Diagnosis of Rubella Immunity in Women of Childbearing Age
Rubella immunity is diagnosed by laboratory confirmation of rubella-specific IgG antibody using a licensed serologic assay, which is the only reliable proof of immune status in women planning pregnancy. 1, 2
Primary Diagnostic Method
Serologic testing for rubella IgG antibody is the gold standard for determining immunity. 2 The most commonly used assays include:
- Enzyme immunoassays (EIAs) are the primary testing method 2
- Any antibody level above the standard positive cutoff value of a licensed assay confirms immunity 2
- Alternative methods include latex agglutination, immunofluorescence assay, passive hemagglutination, hemolysis-in-gel, and virus neutralization tests 2
What Constitutes Acceptable Evidence of Immunity
Women of childbearing age are considered immune if they have any of the following 3, 2:
- Laboratory evidence of rubella IgG antibody (most reliable) 2
- Documentation of at least one dose of MMR or rubella-containing vaccine administered on or after the first birthday 3, 2
- Birth before 1957 - however, this is NOT acceptable for women who could become pregnant 3, 2
Critical Pitfalls to Avoid
Self-reported vaccination history or parental reports are NOT adequate documentation and should never be relied upon for determining immunity. 2 This is a common error that can lead to false reassurance.
Birth before 1957 does NOT provide acceptable evidence of immunity for women of childbearing age, despite being acceptable for other populations. 3, 2 This exception exists because the consequences of rubella infection during pregnancy are so severe.
When to Screen vs. Vaccinate Directly
The decision between serologic screening versus direct vaccination depends on practical considerations:
- Direct vaccination with MMR without prior testing is preferred when serologic testing costs are high or when follow-up for vaccination cannot be reliably assured 2
- Serologic screening before vaccination should only be performed if identified non-immune persons can be reliably vaccinated in a timely manner 2
- Serologic testing is NOT necessary if it might present a barrier to timely vaccination 3, 4
Management of Indeterminate Results
Women with "indeterminate" or "equivocal" serologic test results must be considered susceptible and vaccinated immediately with MMR vaccine without repeat serologic testing. 2 This approach is more cost-effective and ensures protection rather than leaving women vulnerable while awaiting repeat testing.
Prenatal Screening Protocol
All pregnant women without documented immunity should undergo prenatal serologic screening for rubella antibodies. 1, 2, 4 This is critical because:
- Clinical diagnosis of past rubella infection is unreliable 2
- Vaccination history alone is insufficient 2
- Identifying non-immune pregnant women allows for immediate postpartum vaccination 1, 4
Special Populations Requiring Attention
Women born outside the United States, particularly those of Hispanic ethnicity, require special outreach and screening, as recent rubella outbreaks have disproportionately affected these populations who may have grown up in areas without routine rubella vaccination. 1
Standardization Concerns
Be aware that different rubella IgG assays may report different levels of International Units per milliliter (IU/mL) for the same sample, potentially leading to misinterpretation of results. 5 However, any result above the positive cutoff for a licensed assay should be considered evidence of immunity. 2