Comparison of various serological methods and diagnostic kits for the detection of acute, recent, and previous rubella infection, vaccination, and congenital infections

Assessment of IgM enzyme immunoassay and IgG avidity assay for distinguishing between primary and secondary immune response to rubella vaccine

The primary test for the laboratory confirmation of rubella is IgM serology. It is important to distinguish IgM reactivity caused by primary infection from that caused by reinfection or persistence, especially in pregnant women; as termination of pregnancy is considered when primary rubella is diagnosed during the first trimester. In this study, the performance of rubella IgM enzyme immunoassay (IgM-EIA) and rubella IgG avidity assay were compared using well-defined panels of sera from persons vaccinated against rubella and commercial rubella IgM and IgG enzyme immunoassay kits (Dade Behring, Marburg, Germany). The sensitivity and specificity of rubella IgM-EIA were found to be 77.4 and 97.9%, respectively, while the results for rubella IgG avidity assay were 100 and 100%. IgG avidity assay showed higher positive and negative predictive values than the IgM-EIA (100 and 100% compare to 96.9 and 82.9%). In conclusion, the rubella IgG avidity assay is more sensitive and specific than IgM-EIA for differential detection of primary rubella infection from rubella reinfection.

Evaluation of rubella IgM enzyme immunoassays

BACKGROUND

Rubella virus generally causes a mild fever, rash illness similar in clinical presentation to infections by other viruses including measles and parvovirus B19. Rubella infections in pregnant women in the first trimester carry a high risk of congenital rubella syndrome (CRS) which can result in severe congenital defects in the infants. The goal of rubella immunization programs is therefore to eliminate CRS. The primary test for the laboratory confirmation of rubella is IgM serology. It is therefore important to evaluate currently available commercial rubella IgM immunoassays to ensure high quality rubella diagnostic testing.

STUDY DESIGN

In this study, we compared the performance of seven commercial rubella IgM enzyme immunoassays (EIA) (Meddens, Denka Seiken, Behring, Wampole, Captia, Sigma and Abbott Axsym) using well-defined panels of sera from rubella and non-rubella/rash-illness cases.

RESULTS

The Meddens, Denka Seiken, Behring and Wampole rubella IgM EIAs all performed similarly for sensitivity (range of 74.1-76.8%) and specificity (range of 93.9-96.1%). Relative to the other assays, the Axsym had a higher sensitivity (78.9%) but lower specificity (86.5%). The Captia assay had the lowest overall sensitivity (66.4%), while the Sigma assay had a lower specificity (85.6%) in relation to the other assays.

CONCLUSIONS

In conclusion, the Meddens, Denka Seiken, Behring and Wampole rubella IgM EIAs are comparable in their overall performance with respect to sensitivity and specificity.

Rubella vaccines: past, present and future

The association between rubella in pregnancy and congenital anomalies was first reported 50 years ago, by N. McAlister Gregg, an Australian ophthalmologist [1]. During the next 20 years his findings were confirmed by others (reviewed in [2]). However, the first reports of the isolation of rubella virus in cell cultures and development of tests for neutralizing antibodies were not published until 1962 [3, 4]. Subsequent studies conducted in the UK and North America during a pandemic of rubella in 1963–4, were therefore able to make a more accurate estimate of the risks of maternal rubella at different stages of pregnancy. It was estimated that about 30000 rubella-damaged babies were born in the USA alone in 1963–4 [5]. This emphasized the importance of developing a vaccine to prevent infection in pregnancy and thereby, the birth of babies with rubella-induced congenital defects.

A silver enhanced, gold labelled, immunosorbent assay for detecting antibodies to rubella virus

A silver enhanced, gold labelled, immunosorbent assay (SEGLISA) for the detection of IgG antibodies to the rubella virus in human serum was developed. Pre-coated microtitre wells are used as the immobilised base of rubella antigens on to which any rubella antibodies from patient samples will bind. This antigen/antibody complex is then visualised firstly by gold labelled anti-immunoglobulin G, which binds to any human IgG that may be present, and then by silver amplification, resulting in a black permanent deposit on the microtitre well surface. Patient samples (n = 121) were screened using a commercially available enzyme linked immunosorbent assay (ELISA) and an equivalent SEGLISA. Results were comparable but the SEGLISA does not have the disadvantages associated with enzyme labels. The silver deposit may also be read visually or the dried plate may be stored for future reference.

