A positive rubella test result signals that the body has encountered the rubella virus, also known as German measles. The test determines a person’s immunity status, which is particularly important for women of childbearing age or those who are pregnant. While rubella is typically a mild viral infection causing a brief rash and low fever in non-pregnant adults, infection during pregnancy risks severe harm to the developing fetus. The primary goal of testing is to identify protected individuals and quickly identify any active infections.
Understanding the Rubella Test Components
The rubella test, often a blood-based serology panel, measures two specific antibodies produced by the immune system: Immunoglobulin M (IgM) and Immunoglobulin G (IgG). These antibodies appear in a predictable sequence following exposure, acting as markers for the timing of the infection.
Immunoglobulin M (IgM) antibodies are the first type generated, appearing in the bloodstream soon after the onset of infection, often within four days of a rash developing. The presence of IgM indicates a recent or acute infection. These antibodies are short-lived, usually remaining detectable for only six to eight weeks, or up to three months, before their levels drop.
Immunoglobulin G (IgG) antibodies appear slightly later than IgM but are much longer lasting, representing the body’s immunological memory. IgG levels become detectable a few days after IgM appears, peaking within one to two weeks after the rash starts. These antibodies persist in the blood for a lifetime, providing long-term protection against the rubella virus.
Interpreting a Positive IgG Result
A positive result for Immunoglobulin G (IgG) antibodies is a favorable finding, signifying past exposure to the rubella virus. This exposure is typically due to a previous natural infection or a successful response to the measles, mumps, and rubella (MMR) vaccine. The presence of IgG antibodies indicates the immune system has established memory against the virus, meaning the individual is considered protected or immune to future infection.
This immunity is particularly reassuring for pregnant individuals, confirming that protective antibodies are present and capable of preventing the virus from causing an active infection that could be transmitted to the fetus. A positive IgG test means the body is equipped to fight off the virus, eliminating the risk of rubella-related complications during the current pregnancy. Although laboratory-specific thresholds vary, a rubella IgG level above 10 International Units per milliliter (IU/mL) is typically considered sufficient to provide adequate protection. The long-term presence of these antibodies confirms a successful immune response.
Interpreting a Positive IgM Result
The detection of a positive Immunoglobulin M (IgM) result carries a serious implication, as it signifies a recent or acute rubella infection. Because IgM antibodies are the first responders, a positive result suggests the infection occurred within the past few weeks to months. This finding is especially concerning for pregnant women, as a primary rubella infection during gestation, particularly in the first trimester, poses a significant threat to the developing fetus.
The virus can cross the placenta and disrupt organ development, leading to a condition called Congenital Rubella Syndrome (CRS). CRS can result in a range of severe birth defects, including sensorineural deafness, cataracts, congenital heart defects, and developmental delays. The risk of transmission and the severity of fetal damage are highest when the mother contracts the virus early in the pregnancy, making immediate identification of an acute infection a priority. While a positive IgM result points to an acute infection, false-positive results can occur, requiring further confirmatory testing.
Next Steps Following a Positive Diagnosis
The steps taken after receiving a positive rubella test result depend entirely on which antibody was detected. If the result shows a positive IgG but a negative IgM, indicating immunity, no immediate medical action is typically required. This is the desired outcome for those planning pregnancy or already in early pregnancy, confirming protection.
For a positive IgM result, signifying a recent infection, immediate follow-up testing is necessary to confirm the diagnosis and determine the approximate timing. Healthcare providers often order IgG avidity testing, which measures the binding strength of the IgG antibodies. Low-avidity IgG suggests a very recent primary infection, often within the last four to six weeks, while high-avidity IgG points to an infection that occurred further in the past. This timing is critical for assessing the risk of Congenital Rubella Syndrome and guiding counseling for pregnant patients. Non-immune individuals who are not pregnant should receive the MMR vaccine, but this must be postponed until after delivery for those who are currently pregnant.