What is being tested?
Cytomegalovirus (CMV) is a common virus that usually causes no symptoms or only mild illness. CMV testing detects antibodies in the blood that the body produces in response to the infection or detects CMV directly.
In the United States, as many as 60% of people have been exposed to CMV at some point in their life. Almost 1 out of every 3 children have been exposed to CMV by age 5 and more than half of adults are exposed to CMV by age 40.
Most people are infected as children or as young adults, but many don’t know it because CMV usually does not cause noticeable symptoms or only mild illness in otherwise healthy people. Individuals with mild illness may have non-specific signs and symptoms, such as sore throat, fever, tiredness, and swollen glands. In otherwise healthy adults, CMV infection may sometimes cause a flu-like illness or signs and symptoms similar to mononucleosis (mono), such as extreme fatigue, fever, chills, body aches, and/or headaches that usually resolve within a few weeks.
CMV is found in many body fluids during an active infection, including saliva, urine, blood, breast milk, semen, vaginal fluid, and cerebrospinal fluid. CMV easily spreads from person to person through close contact with someone who is infected or by contact with contaminated body fluids or objects, such as diapers or toys.
Once you are infected, CMV remains in your body for the rest of your life without causing symptoms. After your initial “primary” infection resolves, CMV becomes dormant or latent, like other members of the herpes family. However, if your immune system is significantly weakened, the virus can become active again (reactivate) and cause illness.
CMV can cause notable health problems in these situations:
- A pregnant woman who is infected for the first time (primary infection) during pregnancy can pass the infection to her developing baby across the placenta. This can cause serious physical and developmental problems in the baby. 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.
- CMV can cause serious illness and death in people with weakened immune systems, such as those with HIV/AIDS, transplant recipients, and cancer patients receiving chemotherapy. These individuals might experience the most severe symptoms and their CMV infection may remain active. CMV may reactivate in those who were previously exposed. The infection could affect the:
- 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
- Brain, causing encephalitis
- Spleen and liver
- Organ or bone marrow transplants, causing 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 test used?
Cytomegalovirus (CMV) testing is not used to test everyone for a CMV infection. It may be used help diagnose an active, reactivated, or past CMV infection in certain cases, such as:
- Some pregnant women or immune-compromised people with signs and symptoms
- People who may receive an organ or bone marrow transplant
- Newborns with certain birth (congenital) abnormalities
A few different methods of testing may be used depending on the purpose for testing:
Antibody testing (serology)
This type of test detects antibodies in the blood that are produced in response to a CMV infection. It can be used to diagnose a current or past infection by detecting and measuring two classes of CMV antibodies:
- IgM antibodies are produced by the body first in response to a CMV infection. They can be detected in the blood within a week or two after the initial exposure. IgM levels (titers) rise for a short time, then decline and usually fall below detectable levels after a few months. IgM antibody levels rise again when latent CMV is reactivated.
- IgG antibodies are produced several weeks after the initial CMV infection. IgG levels rise during the active infection, then stabilize as the CMV infection resolves and the virus becomes inactive. Once exposed to CMV, you will have some measurable amount of CMV IgG antibody in your blood for the rest of your life, which provides protection from getting another primary infection (immunity).
Health care practitioners compare the absence or presence of IgG and IgM antibodies in the same sample or the amount of antibody present (titer) in samples collected one to three months apart (acute and convalescent samples) to distinguish between active and latent CMV.
Direct detection of CMV
Molecular methods such as polymerase chain reaction (PCR) are used to diagnose congenital infections in newborns and may be used to detect and/or confirm active infections in others. These methods may be used to detect the genetic material (DNA) of CMV (qualitative testing) and/or measure the amount of viral DNA in a sample (quantitative, also called viral load). Molecular methods are more sensitive than culture, and positive and negative results are available in a short period of time.
Immune-compromised people with active CMV may be monitored using a variety of CMV tests. Often, healthcare practitioners monitor the amount of virus present (viral load) to determine a person’s response to antiviral therapy.
Viral cultures are not routinely available but may sometimes be used to detect CMV.
When is it ordered?
CMV tests may be ordered, along with tests for influenza, mononucleosis (mono), and EBV (Epstein-Barr virus), when 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
- Muscle aches
CMV tests may be ordered at regular intervals after a person has received a transplant or when a healthcare practitioner is monitoring the effectiveness of antiviral therapy.
CMV molecular testing may be done for a newborn or infant has:
- Yellowing of the skin and eyes (jaundice)
- Low platelet count
- An enlarged spleen and/or liver
- A small head
- Hearing and vision problems
- Signs of delayed mental development
When a person is a candidate for an organ or bone marrow transplant, CMV antibody testing may be ordered 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 care 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 previously infected with CMV will continue to harbor the virus. The CMV can reactivate intermittently and small amounts of virus can appear in body fluids but not cause 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.
- Tests done in the very early stages of an acute infection could have undetectable levels of CMV antibodies.
A positive CMV IgG and IgM when you have symptoms means it is likely that you have been exposed to CMV for the first time recently or a previous CMV infection has been reactivated. This can be confirmed by measuring IgG levels again 1 to 3 months 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 you have an active CMV infection (primary or reactivated).
A positive CMV IgM and negative IgG means you may have very recently been infected.
Negative or low IgG and/or IgM levels may mean that your symptoms are due to an infection other than CMV or possibly that your 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:
*Result is NOT diagnostic of primary infection
|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
|Past exposure (immunity to primary infection); latent infection
A positive CMV DNA test may mean CMV is present and you have, or the newborn tested, has an active infection. High levels of viral DNA may indicate a more severe infection with serious symptoms. Low levels may indicate a less severe CMV infection, usually one with no symptoms or ones that are mild, or low levels may mean there is no active infection.
Negative results on DNA test may mean you do not have a CMV infection, but it does not rule it out. The virus may be present in very low numbers or may not be present in the sample tested.
When used to monitor effectiveness of treatment, decreasing levels of CMV (viral loads) mean you are responding to antiviral treatment. Levels that do not drop in response to antiviral treatment might mean the virus is resistant to the therapy.
A positive CMV culture may mean you have an active infection. This can often be determined in as little as 1 to 2 days. 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.
Is there anything else I should know?
The CMV test is one of the tests 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, though it may also screen for other infections.
When blood transfusion is needed, certain patients, such as CMV-negative HIV/AIDS patients and CMV-negative heart/lung transplant candidates, should receive blood components that have tested negative for CMV antibodies (so-called CMV seronegative blood products) or products that are leucocytes reduced.
How can I tell if my cytomegalovirus (CMV) has reactivated?
If you are a reasonably healthy person, you will probably not have a symptomatic reactivation or may have mild flu-like symptoms. If you are immune-compromised, you may have more serious symptoms associated with your lungs, digestive tract, or eyes. In this case, it is important to talk to your health care provider about your health concerns.
If I have or had CMV, can I spread it to others?
If you have a new or prior infection with CMV, you can spread it to others even if you aren’t showing signs or symptoms. If symptoms appear, they can develop 9 to 60 days after primary infection. Nevertheless, you must be in close contact with others in order to transmit the virus. It can be spread through several types of body fluids, including saliva, breast milk, vaginal fluids, semen, urine, and blood.
Is there any way to prevent getting CMV?
Currently, there is no vaccine available for CMV. Careful hygiene can help prevent transmission of the virus. But, since CMV is very common, is present in most body fluids, and is passed through close contact, most people are infected early in life. It has been estimated that as many as 70% of children in daycare have been exposed to CMV, and between 8% and 20% of childcare providers contract CMV every year.