How to read the CMV blood tests?
Antibodies are the primary component of the immune system.
The CMV virus blood tests check three parameters, including two types of antibodies:
IGG antibodies and IGM antibodies:
IGG antibodies provide the body with long-term protection after infection, and a positive result for these antibodies in the blood shows that the tested woman was infected with the virus in the past.
IGM antibodies are the antibodies that appear first following primary infection.
In addition, a value called AVIDITY can be tested, which tests the efficiancy of the IGG antibodies, and can therefore help date the estimated time of infection.
The HMOs use different lab kits, and therefore different labs have a different scale for this parameter. Nevertheless, according to all value scales, a high AVIDITY value will be evidence of infection that occurred more than three months before the blood test was performed.
Possible results:
Positive IGG and negative IGM – The woman was infected in the past.
Since it is difficult to detect a non primary infection through blood tests, it is recommended not to repeat testing before or during pregnancy, to avoid unnecessary tests and stress. See non primary infection in the Q&A tab.
Positive IGG and positive IGM –
If the test was performed up to week 8 of pregnancy:
If the avidity value is low, this indicates a new infection and consultation is recommended.
If the avidity value is borderline, the infection occurred less than three months earlier and consultation is recommended.
If the avidity value is high, the infection occurred more than three months earlier and consultation may be sought.
Whenever the IGM result is positive, a Vidas or Additional Assay test should also be performed. These tests are more specific and therefore their result should be the one considered. If these tests are negative, the positive IGM was likely a nonspecific reaction to another virus that is not CMV, causing a false positive result.
It should also be taken into account that IGM levels may remain positive for several months or even years after the initial infection, and this should not be considered significant.
Negative IGG and negative IGM
The woman has never been infected with CMV.
In this case, if she is pregnant or planning a pregnancy in the near future, she is at risk for a primary infection during pregnancy and should take extra precautions.
The recommendations in such a case are:
Provide an explanation about the risk of primary CMV infection and instructions for reducing the risk. See the prevention tab.
Inform women who care for young children (mothers, kindergarten teachers and daycare caregivers) that they are at increased risk of exposure to CMV due to close contact with toddlers bodily fluids, and explain the importance of strict adherence to prevention guidelines.
Repeat the serologic test for IGG and IGM antibodies at weeks 11 to 14 and again at weeks 16 to 19.
Negative IGG and positive IGM –
This result may reflect two possible situations:
A recent CMV infection, in which IGG levels have not yet risen.
In this case, another blood test should be performed after two weeks.
If in the repeat test IGG becomes positive before conception, it is advisable to postpone pregnancy by three months.
A false positive result, due to the sensitivity of the IGM marker, which may incorrectly respond to infections from other pathogens that are not CMV and are not relevant to the fetus.
Additional results of blood tests:
Borderline IGM – can indicate that an active disease is starting or ending. In this case, the additional data need to be examined, in order to determine the stage.
VIDAS or Additional Assay – a more specific method for measuring IGM and IGG antibodies, which makes it possible to obtain more accurate data.
Weak positive IGG and negative IGM
The test should be repeated after two weeks, and if the results remain the same, the woman should be considered negative.
The range of options and the fact that in Israel, many labs carry out the serology tests for the CMV virus, stress how important it is that the reading of these lab test results is made by a person with expertise on the subject, in order to avoid overdiagnosis or underdiagnosis
You can find a list of CMV specialists and CMV clinics under the tab Important Telephone Numbers.
