Cytomegalovirus (CMV) IgG
A blood test for antibodies that show whether your immune system has met cytomegalovirus (CMV), a very common virus, at some point in your life.
AI-verified against cited sources. Awaiting expert reviewAlso called CMV IgG, CMV-IgG, CMV Ab IgGLOINC 5124-3
What it is
Cytomegalovirus is a common virus that spreads through body fluids such as saliva and urine. Most healthy people have no symptoms or a mild illness. After infection the virus stays in the body for life in a dormant state. The immune system makes IgG antibodies, and this test looks for them. A positive result usually means you were infected at some point, often without knowing. About 1 in 3 children are infected by age 5, and over half of adults by age 40.
Why it matters
The test mostly shows whether you have been infected before. That matters most in pregnancy, before a transplant, and for people with a weakened immune system, because CMV can cause more serious illness in these groups. In the US, nearly half of women already have antibodies before their first pregnancy, and 1 to 4 percent of women without them catch CMV during pregnancy. A single IgG result cannot say when the infection happened, and it cannot diagnose CMV in a newborn.
If it’s low
A negative result means no CMV IgG antibodies were found, so you most likely have not been infected before. It can also be negative very early in a new infection, before antibodies appear, and in people whose immune systems make fewer antibodies.
Common causes
- Never having been infected with CMV
- Very early in a new infection, before antibodies have formed
- A weakened immune system that makes fewer antibodies
What can help
- If you are pregnant or planning a pregnancy, ask about hygiene steps that lower the chance of a first CMV infection
- Ask whether a repeat test makes sense after a recent exposure or a flu-like illness
If it’s high
A positive result means antibodies to CMV were found. This usually means you were infected at some point, and the virus stays in the body for life. It does not tell you when the infection happened or whether the virus is active. Being positive is common and is not a problem by itself for most healthy people.
Common causes
- A past CMV infection, which is very common
- A recent infection, in which case your clinician may use other tests such as IgM or IgG avidity to tell
- Antibodies passed from a mother to her newborn
- Donor antibodies from recent immune globulin or blood products
What can help
- Ask whether other tests, such as CMV IgM, IgG avidity or CMV DNA, are needed to tell a recent infection from an old one
- If you are pregnant, ask what your result means for you and your baby
- Ask about everyday hygiene steps that lower the spread of CMV, such as washing hands after changing diapers and avoiding contact with young children's saliva and urine
What can affect your result
- Age: the share of people with antibodies rises through childhood and adulthood
- Where you live: in one 2026 study of a high-prevalence population, about 7 in 10 people had CMV IgG
- Immune function: a weakened immune system may make fewer antibodies
- Recent immune globulin or blood products, which can carry donor antibodies
- The assay: different kits use different cut-offs and units, but IgG results agree well between platforms in a recent four-platform comparison
- Each lab uses its own kit, scale and cut-offs, so a number printed in U/mL or AU/mL from one lab cannot be compared with another lab's.
- Recent treatment with immune globulin or blood products can carry donor antibodies and may cause a positive result.
- In newborns, antibodies passed from the mother are common, so a positive result does not show that the baby is infected (CDC).
Once IgG is positive it generally stays detectable for years, usually for life, because the virus stays in the body after infection.
When to talk to a clinician
Contact a clinician promptly if you are pregnant and develop a flu-like or mononucleosis-like illness (fever, tiredness, sore throat, swollen glands). If you have a weakened immune system, for example after a transplant or on immune-suppressing treatment, contact your care team promptly for fever, new vision changes or other new symptoms. CMV in a newborn is checked with a PCR test on saliva or urine in the first 2 to 3 weeks of life, not with antibody tests.
Questions you could ask
- Should this result be checked together with other tests, such as toxoplasma or rubella?
- Does this result suggest a past infection, and do I need IgM, avidity or CMV DNA testing to be sure?
- Does this result matter for my pregnancy, planned pregnancy or treatment?
- Does it matter that I have not had CMV before, given my pregnancy plans or health?
- Should we repeat this test if I have a new exposure?
Questions people ask
I tested positive for CMV IgG. Does that mean I have an active infection?
Usually not. A positive IgG mostly shows that you were infected at some point, which is true for over half of adults by age 40. It does not show when the infection happened. If a recent infection is a concern, your clinician may order IgM, IgG avidity or CMV DNA.
Is a negative CMV IgG good or bad?
It is neither. It means you most likely have not met the virus yet. That can matter in pregnancy or before a transplant, because catching CMV for the first time during pregnancy can pass it to the baby. Your clinician can explain what it means for you.
Why does my report show a number with units like U/mL or AU/mL?
Many labs print an antibody level alongside positive or negative. The scale and cut-offs belong to that lab's test kit, so the numbers are not comparable between labs. Use the interpretation printed on your own report.
Show the science and sources
For clinicians and researchers
CMV IgG is measured by automated immunoassays. Mayo uses an electrochemiluminescence immunoassay with a reference of Negative at all ages, and Labcorp used a chemiluminescent immunoassay with Negative below 0.60 U/mL, Equivocal 0.60-0.69 and Positive above 0.69 U/mL on its 2020 page. These cut-offs belong to those assays and cannot be applied to other kits. Labcorp reports LOINC 5124-3 and Mayo reports 22244-8. In a 2026 comparison of four platforms, CMV IgG showed almost perfect agreement (kappa 0.91-0.95) with sensitivity of 96.9-99.1% and specificity of 90.5-98.9%, unlike IgM. IgG alone cannot date an infection. A rise from negative to positive between paired samples shows recent infection, and IgG avidity helps date a first infection, since low avidity is generally associated with an infection in the last 3 months and high avidity excludes onset in the last 3 months. Mayo notes the test is not useful for screening blood or plasma donors or for neonatal screening, and CDC states that congenital CMV cannot be diagnosed with IgG or IgM and needs PCR on saliva or urine in the first 2-3 weeks of life. CMV establishes lifelong latency, so IgG persists after infection.
LOINC 5124-3 (primary), 7852-7, 22244-8, 13949-3. Specimen: serum.
Sources
- Clinical Overview of CMV and Congenital CMV, Centers for Disease Control and Prevention
- Cytomegalovirus Antibody, IgG, Serum (test CMVGS), Mayo Clinic Laboratories
- Cytomegalovirus (CMV) Antibodies, IgG (test 006494), Labcorp (2020)
- Consistent performance of cytomegalovirus IgG assays across platforms contrasts with marked IgM variability, supporting IgG avidity-based infection classification in a high-prevalence population, Scientific Reports (2026)
- Pitfalls in the Serological Diagnosis of Primary Human Cytomegalovirus Infection in Pregnancy Due to Different Kinetics of IgM Clearance and IgG Avidity Index Maturation, Diagnostics (2021)
- LOINC codes verified with the NLM Clinical Tables LOINC search (e.g. 5124-3), U.S. National Library of Medicine
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.