Cytomegalovirus (CMV) Testing

Section Editor: Rodney E. Rohde PhD
June 3, 2026

Cytomegalovirus, commonly abbreviated as CMV, is a very common virus. Most people are infected with it at some point in life, often in childhood, and in healthy people, it usually causes no symptoms or only a mild illness. After an infection, CMV stays in the body for life in an inactive form, much like other viruses in its family. For most people, a CMV result is reassuring and needs no action. CMV matters most in three situations: during pregnancy, in newborns, and in people with weakened immune systems.

This article explains the tests used to check for CMV, including antibody tests, an avidity test, and a viral load test, and what the wording on your report means, so you can better understand a report you have received.

What is CMV, and who needs testing?

CMV is a member of the herpesvirus family. In healthy children and adults, a CMV infection often causes no symptoms or a mild illness similar to mononucleosis (mono), with tiredness, fever, and a sore throat. Once you have been infected, the virus remains in your body for life and can occasionally become active again, usually without causing any problems.

Because CMV is so common and usually harmless, testing is not done for most people. It is most often done in three situations:

During pregnancy, a new CMV infection can sometimes pass to the baby.
In newborn babies, to check for an infection that happened before birth.
In people with a weakened immune system, such as those who have had an organ or bone marrow transplant, CMV can become active and cause illness.

CMV antibody tests (IgM and IgG)

Antibodies are proteins that the immune system makes in response to a virus. The two CMV antibodies most often tested are IgM and IgG.

CMV IgM — Tends to appear early in an infection, so that a positive result can point to a recent or current infection. On its own, however, CMV IgM is not a reliable indicator of timing because it can remain positive for months, may be falsely positive, and can reappear if a past infection reactivates.
CMV IgG — Appears later and then stays positive for life. A positive result means you have been infected with CMV at some point.

Because CMV IgM cannot reliably tell a brand-new infection apart from an older one, an additional test called the avidity test is often used to help.

The IgG avidity test

Avidity describes how tightly IgG antibodies bind to the virus. Early after an infection, IgG binds loosely, which is called low avidity. Over the following months, it matures and binds more tightly, which is called high avidity. This change over time is what makes the test useful, as explained in the article on IgM and IgG.

A low-avidity result supports a recent infection, usually within the last few months. A high-avidity result supports an infection that happened longer ago. This is especially helpful in pregnancy, where knowing whether an infection is recent or old changes what happens next, and a high-avidity result is generally reassuring because it suggests the infection was not recent.

CMV viral load (PCR) test

The viral load test, also called a CMV DNA test or a PCR test, looks directly for the virus’s genetic material and measures how much is present. Unlike the antibody tests, which look for your body’s response, the viral load test shows whether the virus itself is active right now.

It is used mainly in two situations: to diagnose a CMV infection in a newborn, often from a urine or saliva sample in the first weeks of life, and to detect and monitor active CMV in people with a weakened immune system, including guiding treatment and checking how well it is working.

How to read your results together

The meaning of your result depends on which tests were done and how they fit together. The points below cover the most common patterns.

IgM negative, IgG negative — No sign of past or current CMV infection. In pregnancy, this means there is no immunity yet, so steps to avoid infection are worth discussing.
IgM negative, IgG positive — A past CMV infection. The virus was encountered at some time before and is not causing a new infection.
IgM positive, IgG negative — A recent (new) infection.
IgM positive, IgG positive — Could be a recent infection, a past infection becoming active again, or IgM lingering from an earlier infection. An avidity test often helps distinguish these.
Low IgG avidity — Supports a recent infection, usually within the last few months.
High IgG avidity — Supports an infection that happened more than a few months ago.
Viral load detected — The virus is active. This result matters most for newborns and people with weakened immune systems.

CMV in pregnancy and newborns

CMV is the most common infection passed from a mother to her baby during pregnancy. The risk of passing it on is higher when a mother has a new, first-time CMV infection during pregnancy, which is why antibody avidity and sometimes viral load tests are used together to estimate whether the infection is recent.

Most babies exposed to CMV are born healthy. Some, however, can cause hearing loss or developmental problems, which is why testing and follow-up matter. In a newborn, an infection that happened before birth (called congenital CMV) is diagnosed by finding the virus with a viral load (PCR) test, usually on urine or saliva, within the first few weeks of life. Testing this early is important because after that point, it becomes difficult to distinguish an infection from before birth from one caught afterward. Antibody tests are less useful in newborns because IgG can cross from the mother to the baby, as explained in the article on IgM and IgG. A specialist usually guides care in these situations.

What happens after these tests

CMV testing shows where you stand with the virus and informs the decisions you and your healthcare team make together, rather than dictating them on its own.

In a healthy person, a CMV infection usually clears on its own and needs no specific treatment. In pregnancy, results are interpreted by your provider, often together with a specialist in pregnancy care, to estimate the timing of any infection and to plan monitoring. In a newborn with confirmed congenital CMV, babies are followed closely, including hearing checks, and some are treated with antiviral medicine, guided by a specialist. In a person with a weakened immune system, active CMV may be treated with antiviral medicines and followed by repeat viral load tests to assess treatment response.

Questions to ask your doctor

Based on my results, do I have a recent CMV infection, a past one, or none at all?
What did my avidity test show about when the infection may have happened?
If I am pregnant, what do these results mean for my baby and me, and should I see a specialist?
How can I lower my chance of catching CMV during pregnancy?
If my newborn is being tested, how and when is congenital CMV confirmed?
I have a weakened immune system. What does my viral load mean, and do I need treatment?
Could my CMV IgM be a false positive or leftover from a past infection?
Will I need to repeat tests to follow this over time?
Is CMV likely to cause me any problems in the future?
Who should I contact with questions about my results or my care?