How Is CMV Diagnosed?

The highest concentration of CMV is usually found in a woman's urine, saliva, and breast milk. The most effective means of diagnosing the infection is for your doctor to take a urine or blood sample and test it for either the virus itself or for the antibodies to the virus. To check for fetal infection, your doctor tests your baby's blood or amniotic fluid by one or more of the following tests:

  • in recent years, much attention has focused on analysis of amniotic fluid and fetal serum as a means to diagnose CMV infection in the fetus. Several authors have compared the relative value of the following diagnostic tests: viral cultures of the amniotic fluid and or fetal serum in a laboratory, then looking under a microscope for the virus itself;
  • identifying anti-CMV IgM in fetal serum (identifying the antibodies in your baby's blood that fight CMV infection);
  • determining the IgM concentration in fetal serum; and
  • assessing your baby's liver function tests.

At this point, most experts agree that testing a baby's amniotic fluid produces more accurate results than testing a baby's blood. These reports uniformly have supported the superiority of amniotic fluid analysis (viral culture or nucleic acid probe for viral antigen) in confirming the diagnosis of congenital CMV infection. A sample of amniotic fluid can be obtained for analysis through a procedure called amniocentesis. In this procedure, a small needle (about the size of a pencil point) is inserted through the mother's skin into the amniotic cavity sac. The risk of spontaneous abortion (miscarriage) from this procedure is approximately about one in 200. However, to extract a sample of your baby's fetal blood, your doctor must insert a needle into the umbilical cord (cordocentesis), which is much more difficult. Fetal blood sampling is very difficult when you are less than 20 weeks pregnant. Amniocentesis, however, can be performed easily when you are between 15 and 16 weeks pregnant.

Technical Information About Diagnostic Tests.

Techniques such as polymerase chain reaction (PCR) can identify the DNA of a virus within 24 hours. Antibody tests are also helpful in diagnosing CMV. During an initial infection, the viral-specific IgM antibody can be found in a patient's blood. Over the next 30 to 60 days, these levels decline. Though there is no absolute IgG antibody test that can clearly differentiate a new infection from a recurrent one, a fourfold or greater change in a patient's IgG level generally implies a recent, acute infection.