Cytomegalovirus (CMV)

No special preparation is needed. The sample required depends on why the test is being done and what your doctor suspects. 

• Blood sample: used for antibody testing (IgG and IgM) and for detecting CMV DNA (PCR) — taken from a vein in the arm 
• Urine sample: commonly used to test newborns for CMV 
• Other samples: in some situations — particularly in immunocompromised patients — samples from the lungs (sputum or bronchoalveolar lavage), cerebrospinal fluid, or other tissues may be needed. Your doctor will explain what is required 

What the test measures 

There are two main approaches to CMV testing, and they answer different questions. 

Antibody tests (IgM and IgG) 

When your body is infected with CMV, your immune system produces proteins called antibodies to fight the virus. CMV antibody tests detect two types: 

• CMV IgM: produced first, usually within one to two weeks of initial infection. Levels rise then fall over several months. IgM may also appear if a dormant infection reactivates 
• CMV IgG: produced one to three weeks after infection and remains detectable for life. A positive IgG result means you have been exposed to CMV at some point. A rising IgG level (measured on two samples taken two to three weeks apart) confirms a recent active infection 

By looking at IgM and IgG together — and whether IgG is rising — doctors can tell whether a CMV infection is new (primary), old (past exposure), or reactivated. 

Viral detection (PCR / DNA test) 

PCR (polymerase chain reaction) tests detect CMV’s genetic material (DNA) directly in blood, urine, or other body fluids. A positive PCR confirms the virus is present. The test can also measure how much virus is present — this is called the viral load. 

Viral load testing is particularly important in immunocompromised patients. A high or rising viral load suggests active infection. A falling viral load after treatment shows the antiviral therapy is working. 

CMV results can be complex to interpret and must always be considered alongside your symptoms and medical history. 

A positive IgG result alone simply means you have been exposed to CMV at some point in your life — it does not mean you have an active infection. 

Your doctor will explain what your specific result means for you. 

What your results mean 

For healthy adults with symptoms 

If both IgM and IgG are present, this suggests a recent primary infection or a reactivated infection. A second IgG blood test taken two to three weeks later helps confirm this — a fourfold rise in IgG level between the two samples confirms active infection. 

If only IgM is positive, this may mean a very recent infection (before IgG has had time to develop). If both are absent, CMV is unlikely to be the cause of your symptoms, though a repeat test may occasionally be needed. 

For pregnant women 

A positive IgG result with no IgM generally means you had CMV in the past and are now immune — this is reassuring, as reactivation during pregnancy rarely causes harm to the baby. A positive IgM alongside rising IgG suggests a new (primary) infection during pregnancy, which carries a higher risk of passing CMV to the unborn baby. 

If primary CMV infection is confirmed during pregnancy, your doctor or midwife will discuss what this may mean for your baby, the level of risk, and any further monitoring or specialist assessment that may be needed. 

For newborns and infants 

If a newborn has CMV IgM antibodies in their blood, this confirms congenital CMV infection (infection present from birth). Urine PCR testing in the first three weeks of life is the most reliable way to confirm congenital CMV. 

Many babies with congenital CMV have no obvious problems at birth but may develop hearing loss or other complications as they grow. Early detection allows monitoring and, where appropriate, treatment. 

For immunocompromised patients 

In people with a weakened immune system, antibody responses may be lower than expected, making antibody tests less reliable. Viral DNA testing (PCR) is therefore the preferred approach. Results are monitored regularly to detect early reactivation and to guide antiviral treatment decisions. 

A positive PCR with a high viral load confirms active CMV disease. A viral load that falls in response to antiviral treatment confirms the therapy is working. A viral load that does not fall may suggest the virus is resistant to the treatment being used.

For most healthy adults, CMV infection resolves on its own within a few weeks and no specific treatment is needed. Rest and managing symptoms is usually sufficient. 

If CMV is confirmed in a pregnant woman for the first time, she will usually be referred to a specialist in fetal medicine for further assessment, counselling, and discussion of options. This may include detailed ultrasound scanning and, in some cases, amniocentesis (testing the amniotic fluid) to check whether the baby has been infected. 

If congenital CMV is confirmed in a newborn, the baby will be referred to a paediatrician with expertise in this area. Some babies may benefit from antiviral treatment in the first months of life. All babies with confirmed congenital CMV should have their hearing monitored regularly. 

For immunocompromised patients with active CMV, antiviral treatment is usually started promptly. Regular viral load testing guides how long treatment continues. Your specialist team will manage this closely alongside your other care. 

• Weakened immune system: people with HIV, transplant recipients, or those on immunosuppressant medicines may produce lower-than-expected IgM and IgG levels even during active infection, making antibody tests less reliable 
• Organ transplant recipients: interestingly, solid organ transplant recipients tend to produce a stronger and longer-lasting IgM response than other patients, which can affect result interpretation 
• Very early infection: if the test is done very shortly after exposure, antibodies may not yet have had time to develop — a repeat test after two to three weeks may be needed 
• IgM cross-reactions: CMV IgM tests can occasionally give a positive result due to other infections (such as Epstein-Barr virus), so results must be interpreted carefully alongside symptoms and other tests 
• Sample type: CMV may be present in some body fluids but not others — a negative result in one type of sample does not completely rule out infection 
• Viral shedding in healthy people: healthy people who have had CMV in the past can intermittently shed small amounts of virus into body fluids without symptoms — a low-level positive PCR does not always indicate active disease 

• Epstein-Barr virus (EBV) antibodies — EBV causes a similar glandular fever-like illness and is often tested alongside CMV 
• SCORTCH test — a panel of tests for infections that can affect the unborn baby, including CMV, rubella, toxoplasmosis, and herpes 
• Herpes simplex virus testing — CMV belongs to the same virus family as herpes and may be tested alongside it 
• Chickenpox and shingles tests (varicella zoster virus) — another member of the herpes virus family sometimes investigated alongside CMV 
• Full blood count — to check for anaemia or abnormalities in white blood cells, which can occur with CMV infection 

CMV is a member of the herpes virus family and is one of the most common viruses worldwide. Once infected, the virus stays in the body for life in a dormant (inactive) state. It rarely causes problems in healthy people but can be serious during pregnancy or in people with a weakened immune system. There is no vaccine against CMV