. The purpose of this study was to compare several commercially available methods for the determination of rubella immunity, includ- ing the fully automated IMx immunoassay ana- lyzer. MATERIALS AND METHODS Specimens A total of 190 serum specimens submitted to the clinical microbiology laboratory at Harrisburg Hospital and Polyclinic Medical Center were tested by 5 methods. The serum samples were obtained from specimens submitted for rubella serology. Sera were tested on receipt in the laboratory by fluores- cence immunoassay (FIAX, Whittaker Bioprod- ucts, Waldersville, MD) and latex agglutination (Rubascan, Becton Dickinson Microbiology Sys- tems, Cockeysville, MD) and then were stored at -30C until subsequent testing. The stored sera remained frozen for 6 months and then were tested using the remainder of the assays. IMx The IMx (Abbott Diagnostic Laboratories, North Chicago, IL) is an automated procedure based on microparticle enzyme immunoassay technology. Once the serum sample is placed into the reaction cell of the carousel, all additional steps are per- formed by the instrument. The principle and oper- ation of the IMx have been described previ- ously6,18 and are reviewed briefly here. Once controls and samples have been loaded onto the carousel and the instrument started, the probe/ SAUTTER ET AL. electrode assembly delivers the sample and diluent buffer to the predilution well of the reaction cell. Next, the microparticles coated with rubella virus and an aliquot of the diluted sample are added to the incubation well. If antibodies to rubella are present in the patient’s serum, they will bind to the antigen- coated microparticle forming an antigen-antibody complex. Diluent buffer is added to the reaction mixture, and an aliquot of the antigen-antibody complex is added to the glass fiber matrix. The microparticles bind irreversibly to the glass fiber matrix. The matrix is washed to remove unbound material, and an alkaline phosphatase-conjugated anti-human IgG is dispensed onto the matrix and binds to the antigen-antibody complex. The matrix is then washed again to remove unbound material. The substrate 4-methylumbelliferyl phosphate is then added to the matrix, and the fluorescent prod- uct formed is measured by the optical assembly of the instrument. Those specimens that exhibited val- ues > 10 IU of IgG antibody to rubella virus were considered immune. At the beginning of the study, 6 calibrators were run to establish a calibration curve. Once established, this calibration curve has been shown to be stable for at least 2 weeks. Posi- tive and negative controls were included in each run. FlAX Fluorescence immunoassay was performed using Rubella G kits supplied by the manufacturer. The method was performed according to the manufac- turer’s instructions and has been described previ- ously. s A result lower than 8 indicated susceptibil- ity to rubella and a result greater than 12 indicated immunity. An equivocal zone of 8-12 has been established by the manufacturer to avoid false posi- tive readings, and serum specimens in this range were retested. Latex Agglutination Latex agglutination was performed using the Rubascan and Rubalex (Wellcome Diagnostics, Re- search Triangle Park, NC). Both procedures were tested according to the manufacturers’ instructions and have been described previously. 2’6 Serum spec- imens tested with the Rubalex method were diluted 1:10 prior to testing. With the Rubascan method, serum samples were tested undiluted and diluted 1:10 as recommended by the manufacturer. INFECTIOUS DISEASES IN OBSTETRICS AND GYNECOLOGY JOe)