Development of automated immunoassays for immune status screening and serodiagnosis of rubella virus infection

The fully automated IMx immunoassay analyzer was used to develop a system for the detection of IgG and IgM antibodies to rubella virus for immune status screening and diagnosis of primary infections. Reagents and assay protocol software were developed using rubella virus sensitized microparticles as the solid phase to capture specific antibodies from serum samples. Anti-human IgG or IgM antibody coupled to alkaline phosphatase enzyme followed by methylumbelliferyl phosphate substrate was used to detect the presence or absence of antibodies specific to the antigens on the solid phase. To evaluate the efficacy of the IMx rubella IgG assay, immune status screening was performed with a clinical patient population of 501 sera. When compared to an IgG specific enzyme immunoassay and passive hemagglutination assay the agreement was greater than 99%. The IMx rubella IgM assay was utilized to determine the presence of rubella specific IgM antibodies in 462 sera. These results were compared to IgM specific enzyme immunoassay results and also demonstrated greater than 99% agreement. Seroconversion following rubella vaccination of susceptible individuals was demonstrated by IgG and IgM antibody responses as early as two weeks postvaccination. In addition to automation, the IMx system offers rapid assay times and calibration curve storage without sacrificing clinical efficacy.

Detection of rubella virus antigen by one-step time-resolved fluoroimmunoassay and by enzyme immunoassay

A one-step time-resolved fluoroimmunoassay (TR-FIA) and a conventional two-step enzyme immunoassay (EIA) for the detection of rubella virus antigen were developed. Two noncompetitive mouse monoclonal antibodies reactive with epitopes on the E1 polypeptide of rubella virus served as immunoreagents. One of the monoclones (7A6) was used for coating the solid phase, and the other (2C3) was labeled with either Europium chelate or with horseradish peroxidase. For TR-FIA, the specimen was incubated simultaneously with the label at 4 degrees C overnight. EIA required an overnight incubation with the specimen and after washing another 1 hr of incubation at 37 degrees C with the conjugate. The sensitivity of TR-FIA was 10 pg in an assay volume of 100 microliters, and the sensitivity of EIA was between 50 and 100 pg. Antigens could be detected by TR-FIA in supernatant of cultures of Vero cells 48 hr after inoculation with approximately 1 TCID50, while cytopathogenic effect (CPE) at that time was detected only in cultures inoculated with 10(5) TCID50 or more. Virus mixed with human amniotic fluid containing antirubella-specific IgG was detectable after an incubation at 37 degrees C for 5 days. The assays may find applications in prenatal diagnosis of intrauterine rubella infection, in early identification of viral antigens in cell culture and in monitoring production, concentration, and purification of rubella antigen for antibody assays.

Rubella IgG total antibody avidity and IgG subclass-specific antibody avidity assay and their role in the differentiation between primary rubella and rubella reinfection

The total rubella IgG antibody avidity ELISA, as described by Hedmann for differentiation between acute primary rubella and reinfection, persisting or non-specific IgM antibodies were evaluated in 212 sera of four categories of patients with known history (n = 149) and in one group (n = 39) of patients with a history of unclear nature. The development of the IgG ratio (avidity) from low (less than 30%) to higher levels (greater than 30%) was determined in 146 sera from 96 cases with acute primary rubella between days 1 to 256 after onset of exanthem, with a borderline between low (less than 30%) and high (greater than 30%) avidity after four weeks after onset of exanthem. From seven days to 14 years following vaccination, the IgG ratio increased more slowly following vaccination, with a useful borderline at 2 1/2 months. In 80 sera from 13 women with reinfection during pregnancy, six infants with rubella embryopathy, and 19 pregnant women with rubella IgM antibodies detected during antenatal screening, the total IgG ratio ranged between 48% and 100%, indicating a previous infection. The diagnostic value of the IgG subclass-specific antibody avidity assay was investigated in 40 of the 251 sera. The results confirmed those of the total IgG avidity assay. Both tests are of additional help for differentiating serologically primary rubella from reinfection or recent from previous vaccination.

Recent rubella virus infection indicated by a low avidity of specific IgG

Rubella-specific IgG in acute-phase sera produces a characteristically altered zone termed "soft hemolysis" in the radial hemolysis test. Here, the soft hemolysis was shown to be a product of the purified IgG1 subclass isolated from acute-phase sera. In contrast, ordinary hemolysis was produced by IgG1 isolated from sera of previous rubella immunity, indicating that the subclass composition of IgG was not involved in the mechanism of soft hemolysis. A novel type of solid-phase immunoassay was developed for the avidity of virus-specific IgG. Acute-phase IgG (with soft hemolysis) was dissociated from rubella antigen in an enzyme immunoassay (EIA) test by hydrogen-bond disrupting agents under conditions where IgG of previous immunity (showing ordinary hemolysis) remained mostly bound. These data suggest that the mechanism of soft hemolysis is the avidity of rubella-specific IgG. The new quantitative avidity EIA was tested with sera taken from 169 subjects. Recent infection could be shown from sera taken weeks or months after primary rubella.

Evaluation of a latex agglutination test for screening antibodies to rubella virus

The performance of a commercial latex agglutination test for screening rubella antibodies was investigated, using two panels of selected serum samples especially designed for a careful evaluation of sensitivity. Three false negative results were obtained on samples with very low levels of antibody. However, the test had higher sensitivity than the hemagglutination inhibition test and fluoroimmunoassay when samples were tested undiluted. False positive results were not obtained, and prozone reactions were not observed. The test is a very practical method for screening rubella antibodies in primary health care units.

