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If your CMV IgG test comes back positive, it tells you one straightforward thing: at some point in your life, your body encountered cytomegalovirus and mounted an immune response. CMV IgG (cytomegalovirus immunoglobulin G) is an antibody your immune system produces after CMV infection, and once it appears, it stays in your blood for life. A positive result does not mean you are sick right now. It means your immune system has a memory of this virus.
For most healthy adults, a positive result is clinically unremarkable. CMV is extremely common. In the United States, roughly 36% of children ages 6 to 11 have already been infected, and that number climbs above 90% in adults over 80. If you are generally healthy, this virus lives quietly in your cells without causing symptoms.
The reason this test matters is context. Your CMV serostatus becomes important information in specific situations: before an organ or stem cell transplant, during pregnancy, if you are living with HIV, or if you are on medications that suppress your immune system. In those settings, knowing whether you carry this virus can shape critical medical decisions.
CMV IgG is a yes-or-no test in practice. A positive result means you have been infected at some point, whether that was last year or decades ago. A negative result means your immune system has no record of CMV exposure. The test does not tell you when infection occurred, and it cannot distinguish between a virus sitting dormant and one that has reactivated.
If you need to know whether an infection is recent, a more specialized test called IgG avidity testing is required. This measures how tightly your antibodies bind to the virus. Loosely binding (low-avidity) antibodies suggest a recent infection, typically within the past 3 to 5 months. Tightly binding (high-avidity) antibodies suggest infection occurred more than 3 to 4 months ago.
One important caution: serial CMV IgG measurements over time are not reliable for tracking infection. Different lab platforms can produce results that vary by as much as 185-fold for the same blood sample. If your doctor suggests repeating the test to monitor changes, the results may reflect differences between assays rather than changes in your immune status.
For infants under 12 months, a positive CMV IgG may simply reflect antibodies passed from the mother during pregnancy rather than the baby's own infection.
CMV is a virus that coexists peacefully with a healthy immune system. The trouble begins when immune defenses weaken. In those circumstances, the virus can reactivate from its dormant state and cause serious organ damage. The complications come from the virus itself, not from the antibodies the test measures.
Transplant recipients: This is the group where CMV serostatus carries the most weight. If you are awaiting a transplant and test negative for CMV IgG but receive an organ from a donor who is positive, you face the highest risk category for CMV complications. The virus can cause direct problems like pneumonia, hepatitis, and gastrointestinal disease, and indirect ones like increased risk of organ rejection and secondary infections. Among stem cell transplant recipients who develop CMV disease affecting an organ, mortality is severe: roughly 62% for pneumonia, 57% for disease that spreads to multiple organs, and 40% for brain involvement.
People living with HIV: Before effective antiretroviral therapy existed, CMV retinitis (infection of the retina) affected approximately 30% of people with AIDS. Modern HIV treatment has reduced this by more than 95%. CMV end-organ disease primarily threatens those whose CD4 counts have fallen very low, but relapses have been documented even after immune recovery, at CD4 counts as high as 1,250 cells per cubic millimeter.
Critically ill and ICU patients: CMV can reactivate in up to 30% of critically ill patients in the ICU, even those who were previously healthy. Risk factors include septic shock, low white blood cell counts, high-dose steroid use, multiple blood transfusions, and COVID-19. Reactivation in this setting is associated with longer time on mechanical ventilation, extended hospital stays, and roughly double the mortality compared to ICU patients without reactivation.
People on immunosuppressive medications: If you take immune-suppressing drugs for conditions like rheumatoid arthritis or lupus, CMV reactivation risk increases. Higher cumulative corticosteroid doses, older age, low blood albumin levels, elevated creatinine, and cyclosporine use are all associated with greater risk.
If you are pregnant or planning to become pregnant, your CMV status has a unique significance. Congenital CMV infection, meaning the virus passes from mother to baby during pregnancy, occurs in roughly 0.2% to 2.2% of all newborns. It is the leading nongenetic cause of childhood hearing loss.
A first-time CMV infection during pregnancy (called a primary infection) carries a 30% to 40% chance of transmitting the virus to the fetus. The timing matters enormously. Transmission rates are actually lower early in pregnancy (about 37% in the first trimester) and higher later (about 66% in the third trimester). But the consequences are most severe when infection occurs earliest: neurologic damage and hearing loss affect roughly 29% of babies infected around the time of conception and 19% of those infected in the first trimester.
If you are CMV IgG negative and pregnant, you have never been exposed and are therefore susceptible to a primary infection. If you are CMV IgG positive, you have some existing immunity, though reinfection or reactivation can still occasionally cause fetal transmission. When acute infection is suspected during pregnancy, IgG avidity testing is essential to help determine whether infection is recent.
Routine CMV IgG screening is not recommended for the general population, and guidelines specifically advise against it even for most people living with HIV, because seroprevalence is so high that the result rarely changes management. The test becomes actionable in targeted situations.
Certain lab factors can occasionally affect CMV IgG results, so confirming unexpected findings with your clinician is reasonable.