Rubella Test

How is it used?

The rubella test is used to detect antibodies in the blood that develop in response to a rubella infection or immunization. Rubella testing may be used to:

• Confirm the presence of adequate protection against the rubella virus (immunity)
• Detect a recent or past infection
• Identify those who have never been exposed to the virus and those who have not been vaccinated
• Verify that all pregnant women and those planning to become pregnant have a sufficient amount (titer) of rubella antibodies to protect them from infection

Rubella is a viral infection that is usually mild and marked by fever and rash that last about 2 to 3 days. The infection usually resolves without treatment. However, if a pregnant woman contracts it for the first time during the first three months of her pregnancy, rubella can cause serious complications in the developing baby (fetus). (For more on this, read the "What is being tested?" section.)

A rubella test may be ordered for a person, pregnant or not, who has symptoms that a health practitioner suspects are due to a rubella infection. It may also be ordered for a newborn who is suspected to have become infected during pregnancy or that presents with congenital birth defects that a health practitioner suspects may be due to a rubella infection.

There are two types of rubella antibodies that lab tests can detect, IgM and IgG:

• The first type to appear in the blood after exposure is the IgM rubella antibody. The level of this protein rises and peaks in the blood within about 7 to 10 days after infection and then tapers off over the next few weeks, except in an infected newborn, in whom it may be detected for several months to a year.
• The IgG rubella antibody takes a bit longer to appear than the IgM, but once it does, it stays in the bloodstream for life, providing protection against re-infection. The presence of IgM rubella antibodies in the blood indicates a recent infection while the presence of IgG antibodies may indicate a recent or past rubella infection, or indicate that a rubella vaccine (a measles, mumps, rubella vaccine) has been given and is providing adequate protection.

The IgM rubella test is the standard test for the rapid laboratory diagnosis of rubella. Detection of a rise in IgG rubella in blood samples collected when a person is acutely ill and then as the person begins to recover (convalescent phase) can be used to confirm infection. The antibody tests vary among laboratories and the state health department can provide guidance on available laboratory services and preferred tests.

When is it ordered?

The IgG rubella test is ordered when a woman is pregnant or is planning on becoming pregnant. It is ordered whenever a check for immunity against rubella is required. IgM and IgG rubella tests may be ordered when a pregnant woman has signs and symptoms that may indicate a rubella infection.

Some signs and symptoms include:

• Mild fever
• A pink rash that begins on the face and then spreads downward to the body and then the legs and arms; once it begins to spread to the body, the rash may disappear from the face.
• Runny or stuffy nose
• Red or inflamed eyes
• Aching joints
• Swollen lymph nodes

Since many conditions can cause similar symptoms, a health practitioner will need to order the tests in order to confirm the diagnosis.

IgM and IgG tests may be ordered for a newborn when the mother was diagnosed with a rubella infection during pregnancy and/or when a newborn is born with birth defects such as hearing loss, heart defects, or cloudy lens of the eyes (cataracts) that could be due to congenital rubella syndrome (CRS).

Since rubella antibodies take some time to appear after infection, the tests may be repeated after day 5 of illness onset (IgM) and 7-21 days after the first specimen (IgG) to see if the antibody levels have become detectable (when initially absent) and to determine whether the levels are rising or falling over time.

This test may be required of a healthcare worker or a person starting college and is still ordered for women in some states as part of the blood testing required to obtain a marriage license.

What does the test result mean?

Adult or Child

In an adult or child, the absence of IgG rubella antibodies means that the person likely has not been exposed to the rubella virus or been vaccinated and is not protected against it.

The presence of IgG antibodies but not IgM antibodies indicates a history of past exposure to the virus or vaccination and indicates that the person tested should be immune to the rubella virus.

The presence of IgM antibodies, with or without IgG antibodies, in a child or adult indicates a recent infection with the rubella virus.

Newborn

The presence of IgG antibodies, but not IgM antibodies, in a newborn means that the mother's IgG antibodies have passed to the baby in utero and these antibodies may protect the infant from rubella infection, although they should decrease by four- to eight-fold by three months of age and should disappear by 6 to 12 months of age.

The presence of IgM antibodies in a newborn indicates that the baby was infected during pregnancy because the mother's IgM antibodies do not pass to the baby through the umbilical cord.

The table below summarizes some results that might be seen:

Because rubella incidence is low, a person may have a false-positive test for IgM rubella antibodies due to infection with other viruses or the test components are cross-reacting with other proteins in the body. To confirm the IgM result, the health practitioner may order an IgG test to establish a baseline level of antibody and may repeat the IgG test again (tested together with the first specimen) in 7-21 days to look for a significant increase in the amount (titer) present, indicating a recent rubella infection.

Is there anything else I should know?

In addition to antibody tests, the evidence of recent rubella infection is a positive viral culture for rubella or detection of rubella virus by testing for the genetic material (RNA) of the rubella virus in a body fluid sample, such as a throat swab. Because these procedures are complex, time-consuming and expensive, most diagnostic laboratories refer this testing to a reference lab such as the Centers for Disease Control and Prevention (CDC).

The number of cases of rubella and congenital rubella is monitored by the CDC to track the effectiveness of the rubella vaccine and to detect any outbreaks of the disease.