Prenatal diagnosis of intrauterine rubella

The results of prenatal diagnosis of fetal rubella infection by specific IgM antibody detection in fetal infection by specific IgM antibody detection in fetal blood obtained in the 21st to 23rd weeks of pregnancy of women with different types of rubella problems during pregnancy (categorised in four groups) showed that in 28 of 31 cases the diagnosis apparently assisted in the correct management of the pregnancy. However, in two cases with negative IgM findings in the fetal blood, children with rubella embryopathy were born, and in one case with positive IgM antibody findings, the child was infected but healthy. From this experience we recommend that in all cases of symptomatic rubella infection in the first 12 weeks, termination of pregnancy should be considered without prenatal diagnosis. Prenatal diagnosis, if considered at all, should be limited to "symptomless" acute rubella infection and reinfection in the first 12 weeks and to symptomatic rubella infection in the 12th to 17th weeks of pregnancy.

Accuracy of single radial hemolysis test for rubella immunity when internal reference standards are used to estimate antibody levels

The accuracy and reproducibility of antibody levels obtained by single radial hemolysis with six internal reference sera were evaluated. The test was performed in a clinical laboratory for routine assessment of immunity to rubella infection over a period of 1 year. A linear relationship exists between the antibody titer (expressed in log dilution) and zone diameters. In 43 of 44 test runs the correlation coefficient of the standard curve was over 0.990. Prediction limits of 95% around the curve showed that on replication of the test, zone diameters could be found within less than half a doubling-dilution step. The antibody level can thus be determined more accurately by the single radial hemolysis test than in the conventional hemagglutination inhibition test. This is particularly important in assessing immunity when antibody titers are low, since the hemagglutination inhibition test is less reliable then. The use of standard sera calibrated international units would render results of different laboratories comparable and allow standardization at threshold values.

Detection of IgM antibodies against rubella virus: comparison of two indirect ELISAs and an anti-IgM capture immunoassay

A commercial antibody capture enzyme immunoassay (Rubenz M) was compared to two commercial indirect enzyme immunoassays (Enzygnost IgM, Rubazyme-M) for the detection of rubella-specific IgM. Five hundred and fifty-two sera collected between the day of onset and 272 days after the onset of the exanthem of primary rubella were tested. Rubenz M was more sensitive early and late after the onset of the exanthem than the two indirect ELISAs. Rubenz M also appeared more sensitive when 240 sera were examined from patients with possible rubella in pregnancy, reinfection in pregnancy, suspected intrauterine infection, and recent vaccination. However, 5.5% of 968 pregnant women with no history of rubellalike symptoms or recent vaccination, the majority with elevated HAI titers, gave a low-positive or borderline result with Rubenz M. None of these women delivered a congenitally infected child. Therefore, borderline and low-positive results must be interpreted with caution, as for any assay for rubella-specific IgM.

Detection of specific IgG and IgM antibodies in the haemagglutination inhibition test and the enzyme-linked immunoassay for the diagnosis of rubella infection

A study has been carried out on the sera of 710 women who wished to know their state of rubella immunity using haemagglutination inhibition (HAI) and enzyme-linked immunoassay (ELISA) techniques. The majority of the women presented no symptoms. The ability of HAI to detect low antibody levels (1:8, 1:16) appears to be greater than that of Rubazyme ELISA IgG, employing the recommendations of the manufacturers. The correlation between HAI and IgG values above HAI titres of 1:16 is nearly 100%. In an additional study of 17 primary infections in pregnant women with definite rubella symptoms, the total titre of antibodies was determined by HAI and IgG and specific IgM with ELISA Rubazyme in successive samples. In four cases, IgM was clearly positive and no increase in either HAI or IgG antibodies could be demonstrated over three successive samples taken at an interval of 15 to 20 days. Therefore, we consider it necessary to determine IgM antibodies (evaluating these in the absence of the rheumatoid factor) in every doubtful case occurring in pregnant women, irrespective of whether clinical signs are present or not.

Rubella embryopathy after previous maternal rubella vaccination

This report concerns a boy with congenital rubella infection and features of the classical and expanded rubella syndrome who was born to a mother who had been successfully vaccinated with Cendehill vaccine seven years previously. The diagnosis of rubella embryopathy was confirmed by demonstrating rubella-specific serum IgM antibodies using four different methods, by the persistence of rubella HAI and IgG antibodies in serum taken between three and eight months of age and by the isolation of rubella virus from throat secretion, urine and blood mononuclear cells. The child died at eight-and-a-half months of age. This case is discussed in relation to the persistence of vaccine-induced immunity with particular respect to the protective quality of low levels of antibodies against intrauterine infection in the event of re-infection during pregnancy.