What is CMV IgM?
CMV IgM is a type of antibody that is formed early in an infection with Cytomegalovirus (CMV). These antibodies are produced by the immune system as a first line of defense against the virus. CMV belongs to the herpesvirus family and is very common, but the infection is often asymptomatic in healthy individuals. When CMV is activated in the body, either during a primary infection or during reactivation of latent virus, the levels of CMV IgM in the blood rise. The presence of CMV IgM can therefore be an indication of an ongoing or recent infection.
How does CMV IgM work in an infection?
IgM antibodies are the body's rapid response to an infection. In a primary infection with CMV, IgM is usually produced within 1 to 2 weeks after infection. The antibodies help fight the virus by binding to virus particles and activating parts of the immune system, including the complement cascade. CMV IgM may also be seen with reactivation of a previous infection, especially in people with compromised immune systems. Because CMV can remain latent in the body for a long time, it is important to interpret IgM results with caution and in conjunction with other laboratory markers.
When is CMV IgM tested?
A CMV IgM blood test is used to identify a current or recent CMV infection. The test is particularly important in:
- Pregnancy – to detect a primary infection in the mother, which poses a risk to the fetus.
- Organ transplantation – to assess the risk of CMV complications in the recipient.
- Investigation of fever, lymph node enlargement or liver involvement – especially in patients with a weakened immune system.
The CMV IgM test is often performed together with CMV IgG and sometimes an avidity test for IgG, which can help determine how recent the infection has occurred.
Diagnostic value of CMV IgM
Positive CMV IgM indicates a current infection, but it is not always synonymous with a recent primary infection. IgM can persist in the blood for several months after infection and can also appear during reactivation or cross-reaction with other viruses. Therefore, complementary tests are often used to confirm the phase of the infection. A simultaneous positive CMV IgG with low avidity strengthens the suspicion of a primary infection.
CMV IgM and pregnancy
In pregnant women, the analysis of CMV IgM is of great importance. A primary CMV infection during pregnancy can lead to transmission to the fetus and cause congenital CMV infection, which in some cases can result in hearing loss, growth retardation or neurological damage. If CMV IgM is positive during pregnancy, further investigation with CMV IgG and avidity testing should be performed to determine the risk to the fetus and the need for follow-up or treatment.
Interpretation and normal values
CMV IgM results are reported as positive, negative or borderline, depending on the concentration in the blood. Normal values vary somewhat between laboratories, but a negative result usually excludes an ongoing infection. A positive test should always be followed up with an assessment of CMV IgG levels and the patient's symptoms. It is also important to note that some test variability and false positive results may occur, especially in pregnant women or patients with other viral infections.
Clinical use of CMV IgM
CMV IgM is a central tool in clinical microbiology to:
- Identify acute CMV infections in diffuse symptoms of infection.
- Make a diagnosis of suspected congenital CMV in newborns.
- Assess infection status in immunosuppressed patients before and after transplantation.
- Determine infection risk in pregnant women during serological screening or suspicion of primary infection.
The interpretation of CMV IgM should always be done in combination with other serology, clinical signs and possible virological analysis (PCR or antigen detection).