Rubella testing in pregnancy

• Testing is recommended for all cases of possible rubella by simultaneous testing for rubella-specific IgG (or total rubella antibody) and IgM.
• When reporting the results of rubella serology, the laboratory must advise on any further sera/follow-up required, and give a definitive conclusion of their investigations, eg "No evidence of recent primary rubella".
• If Ix begin >4/52 after the onset of the rash:
• Rubella-specific IgG (+), IgM (-), rubella as a cause of the rash illness cannot be excluded serologically unless seroconversion has investigated. An assessment of probabilities has to be made based on recent epidemiology of rubella in the community, past history of vaccine and testing, characteristics of illness, etc.
• Rubella-specific IgM (-). Although positive rubella IgM results which do not reflect recent rubella (primary or reinfection) ("false positive") are infrequent, the control of rubella in the UK means that most rubella-specific IgM positive results do not reflect recent rubella.
• No woman in the first 20 weeks of pregnancy should have rubella diagnosed on the basis of a positive rubella-specific IgM alone. Results must be interpreted in relation to full clinical and epidemiological information.
• Unless seroconversion has been shown, further testing by alternative rubella-specific IgM tests and measuring the strength of binding of specific IgG (avidity) is advised. IgG avidity is low soon after a primary infection, but matures over a few weeks to become more strongly binding. If rubella-specific IgM positivity reflects a recent rubella episode (whether primary or reinfection), the degree of reactivity will usually change over the period of a few weeks, rather than persisting at a similar level.

AMH