RUBELLA ANTIBODIES - serum

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Specimen: 5-10 mL blood in plain tube. An acute phase sample should be taken within a few days of onset of the rash; convalescent (after 10-14 days) samples may also be required for interpretation. Cord blood, if congenital rubella suspected.

Method: HAI, EIA.

Application: To support a clinical diagnosis of rubella (German measles); determination of immune status, esp before or during pregnancy and after vaccination; diagnosis of congenital infection.

Interpretation: IgM antibody becomes positive approximately 7 days after onset of the illness but will become undetectable after 8 weeks. A four-fold rise in IgG titre between acute and convalescent samples indicates recent infection. IgM antibodies in maternal serum indicate high fetal risk, particularly in the first and second trimester. Detection of IgM antibody in cord blood suggests congenital infection, but false positives can occur. False negatives can also occur, particularly if rubella has occurred late in pregnancy, as the fetus has had insufficient time to mount an immune response. In this circumstance, serial IgG titres may be helpful as maternal IgG will clear between 6-12 months.

Reference: Chernesky MA and Mahony JB. In: Murray PR et al eds. Manual of Clinical Microbiology. 6th ed. ASM Press 1995.

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