Understanding Cytomegalovirus IgG: What Your Blood Test Means
CMV IgG positive means a past cytomegalovirus infection, now dormant. What the CMV IgG test shows, how to read it with IgM, and pregnancy notes.
A positive CMV IgG means you were infected with cytomegalovirus (CMV) at some time in your life, and the virus now stays in your body in a dormant (inactive) form. It is a very common result: in the United States more than half of adults have been infected by age 40, usually without ever noticing. CMV IgG on its own does not show when the infection happened or whether it is active now; for that, doctors look at CMV IgM, IgG avidity, or repeat testing. For most healthy people a positive result needs no treatment; it matters mainly in pregnancy, for transplant patients, and for people with a weakened immune system.
What the CMV IgG test measures
CMV is a common virus that infects people of all ages, most often in childhood. It spreads through body fluids such as saliva, urine, blood, tears, semen and breast milk, through sexual contact, breastfeeding, organ transplants and blood transfusions, and from a pregnant person to the baby. Once CMV is in the body, it stays for life and can reactivate; a person can also be re-infected with a different strain.
Antibodies are proteins the immune system makes against a germ. The CMV antibody test looks for two kinds:
- IgG – appears after an infection and normally stays for life. It is the marker of past exposure.
- IgM – the immune system usually makes IgM when it first meets an infection, so it can suggest a recent infection. However, IgM can also appear when an old infection reactivates or when a new strain is caught, so a positive IgM alone cannot prove a first-time infection.
The test is a simple blood draw from a vein in the arm. No special preparation, such as fasting, is needed.
Why and when the test is ordered
- Pregnancy, especially when a pregnant person has symptoms that suggest an active infection.
- Organ or bone marrow transplant recipients, people with HIV or other causes of a weakened immune system, who are at high risk of serious CMV illness.
- Symptoms suggesting active infection in someone at higher risk: fever, sore throat, swollen glands, tiredness, muscle aches or headache.
- Newborns are not diagnosed this way. Congenital CMV is diagnosed by a PCR test on the baby's saliva (confirmed on urine), not by antibody tests, because a baby can carry the mother's IgG; antibody testing is reliable only from about 12 months of age.
How to read the result
| CMV IgG | CMV IgM | Usual interpretation |
|---|---|---|
| Negative | Negative | No past infection detected; you can still catch CMV in future |
| Positive | Negative | Past infection at some point in life; the virus is dormant. The most common positive pattern in adults |
| Positive | Positive | Possible recent infection, but also seen with reactivation or reinfection; avidity or repeat testing helps |
| Negative | Positive | Possible very early infection; IgM alone cannot confirm it, so repeat testing is needed |
How results are reported and the cut-offs used vary between laboratories, so use the interpretation on your own report. If IgG is clearly higher on a repeat test about three months later, that may suggest an active infection.
Common reasons for a positive CMV IgG
- Past infection, usually in childhood or early adulthood – by far the most common reason. In the United States nearly 1 in 3 children is infected by age 5.
- Reactivation of the dormant virus – IgG is already positive; reactivation rarely causes disease unless the immune system is suppressed by medicines or illness.
- Recent first-time infection – if a previous test was negative and the new one is positive (seroconversion), that is clear evidence of a recent infection.
- Antibodies from the mother – a baby under 12 months may carry the mother's IgG without being infected.
Symptoms
A positive IgG causes no symptoms. Most people infected with CMV never have symptoms; some have a mild illness with fever, sore throat, tiredness or swollen glands, and occasionally CMV causes a mononucleosis-like illness or hepatitis. Serious illness is mainly a problem for people with weakened immune systems and for babies infected before birth.
What usually happens next
For most healthy, non-pregnant adults, nothing further is needed. When the timing matters, a clinician may order:
- CMV IgM together with IgG.
- CMV IgG avidity – after a first infection, IgG binds weakly (low avidity) and becomes stronger over the following months. A positive IgM with low avidity is considered reliable evidence of a recent primary infection.
- Repeat IgG one to three months later to look for seroconversion or a rising level.
- CMV PCR (viral DNA) on blood or other samples in transplant recipients and other immunocompromised people, and on saliva or urine in newborns.
Frequently asked questions
What does CMV IgG positive mean?
It means you have had a CMV infection at some point and the virus is dormant in your body. It does not tell you when you were infected, and on its own it does not mean you are ill now.
Is a positive CMV IgG bad?
For most people, no. It is a common result and the dormant virus rarely causes disease in a healthy immune system. It becomes important in pregnancy, before and after a transplant, and for people with HIV or other immune problems.
What does CMV IgG positive mean in pregnancy?
It usually means you were infected before the pregnancy. The risk of passing CMV to the baby after a non-primary infection (reactivation or reinfection) is about 3%, much lower than after a first infection during pregnancy, which carries a 30 to 40% risk in the first and second trimesters and 40 to 70% in the third. Your clinician may check IgM and avidity to judge timing. Routine CMV screening in pregnancy is not recommended in the United States.
What does IgG positive and IgM negative mean for CMV?
This is the usual pattern of a past infection with no sign of a recent one.
Does a positive CMV IgG mean I am immune?
Not fully. Antibodies do not stop the dormant virus from reactivating, and you can be re-infected with a different strain.
Can I pass CMV to others?
CMV spreads through body fluids, most often through direct contact with the saliva or urine of babies and young children, and also through sexual contact, breastfeeding, transplants and transfusions. A positive IgG does not by itself show whether you are carrying active virus right now.
When to see a doctor
It is worth discussing a positive CMV IgG with your doctor if you are pregnant or planning a pregnancy, are waiting for or have had a transplant, have HIV, or take medicines that weaken the immune system. Also talk to your doctor if you have a fever, swollen glands or tiredness that lasts. If you have severe breathing difficulty, confusion, or sudden vision loss while your immune system is weakened, seek urgent care.
Sources
- Laboratory Testing for CMV and Congenital CMV — U.S. Centers for Disease Control and Prevention
- About Cytomegalovirus — U.S. Centers for Disease Control and Prevention
- Clinical Overview of Cytomegalovirus — U.S. Centers for Disease Control and Prevention
- Cytomegalovirus (CMV) Tests — MedlinePlus Medical Test (U.S. National Library of Medicine)
- CMV blood test — MedlinePlus Medical Encyclopedia (U.S. National Library of Medicine)
Decisions about your health should always be made in consultation with your healthcare provider.