A positive CMV IgG result stays positive for life. Once you’ve been infected with cytomegalovirus, your body produces IgG antibodies that remain detectable indefinitely. This isn’t a test result that fades over time or needs to be rechecked for confirmation. A positive result today means you were infected at some point in the past, and your immune system continues to recognize the virus.
Why CMV IgG Never Goes Away
The reason your IgG stays positive is that CMV itself never actually leaves your body. After the initial infection, the virus goes dormant inside certain bone marrow cells called myeloid progenitor cells. It essentially hides there by shutting off its own genes, making itself invisible to your immune system while keeping its genetic material intact inside the cell nucleus. These host cells have long lifespans, giving the virus a stable, long-term home.
Because the virus remains in your body in this dormant state, your immune system keeps producing antibodies against it. The virus can occasionally reactivate at low levels, which further reminds your immune system to maintain its defenses. A 27-year longitudinal study published in Clinical & Translational Immunology confirmed that people who were CMV-seropositive at the start of the study remained seropositive throughout the entire follow-up period. A separate 12-year study tracking older women found that anti-CMV IgG titers did not change meaningfully over the entire observation period. The antibody levels held steady year after year.
How IgG Differs From IgM
If you’re looking at lab results and wondering about the difference between IgG and IgM, they tell you different things. IgM antibodies appear first during a new infection and typically fade within weeks to a few months, though in some people they can linger longer or reappear during viral reactivation. IgG antibodies develop shortly after IgM but persist permanently.
The practical takeaway: a positive IgM with a negative IgG suggests a very recent, new infection. A positive IgG with a negative IgM means you were infected in the past and your body has long since controlled the virus. A positive result for both can mean either a recent infection or a reactivation, and further testing may be needed to sort that out.
How Doctors Tell Old Infections From New Ones
Since IgG stays positive forever, a single positive result can’t tell you when you were infected. That’s where avidity testing comes in. Avidity measures how tightly your IgG antibodies bind to the virus. In the first few months after infection, IgG binds weakly (low avidity). Over about five to six months, the binding strength gradually matures to high avidity.
The typical pattern looks like this: for the first three to four months after a primary infection, avidity remains low. It then moves into an intermediate range for one to two months before reaching full maturity at around five to six months. After that point, avidity stays high permanently. So if your IgG avidity is high, your infection is at least six months old. Low avidity suggests the infection happened within the past few months. This distinction matters most during pregnancy, where the timing of infection affects risk to the fetus.
What Your Lab Numbers Mean
CMV IgG results are reported in International Units per milliliter (IU/mL). The standard cutoffs used by most labs are: below 0.4 IU/mL is negative, 0.4 to 0.6 IU/mL is equivocal (borderline), and above 0.6 IU/mL is positive. An equivocal result typically calls for retesting in two to three weeks to see if levels are rising.
The specific number above the positive threshold doesn’t carry much clinical meaning on its own. Higher titers don’t necessarily mean a more active or dangerous infection. Research on older adults has shown that IgG levels remain remarkably stable over years and are not a useful tool for monitoring how active the virus is at any given time. In other words, the number tells you that you’ve been infected, but not much else about what the virus is doing right now.
Why It Matters During Pregnancy
Pregnancy is the most common reason people search for information about CMV IgG status. A woman who is already IgG-positive before becoming pregnant has existing immunity, which provides significant (though not complete) protection against passing the virus to her baby. In one prospective study of seropositive pregnant women, the rate of congenital CMV was 6.9%, meaning the vast majority of babies born to IgG-positive mothers were unaffected.
The higher-risk scenario is when a woman who has never been infected (IgG-negative) catches CMV for the first time during pregnancy. A first-time infection during pregnancy carries a greater chance of transmission to the fetus and a higher risk of complications. This is why some providers check CMV IgG status early in pregnancy or before conception: a positive result is actually reassuring in this context, because it means prior immunity is already in place.
How Common Is a Positive Result?
CMV is extremely widespread. In the United States, seroprevalence varies by age and demographic group, but a significant portion of the adult population tests positive. Among children aged one to five, prevalence increased from 21% to 29% between 2011 and 2020, and rates climb steadily with age. Globally, the majority of adults have been infected by middle age. The annual rate of new infections among previously negative adults is relatively low, around 0.56% per year in one long-term study, with women seroconverting at roughly three times the rate of men.
Most people acquire CMV without ever knowing it. The initial infection in healthy individuals is usually mild or completely silent, producing no symptoms at all. By the time you see a positive IgG on a lab report, the acute phase is long over, and the result simply reflects your immune history with a virus your body successfully controlled.