Rubella‐continuing problems

Despite the availability of safe and effective attenuated rubellavaccines and sensitive laboratory tests for rubella antibody screening and diagnosis, most clinical virologists continue to rcceive a large number of queries relating to maternal rubella from their obstetric and general practitioner collcagues. Perhaps thc most frequent query relates to the interpretation of conflicting results of rubella antibody screening tests carried out in antenatal clinics. Thus, onc report may state that the patient is immune, obviating the necessity of postpartum vaccination, whereas a second test in a subsequent pregnancy may report that the patient is susceptible. This confuses obstetricians, creates anxiety among patients and casts doubt on the credibility of laboratory tests. The test which is most generally used for rubella antibody screening in the UK is the single radial haemolysis test (SRH); it is cheap, simple and reliable and, unlike the haemagglutination inhibition test (HAl), which it has largely replaced. does not give false-positive results. However, false-negative results occur due to the presence of ‘blocking factors’ in 1-3% of sera. What concentration of antibody should be acceptable as indicative of rubella immunity? Although it is probable that any concentration of naturally acquired antibody is protective, in that it will prevent viraemia with its attendant risk of fetal infection, this may not necessarily apply to vaccine-induced antibody (O’Shea el ul. 1983). The presence of relatively low concentrations of rubella antibody is usually responsible for contradictory reports in successive tests. Most laboratoriesaccept alcvelof 1Si.u. imlas protective although this level is arbitrary and perhaps conservative. There is no difficulty in interpreting reports on patients whose sera have concentrations in excess of 15 i.u. /ml (immune); following naturally acquired infection, such levels are likely to persist for periods of at least 18 years, and usually for very much longer, even for life. However, following rubella immunization, about 10% of vaccinces have concentrations of rubella antibody which decline to <15 i.u. /ml over a 16-18 year period (O’Shea rt nl. 1982. 1984; Zealley & Edmond 1982). Interpreting reports on sera obtained from patients without detectable rubella antibody who are clearly rubella-susceptible and should bc offered rubella vaccination postpartum is also straightforward. Problenis tend t o arise when sera have antibody concentrations just at, or below, the 15 i.u. /ml ‘cut-off’, since the result may be reported as immune on one, and susceptible on another occasion. This phenomenon may be due either to the concentration of antibody declining between successive tests, or variation in the sensitivity of the test procedure itself. Confusion could, to a large extent, be overcome if laboratory workers ensured that results were not merely reported as ‘>15 i.u. /ml immune’ or ‘<1S i.u. /ml susceptible‘ but were qualified by including appropriate standardized statements in the following terms (Pattison 1982): (1) SRH->15 i.u. /ml-rubella antibodies present. Immune. Vaccination not required. (2) SRH-no antibodies detected. Susceptible. Vaccination recommended. (3) SKII-<15 i.u. /ml-low level of rubella antibody detected. Regard as susceptible. Vaccination recommended. Although all obstetric units have access to a rubella antibody screening service, they must strive to improve delivery of rubella vaccine to rubella-susceptible patients postpartum. Thus, if details of rubella immunity are not displayed prominently in the patient’s notes, identification of those who should be offered rubella vaccine may be poor. Some patients may be discharged from hospital rapidly and their general practitioners may fail to offer rubella vaccination at home. Rubella-susceptible patients who miscarry or have ectopic pregnancies may be admit-