Rubella IgM and IgG (German Measles)
Definition
Two types of antibody against the rubella virus are measured:
| Antibody | What it shows | How long it persists |
|---|---|---|
| IgG | Immunity, past infection or vaccination | Can last a lifetime |
| IgM | Acute (new) infection | Detected 4 to 30 days after the rash, persists for up to 3 months |
Purpose and correct use: the two tests serve different purposes and must not be confused:
IgG, for assessing immunity:
Rubella IgG serology should be used to assess rubella immunity, including before, during and after pregnancy. Immunity threshold: >10 IU/mL (the cut-off used in the USA)
IgM, only for confirming suspected cases:
Detection of rubella IgM by enzyme immunoassay (EIA) is used to confirm cases of suspected acute rubella infection and congenital rubella syndrome (CRS).
Risks and Limitations
Important warning, which must be included in the glossary:
Routine IgM screening is not recommended in pregnant women.
Reason: false positive results can occur in people without symptoms and can be misleading.
Because no test is 100% specific, serological tests in cases that are not rubella occasionally produce a false positive IgM result.
A specific cause of false positives: the presence of rheumatoid factor (RF) can lead to a false positive IgM. (See the RF entry, and remember that RF can also be present in healthy people.)
IgG avidity test: distinguishes recent exposure from old exposure. Low avidity indicates recent infection (detectable for up to 4 months), high avidity indicates old infection or vaccination.