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 efficacy 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 and is not currently ill. Even if the virus in her body is reactivated, or if she contracts another strain of the virus, the risk for fetal infection and illness is low, so most doctors will recommend not to conduct any special monitoring. See non-primary infection in the Q&A tab.
Positive IGG and positive IGM – a result that the woman was probably infected recently. Why “probably” and not certainly? Since the IGM value can remain positive for several months and even years after the primary infection. In addition, there may be a non-specific reaction from another virus other than CMV, which can result in a false positive of IGM value.
Negative IGG and negative IGM – the woman has never been infected with the CMV virus, and if she is pregnant, or plans a pregnancy in the near future, she is at risk for primary infection during pregnancy and has to take precautions.
In such a case, the Ministry of Health guidelines are as follows:
- Providing an explanation on the risk for primary infection with the CMV virus and instruction on how to reduce the risk for infection.
- Informing women tending to infants (mothers, kindergarten teachers and caretakers in nurseries and kindergartens) of the increased risk for exposure to the CMV virus due to the close contact with infants. Additionally, providing an explanation on the importance of close adherence with the instructions for the prevention of infection. See more details under the Q&A tab.
- Repeating the serology test for CMV virus IGG and IGM antibodies every 4-6 weeks until gestational week 30.
Negative IGG and positive IGM – this result can represent two conditions:
- New infection with the CMV virus, so that the IGG antibodies have not had time to rise yet (in this case, another blood test needs to be performed 2-4 weeks later).
- A false positive, resulting from the sensitivity of the IGM parameter, which can erroneously react to infection with other infectious agent other than the CMV virus, and which are not relevant for the fetus during pregnancy.
Additional results of blood tests:
Borderline IGM – can indicate the beginning or end of an active disease. In this case, the additional data need to be examined, in order to determine the case.
VIDAS or Additional Assay – a more specific method for measuring IGM and IGG antibodies, which makes it possible to obtain more accurate data.
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 tests’ results is made by a person with expertise on the subject, in order to avoid overdiagnosis or underdiagnosis
You can find the list of expert doctors and the CMV clinic under the tab Important Telephone Numbers.
