Dr Leigh Martens

General Practitioner

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Blood-tests Explained


Disclaimer:

This website is intended to assist with patient education and should not be used as a diagnostic, treatment or prescription service, forum or platform. Always consult your own healthcare practitioner for a more personalised and detailed opinion

CMV Tests


At a Glance

Why Get Tested?

If your health practitioner suspects that you presently have, or recently had, a cytomegalovirus (CMV) infection or if it is important to know if you have ever had a CMV infection, such as prior to receiving an organ transplant

When to Get Tested?

When a young adult, a pregnant woman, or an immune-compromised person has flu-like symptoms that suggest a CMV infection; when a newborn has multiple congenital abnormalities, unexplained jaundice or anemia, and/or when an infant has seizures or developmental problems that may be due to CMV; prior to receiving an organ transplant

Sample Required?

A blood sample drawn from a vein in your arm for CMV antibody testing; to detect the virus itself, the sample may be blood, urine, sputum, amniotic fluid, cerebrospinal fluid, duodenal fluid, or other body tissue

Test Preparation Needed?

None

The Test Sample

What is being tested?

Cytomegalovirus (CMV) is a common virus that occurs widely throughout the population but rarely causes symptoms. In the United States, as many as 50-85% of adults have been infected with CMV. Most people are infected as children or as young adults and do not experience any significant symptoms or health problems.

CMV testing involves either a measurement of CMV antibodies, immune proteins produced in response to CMV exposure, or the detection of the virus itself. The virus can be identified during an active infection by culturing CMV or by detecting the virus's genetic material (its DNA) in a fluid or tissue sample.

CMV is found in many body fluids during an active infection, including saliva, urine, blood, breast milk, semen, vaginal secretions, and cerebrospinal fluid. It is easily transmitted to others through close physical contact or by contact with infected objects, such as diapers or toys. After the initial "primary" infection has resolved, CMV becomes dormant or latent, like other members of the herpes family. Cytomegalovirus remains in a person for the rest of the person's life without causing any symptoms unless the person's immune system is significantly weakened. If this happens, the virus can reactivate.

CMV can cause notable health problems in three situations:

  • In young adults, primary CMV infection may cause a flu-like or mononucleosis-type illness. This condition, which causes symptoms such as extreme fatigue, fever, chills, body aches and/or headaches, usually resolves within a few weeks. 
  • In infants, primary CMV infection may cause serious physical and developmental problems. This occurs when a woman is infected for the first time (primary infection) during pregnancy and then passes the infection to her developing baby across the placenta. Most newborns (about 90%) who are infected appear healthy at birth but may develop hearing or vision problems, pneumonia, seizures, and/or delayed mental development a few months later. A few babies may be stillborn, while others may have symptoms at birth such as jaundice, anemia, an enlarged spleen or liver, and a small head.
  • In those with weakened immune systems, CMV could cause serious illness and death. This includes those with HIV/AIDS, those who have had organ or bone marrow transplants, and those undergoing chemotherapy treatment for cancer. People with compromised immune systems who become infected for the first time (primary infection) might experience the most severe symptoms and their CMV infection may remain active. Those who have been exposed to CMV previously may reactivate their infection. This could affect their eyes (causing inflammation of the retina, which can lead to blindness), digestive tract (causing bloody diarrhea and abdominal pain), lungs (causing pneumonia with a non-productive cough and shortness of breath), and brain (causing encephalitis). There can also be spleen and liver involvement, and those who have had organ or bone marrow transplants may experience some degree of rejection. Active CMV also further depresses the immune system, allowing other secondary infections such as fungal infections, to occur.

How is the sample collected for testing?

The sample required depends on the type of testing. Antibody testing requires a blood sample, obtained by inserting a needle into a vein in the arm. Viral detection may be done on a variety of samples, including urine, blood, or sputum. Some samples may require a special procedure to collect, such as amniotic fluid, duodenal fluid, cerebrospinal fluid, or body tissue (biopsy).

