Interpretation of Laboratory Tests
Serologic tests that detect CMV antibodies (IgM and IgG antibody to CMV) are widely available from commercial laboratories. The enzyme-linked immunosorbent assay (ELISA) is the most common serologic test for measuring antibody to CMV.
- A positive test for CMV IgG indicates that a person was infected with CMV at some time during their life, but does not indicate when a person was infected. This applies for persons ≥12 months of age when maternal antibodies are no longer present.
- Measurement of CMV IgG in paired samples taken one to three months apart can be used to diagnose primary infection; seroconversion (1st sample IgG negative, 2nd sample IgG positive) is clear evidence for recent primary infection.
- The presence of CMV IgM cannot be used by itself to diagnose primary CMV infection because IgM can also be present during secondary CMV infection. IgM positive results in combination with low IgG avidity results are considered reliable evidence for primary infection, with limitations of avidity testing described below.
- IgG avidity testing may provide useful information regarding timing of infection. IgG antibodies produced following primary CMV infection have low avidity (low binding strength). Two to four months following infection, IgG antibodies mature to high-avidity (high binding strength). Therefore, avidity assays can be used to assess low avidity (which indicates recent infection) versus high avidity (past infection). However, not all avidity assays have been validated and, therefore, should be interpreted with caution. The review by Prince and Lapé-Nixon (CVI, 21:1377, 2014)External lists commercial tests for CMV IgG avidity that can be purchased in the United States, and describes challenges with interpreting CMV IgG avidity testing results.
The standard laboratory test for diagnosing congenital CMV infection is polymerase chain reaction (PCR) on saliva, with urine usually collected and tested for confirmation. The reason for the confirmatory test on urine is that most CMV seropositive mothers shed CMV virus in their breast milk. This can cause a false-positive CMV result on saliva collected shortly after the baby has breast fed.
Specific steps for appropriate collection of saliva samples from a baby are as follows:
- Collect a saliva specimen more than one hour after breastfeeding and within three weeks of birth, because detection of CMV after three weeks could be the result of post-partum infection.
- Insert a sterile cotton or polyester swab into the baby’s mouth between the gum and cheek and swirl for several seconds.
- Remove the swab and place into a buffer formulated for PCR diagnostic testing (several are available). If CMV is present, it will leach from swab to the liquid.
The liquid is processed according to manufacturer’s instructions, and PCR testing is performed according to the protocol in the laboratory. Specific procedures and interpretation of tests vary according to the laboratory.
Currently, testing of newborns for CMV is not routinely performed, though some states perform targeted CMV testing of newborns who fail the hearing screening. CDC is currently studying whether dried blood spots (DBS), which are already collected on almost all newborns, can identify the majority of children most likely to suffer long-term health problems from congenital CMV.