Rubella Avidity (IgG)

Overview

Rubella, caused by the Rubella virus (family Togaviridae, genus Rubivirus), is a contagious viral disease transmitted via respiratory droplets. While often mild, it can lead to severe consequences during pregnancy, especially Congenital Rubella Syndrome (CRS).

The Rubella Avidity (IgG) Test measures the binding strength of IgG antibodies to rubella antigens. IgG avidity matures over about six months following primary infection, transitioning from weak-binding to strong-binding antibodies. Thus, the test is particularly useful for distinguishing recent rubella infection from past exposure or vaccination.

1. Low IgG avidity (<40%) → Suggests recent or primary infection with high CRS risk.
2. Intermediate IgG avidity (40–60%) → Requires repeat testing for confirmation.
3. High IgG avidity (>60%) → Indicates past infection or vaccination, associated with low CRS risk.

Testing is primarily recommended for pregnant women suspected of rubella infection, as it helps assess the likelihood of fetal exposure.

Symptoms

Rubella infection, which leads to IgG antibody production detectable in this test, typically presents with:

1. Systemic Symptoms: Fever, headache, malaise, and general discomfort.
2. Respiratory Signs: Mild cough, runny nose, and sore throat.
3. Ocular Signs: Conjunctivitis or mild pink eye (redness and swelling of the white of the eye).
4. Lymphatic Signs: Swollen and enlarged lymph nodes, especially behind the ears and neck.
5. Skin Symptoms: Rash that begins on the face and spreads to the body, usually lasting three days.

Importantly, 25–50% of infected individuals are asymptomatic, yet remain contagious, spreading the virus up to one week before and seven days after rash onset.

Causes

The underlying cause of positive rubella IgG avidity test results is infection with the rubella virus or immunity due to vaccination.

1. Viral Infection Pathway:
   1. Transmission occurs through respiratory droplets or direct contact.
   2. Virus replicates in the nasopharynx and lymph nodes, followed by viremia.
   3. In pregnancy, transplacental spread to the fetus occurs, potentially causing CRS.
2. Immune Response:
   1. During recent infection, IgG antibodies are immature and weak-binding, leading to low avidity results.
   2. With time or after vaccination, IgG antibodies mature, producing high avidity results.

This maturation process forms the basis for the clinical utility of the IgG avidity test.

Risk Factors

Individuals at higher risk of rubella infection and thus requiring avidity testing include:

1. Pregnant Women: Especially those in the first trimester, where maternal infection has the highest risk of CRS in the fetus.
2. Unvaccinated Individuals: More prone to contracting rubella and spreading it.
3. Infants Born to Infected Mothers: CRS risk is directly related to maternal infection timing.
4. Healthcare Workers: At higher risk due to frequent exposure to potentially infected patients.
5. Close-Contact Groups: Children in daycare or school settings, where rubella can spread easily.
6. Women of Childbearing Age: Those without documented vaccination or prior rubella infection history.

Prevention

While rubella infection cannot always be avoided, the risk of transmission and CRS can be minimized through preventive strategies:

1. Vaccination:
   1. MMR (Measles, Mumps, Rubella) vaccine is the most effective preventive tool.
   2. It induces long-term high avidity IgG antibodies, protecting against future infections.
   3. Women should receive vaccination before pregnancy; it is contraindicated during pregnancy.
2. Screening and Testing:
   1. IgG avidity testing during pregnancy helps differentiate recent from past infections.
   2. Pregnant women with low avidity results require careful monitoring and follow-up testing.
3. Sample Collection and Handling:
   1. Collect 3 mL of blood in a red-capped plain tube.
   2. Serum should be separated quickly and stored at 2–8°C for up to 72 hours, or frozen at –20°C or lower for longer preservation.
   3. For long-term storage (≤ –70°C), specimens must be transported on dry ice.
4. Hygiene and Infection Control:
   1. Encourage respiratory hygiene (covering coughs, frequent handwashing).
   2. Isolate suspected cases during the contagious period to prevent spread.
5. Genetic Counseling and Awareness:
   1. Educating women of reproductive age about rubella risks and vaccination before conception reduces CRS incidence.