A positive rubella test result indicates one of two things: either a person is protected from the German measles virus, or they have a current or very recent infection. While rubella is a mild viral illness in children and non-pregnant adults, it can cause serious complications if contracted during pregnancy. Because of this risk, testing for rubella antibodies is a standard procedure, particularly for individuals planning pregnancy or those in early gestation. Interpreting the results correctly is fundamental for determining necessary medical management and understanding immunity status.
The Two Markers of Rubella Testing
The rubella test, performed using a blood sample, looks for two distinct types of antibodies produced by the immune system: Immunoglobulin G (IgG) and Immunoglobulin M (IgM). The presence or absence of each type provides a timeline of the body’s interaction with the rubella virus.
Immunoglobulin M (IgM) antibodies are the first line of defense, appearing a few days to a week after the onset of a new infection or recent vaccination. IgM levels are temporary; they rise quickly and typically become undetectable within three months. A positive IgM result indicates a very recent or current active rubella infection.
Immunoglobulin G (IgG) antibodies appear shortly after IgM but persist in the bloodstream for a lifetime. These long-term antibodies confer lasting immunity, protecting the body from future rubella infections. A positive IgG result indicates past exposure to the virus, through natural infection or vaccination, and confirms protective status.
Interpreting a Positive Result: Immunity vs. Recent Infection
A “positive” rubella test result means antibodies against the virus were detected, but it does not automatically mean a person is sick. Interpretation requires differentiating between long-term protection and active disease based on the combination of the two antibody markers. The most common and reassuring positive result is a high IgG level with a negative IgM level.
This scenario indicates that the individual is immune to rubella, possessing protective antibodies from a past infection or a previous vaccination. For instance, an IgG level above 10 International Units per milliliter (IU/mL) is generally considered protective, signifying adequate immunity. No further action is necessary, and the person is safe from contracting the disease.
A much more concerning result is a positive IgM test, often accompanied by a rising IgG titer, which signals an acute or very recent infection. If both IgM and IgG are positive, or if a second test shows a significant increase in IgG levels over a few weeks, a recent rubella infection is confirmed. Because a positive IgM result can sometimes be a false positive, confirmation with a second test or an avidity test may be needed to determine the exact timing of the infection.
Implications of Active Rubella During Pregnancy
The distinction between immunity and recent infection is particularly significant for pregnant individuals due to the high risk of fetal harm associated with active rubella. If a pregnant person contracts the rubella virus, the infection can cross the placenta and transmit to the developing fetus, leading to a condition known as Congenital Rubella Syndrome (CRS). The virus can cause severe birth defects and developmental issues.
The risk to the fetus is most severe if the rubella infection occurs early in the pregnancy, during the period of major organ formation. Infection during the first 12 weeks of gestation carries the highest risk, with up to 85% of newborns affected by CRS. This risk drops to about 50% if the infection occurs between 13 and 16 weeks, and further decreases during the latter half of the second trimester.
CRS can result in a constellation of serious and lifelong defects, collectively known as the classic triad: deafness, cataracts, and congenital heart defects. Other severe complications can include intellectual disabilities, liver and spleen damage, and low birth weight. Furthermore, a rubella infection during pregnancy carries a risk of miscarriage or stillbirth.
Next Steps and Management
Management following a positive rubella test depends entirely on which marker is positive. If the result shows positive IgG and negative IgM, confirming immunity, no medical action is typically required. The individual is protected for the duration of current or future pregnancies.
If the test reveals that a person is susceptible (negative IgG and negative IgM), vaccination with the Measles, Mumps, and Rubella (MMR) vaccine is strongly advised, but this must be done before pregnancy. The MMR vaccine is a live-attenuated vaccine and must not be administered during pregnancy. Women should wait at least four weeks after receiving the vaccine before attempting to conceive.
When a recent infection is confirmed with a positive IgM result in a pregnant individual, immediate consultation with an obstetrician and an infectious disease specialist is necessary. The medical team will monitor the pregnancy closely. Further testing, such as rubella avidity testing or fetal surveillance, may be used to assess the risk to the fetus. Counseling is provided regarding the high risk of CRS and the potential for severe outcomes, which guides decisions about the continuation of the pregnancy.