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

How is it used?

Cytomegalovirus (CMV) testing is used to determine whether someone with signs and symptoms has an active CMV infection. Sometimes it may be ordered to help determine whether someone had a prior CMV infection.

CMV is a common virus that occurs widely throughout the population but rarely causes noticeable symptoms or significant health problems. However, primary CMV infection may cause serious illness and complications in newborns and people with weakened immune systems, such as transplant recipients, cancer patients, people receiving immunosuppressive drugs, and people with HIV. (See the "What is being tested?" section for more on this.)

There are a few different methods of detecting a CMV infection. The choice of tests and samples collected depends on the age of the person, their general health status and symptoms, and on the health practitioner's clinical findings and suspicions of organ involvement.

Antibody testing (serology)
Antibody testing is done on blood samples and can be used to determine if someone has had recent or past exposure. There are two types of CMV antibodies that are produced in response to a CMV infection, IgM and IgG, and one or both may be detected in the blood.

  • IgM antibodies are the first to be produced by the body in response to a CMV infection. They are present in most individuals within a week or two after the initial exposure. IgM antibody production rises for a short time period and then declines. After several months, the level of CMV IgM antibody usually falls below detectable levels. Additional IgM antibodies are produced when latent CMV is reactivated.
  • IgG antibodies are produced by the body several weeks after the initial CMV infection and provide protection from primary infections. Levels of IgG rise during the active infection, then stabilize as the CMV infection resolves and the virus becomes inactive. After a person has been exposed to CMV, the person will have some measurable amount of CMV IgG antibody in their blood for the rest of their life. CMV IgG antibody testing can be used, along with IgM testing, to help confirm the presence of a recent or previous CMV infection.

CMV antibody testing may be used to determine immunity to primary CMV infections for people prior to organ or bone marrow transplantation and for a person diagnosed with HIV/AIDS. Since CMV infection is widespread and causes few problems to those with healthy immune systems, general population screening is rarely done.

Antibody testing and viral CMV detection may be used to help diagnose primary CMV infection in young adults, pregnant women, and some immune-compromised people with flu- or mononucleosis-like symptoms. By comparing the absence or presence of IgG and IgM antibodies in the same sample or the amount of antibody present in samples collected on different days, a health practitioner may be able to distinguish between active and latent CMV.

Direct detection of CMV
Viral detection involves determining the presence of CMV in a blood, fluid, or tissue sample. This can be done either by culturing the virus or by detecting the virus's genetic material (CMV DNA). Viral detection is used to diagnose congenital infections in newborns and may be used to detect and/or confirm active infections in others.

Viral culture is the traditional method of virus detection. Presence of the virus (positive cultures) can often be determined in as little as 1 to 2 days, but cultures that are negative for the virus must be held for 3 weeks to confirm the absence of CMV because the virus may be present in very low numbers in the original sample and/or the CMV strain may be slow-growing.

Molecular methods such as polymerase chain reaction (PCR) may be used to detect and measure the amount of viral DNA in a person's sample. Testing can be qualitative, determining the presence or absence of CMV, or quantitative, measuring the amount of virus present.

Immune-compromised people with active CMV may be monitored using a variety of CMV tests. Often, health practitioners want a quantifiable viral test to be able to track the amount of virus present (viral load). They can use a quantitative test to monitor a person's response to antiviral therapy.

When is it ordered?

CMV tests may be ordered, along with tests for influenza, mononucleosis (mono), and EBV (Epstein-Barr virus), when a young adult, a pregnant woman, or an immune-compromised person has flu- or mono-like signs and symptoms such as:

  • Fatigue, weakness
  • Sore throat
  • Swollen lymph nodes
  • Fever
  • Headache
  • Muscle aches

Other less common but more serious signs include inflammation of the lungs, eyes, liver, spleen, and/or digestive tract.

