Rubella Avidity - IgG – BC275
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Sample Type: Blood (Serum)
Test Code: BC275
The Rubella Avidity - IgG test is a specialized blood test that helps determine the timing of a Rubella (German measles) infection. It measures the binding strength ("avidity") of IgG antibodies. This information is crucial for pregnant women who test positive for Rubella IgG but need to know if the infection was recent or from the distant past.
✔ Helps differentiate a recent primary infection from a past infection during pregnancy.
✔ A key test to assess the risk of Congenital Rubella Syndrome (CRS) in the fetus.
✔ A high avidity result provides reassurance that the infection is not recent.
✔ Ordered when Rubella IgG is positive but IgM is negative or borderline.
The Avidity Test: Solving a Critical Pregnancy Puzzle
A Rubella infection during the first trimester of pregnancy can be devastating, leading to a condition in the baby called Congenital Rubella Syndrome (CRS), which can cause deafness, cataracts, heart defects, and developmental delay. Standard tests check for two types of antibodies: IgM (indicating a very recent infection) and IgG (indicating past infection or immunity).
A dilemma arises when a pregnant woman tests positive for IgG but negative for IgM. Does the positive IgG mean she was vaccinated years ago (safe), or did she have a mild, recent infection a few weeks or months ago where the IgM has already faded (dangerous)? This is the critical question the Avidity test answers.
What is Avidity?
Avidity refers to the binding strength between an antibody and its target antigen. Think of it like the "stickiness" of the antibody. This strength matures over time:
- Low Avidity IgG: These are "young" antibodies produced during the initial phase of an infection. They do not bind very strongly. A low avidity result suggests a **recent primary infection**, likely within the last 3-4 months. This is a high-risk result for a pregnant woman.
- High Avidity IgG: Over several months, the immune system refines its antibody production, creating "mature" antibodies that bind very tightly. A high avidity result indicates a **past infection**, likely more than 4 months ago. For a pregnant woman, this is a reassuring result that effectively rules out a recent infection during the critical period of her pregnancy.
Frequently Asked Questions (FAQ)
This test is ordered for pregnant women who have a positive Rubella IgG result but a negative or unclear IgM result. The avidity test helps your doctor determine *when* you were infected—recently or in the distant past—which is crucial for assessing any risk to your pregnancy.
A high avidity result is good news. It indicates that your IgG antibodies are mature and strong-binding, which means your Rubella infection happened in the distant past (more than 4 months ago). This effectively rules out a recent infection during your pregnancy.
A low avidity result suggests that your IgG antibodies are "young," indicating a recent primary infection (likely within the last 3-4 months). This is a more concerning result during pregnancy, and your doctor will discuss the implications and further monitoring with you.
A standard IgG test just tells you "yes" or "no"—whether the antibodies are present. The avidity test is a specialized follow-up test that measures the *binding strength* of those IgG antibodies to determine if they are from a recent or old infection.