Why Do I Need a Rubella Test?

Medically Reviewed by Jennifer Robinson, MD on February 25, 2024
3 min read

Rubella, also called German measles or 3-day measles, isn't a problem for most people. It causes a mild fever and rash that go away in a few days. Most kids get vaccinated for it with the MMR (measles-mumps-rubella) or MMRV (which also includes chickenpox) shots.

But when you're pregnant, rubella can be very serious. If you get it in the first 4 months, your baby could have eye, hearing, or heart problems or be born too soon.

  • A woman who's having or planning to have a baby
  • A newborn baby whose mom may have had rubella when they were pregnant
  • A baby with birth defects that could be caused by rubella
  • Anyone with rubella symptoms
  • Healthcare workers who haven't had rubella or the vaccine
  • Students starting college

A technician uses a needle to take a small sample of blood from a vein in your hand or arm. You may feel a small skin prick and have a little bleeding or bruising where the needle goes in. Then they'll send your blood to a lab.

A rubella blood test checks to see if you have antibodies to the rubella virus. Antibodies are proteins your immune system makes to help fight infections and keep you from getting sick. They're targeted to specific germs, viruses, and other invaders. Your doctor can tell a lot from the type of antibodies that you have in your blood.

IgM is first on the scene after you get rubella. It sticks around for 7 to 10 days in adults and up to a year in newborns. You'll get this test if your doctor thinks you may have rubella. You might need to have your blood tested in a public health lab in your state.

IgG stays in your bloodstream for life. It means you had either the illness or the vaccine in the past and are now immune to the virus. You're likely to have this test when you need to know that you can't get sick with rubella.

You'll need both tests if you're pregnant and your doctor suspects you have rubella. And your baby will need both tests after birth, too.

A "positive" IgM test, meaning you have IgM in your blood, could be because you've recently been infected. But since rubella isn't a common illness, the test may be a "false-positive" -- you might be infected with a different virus or the test is reacting to other proteins in your blood instead. More tests can confirm the result.

A "negative" IgM test usually means you're not infected. But people with a weak immune system (like someone with HIV or taking medication that suppresses their immune system) may be infected and not able to make enough antibodies to show up in the test.

Here's how your IgG test results stack up:

  • A positive test is 1.0 or higher. That means you have rubella antibodies in your blood and are immune to future infection.
  • A negative test is 0.7 or lower. You have too few antibodies to make you immune. If you have any, they can't be detected.

A score of 0.8 or 0.9 could mean you just had the vaccine and antibodies haven't shown up in your blood yet. Your doctor may ask you to take the test again.

Babies can't get IgM antibodies from their moms, so if a newborn has a positive test, they were infected before or just after birth. A mother's IgG antibodies may protect their baby during pregnancy and for a few months after they're born.