One or more CMV tests may be ordered at intervals when a health practitioner is monitoring the effectiveness of antiviral therapy.

CMV culture or molecular testing may be done for a newborn with jaundice, anemia, an enlarged spleen and/or liver, and a small head; or for an infant with hearing and vision problems, pneumonia, seizures, and/or signs of delayed mental development.

When a person is a candidate for an organ or bone marrow transplant, CMV antibody testing may be ordered as a screening test to determine if the person has been exposed to CMV in the past.

What does the test result mean?

Care must be taken when interpreting the results of CMV testing. A health practitioner evaluates the results in conjunction with clinical findings, including signs and symptoms. It can sometimes be difficult to distinguish between a latent, active or reactivated CMV infection. This is due to several reasons, including:

  • A healthy person who has been infected with CMV at one time will continue to harbor the virus. The CMV can reactivate intermittently, often sub-clinically, shedding small amounts of virus into body fluids but not causing symptoms. 
  • An immune-compromised person may not have a strong antibody response to the CMV infection; the person's IgM and IgG levels may be lower than expected even though the person has an active case of CMV. 
  • The virus may not be present in sufficient number in the particular fluid or tissue tested to able to be detected.

Antibody testing
A positive CMV IgG and IgM in a symptomatic person means it is likely that the person has either recently been exposed to CMV for the first time or that a previous CMV infection has been reactivated. This can be confirmed by measuring IgG levels again 2 or 3 weeks later. A high level of IgG is not as important as a rising level. If there is a 4-fold increase in IgG between the first and second sample, then the person has an active CMV infection (primary or reactivated).

A positive CMV IgM and negative IgG means the person may have very recently been infected.

A negative IgG and/or IgM or low levels of the antibodies in someone who is symptomatic may mean that the person either has a condition other than CMV or that the person's immune system is not responding normally (not producing an adequate amount of antibody even if CMV is present).

The following table summarizes possible antibody testing results:

CMV, IgM CMV, IgG Possible Interpretation
Negative Negative
  • No current or prior infection; no immunity (person is susceptible to primary infection)
  • Symptoms due to another cause
  • OR immune system cannot produce adequate amount of antibody (immunocompromised)
Positive Negative
  • Recent active primary infection
  • OR person re-exposed to CMV
  • OR reactivation of latent CMV

*Result is NOT diagnostic of primary infection
Positive Positive (with four fold increase in titer between first sample and another collected later (acute and convalescent samples) Likely active primary or reactivated latent infection
Negative Positive Past exposure (person is immune from primary infection); latent infection

Viral detection
If a person is symptomatic and the culture is positive for cytomegalovirus, then the person likely has an active CMV infection. If the culture is negative, then the person's symptoms may be due to another cause or the amount of CMV virus in the sample is too low to detect.

If a test for CMV DNA is positive, then CMV is present and the person has an active infection. High levels of viral DNA tend to indicate a more invasive infection accompanied by serious symptoms while low levels indicate a CMV infection, usually one with no symptoms or ones that are mild. Like culture, negative results on a DNA test do not rule out CMV infection; the virus may be present in very low numbers or may not be present in the body sample tested.

When used to monitor effectiveness of treatment, decreasing viral loads reflect a response to antiviral treatment. Levels that do not drop in response to antiviral treatment might reflect a resistance to the therapy being used.

Is there anything else I should know?

CMV is one of the conditions included in a TORCH testing panel. This panel of tests screens for a group of infectious diseases that can cause illness in pregnant women and may cause birth defects in their newborns. TORCH is an acronym for: Toxoplasmosis, Rubella, Cytomegalovirus, and Herpes simplex virus.

When blood transfusion is needed, certain patients, such as CMV-negative HIV/AIDS patients and CMV-negative heart/lung transplant candidates, should receive cellular blood products that have been tested negative for CMV antibodies (so-called CMV seronegative blood products).