CMV IgM (Cytomegalovirus IgM)

What is CMV IgM?

The CMV IgM test measures the IgM-class antibodies that the immune system produces against cytomegalovirus in the early stage of infection. These antibodies usually rise markedly in a first-time, or primary, infection and decrease within a few months. For this reason, the test is used to investigate a CMV infection that has started recently.

However, CMV IgM can become positive not only in a new infection but also when the virus that has remained dormant in the body reactivates, or in reinfection with a different strain. In some people, IgM antibodies can remain positive for months or even longer. In addition, other infections such as Epstein-Barr virus, or factors such as rheumatoid factor, can lead to false-positive results. The antibody level is determined in the laboratory on serum using an ELISA-based method, and the result is usually reported as positive, negative or borderline.

When is the CMV IgM test ordered?

CMV IgM is mostly ordered when a recent infection is suspected, usually together with CMV IgG. In pregnancy, a first-time infection in the mother matters more for transmission to the baby, so correct interpretation of the result is particularly important during this period. In healthy adults, the test is used when investigating the cause of conditions such as prolonged fever and fatigue.

• Suspected CMV infection during pregnancy, or ultrasound findings flagged by the doctor
• Investigation of a new infection in a pregnant woman who previously tested negative for CMV IgG
• An illness with fever, sore throat and swollen lymph nodes when EBV tests are negative
• Fatigue lasting for weeks together with unexplained abnormalities in liver tests
• A febrile illness in pregnant women who work in daycare or childcare

How do you prepare for the CMV IgM test?

There are no food restrictions for CMV IgM, and the blood draw can be done at a time of day that suits you. Sharing with your doctor when your symptoms started, your week of pregnancy if you are pregnant, and the results of any previous CMV tests makes interpretation much easier. Also mention if you have a rheumatic disease or have recently had another viral infection.

If the result is inconclusive, your doctor may ask for the test to be repeated two to three weeks later or for an IgG avidity test. Blood is drawn from a vein in the arm using single-use sterile supplies, and the procedure takes a few minutes. Since frequent checks may be needed during pregnancy, you may prefer to give your sample to a nurse who comes to your home or workplace.

What does the CMV IgM result mean?

A negative result indicates that a recently started CMV infection is unlikely. In this case, your immune status is assessed based on the CMV IgG result: if IgG is positive, a past infection is assumed; if IgG is also negative, it is assumed that you do not yet have immunity against the virus. In the first days of an infection, IgM may not have risen yet; if clinical suspicion persists, your doctor may request a new measurement a few weeks later.

Although a positive result suggests a new infection, it is not definitive proof on its own; reactivation, long-persisting IgM or a false positive are also possible. For this reason, your doctor usually clarifies the situation with additional tests such as an IgG avidity test, a repeat measurement and, if necessary, CMV PCR. Low avidity supports a recent infection, while high avidity supports an older one. In pregnancy, a positive result may require a detailed evaluation by a specialist experienced in the health of mother and baby.

Positivity thresholds and inconclusive result ranges vary between laboratories; age, sex and pregnancy status can also affect the clinical meaning of the result. The laboratory reference you can compare your result with is also given on this page. Especially in pregnancy, no decision should be based on a single result; the result should be interpreted by your doctor together with your symptoms and other tests.