Indications for Testing
CMV is a common virus. Most people with this infection have no symptoms. However, the following populations warrant consideration for laboratory testing :
- Immunocompetent patients presenting with mononucleosislike illness after testing negative for Epstein-Barr virus (EBV)
- Immunocompromised patients presenting with febrile illness, hepatitis, and colitis
- Pregnant women with possible active CMV infection (infection immediately before or during the first trimester of pregnancy increases the risk of cytomegalic inclusion disease in the fetus)
- Neonates with congenital syndromes (eg, hepatosplenomegaly, microcephaly, sensorineural deafness) suggestive of CMV
- Patients receiving organ transplants
Molecular testing (eg, quantitative PCR) for viral DNA is the preferred test for detecting CMV infection because it
- Quantifies viral load
- Provides rapid and early detection
- Offers higher sensitivity than culture
PCR quantitative testing is used for both pre- and posttransplant testing for CMV status. Serology is not recommended for diagnosis in this population.
Serologic assays are the primary tools for assessing primary CMV infections during pregnancy. Testing for maternal CMV infection generally occurs after concerning ultrasound findings. For women with possible primary CMV infections during pregnancy, diagnosis should be based on either IgG seroconversion or positive CMV IgM and IgG and low IgG avidity.
Interpretation of CMV Serology Test Results
||Suggests past infection
Only applicable for persons ≥12 mos of age (when maternal antibodies are no longer present)
|Paired CMV IgG samples (taken 1-3 mos apart)
||Can be used to diagnose primary infection
Seroconversion (initial IgG-negative sample followed by positive second sample) offers clear evidence for recent primary infection
||In isolation, not helpful in diagnosing primary CMV infection because IgM can also be present during recurrent CMV infection
|Source: CDC, 2016
CMV IgG avidity testing aids in distinguishing between primary and recurrent CMV infection in pregnant women after initial serologic testing.
Interpretation of CMV IgG Avidity Test Results
|Low IgG avidity
||Suggests primary CMV infection occurred within the last 2-4 mos
During first trimester, patients should seek consultation with obstetrician experienced with congenital CMV infections
|Intermediate IgG avidity
||Clinical relevance undetermined
Lower risk of intrauterine transmission
|High IgG avidity
||Suggests past infection
|Note: Since not all avidity assays have been validated, they should be interpreted with caution.
Source: CDC, 2016
CMV can be detected in the amniotic fluid of infected fetuses by either culture or PCR. PCR testing of amniotic fluid is most sensitive at >21 weeks gestation. The detection of CMV in amniotic fluid does not predict the severity of congenital CMV infection.
PCR testing of saliva can also identify CMV infection. Following positive results, PCR testing of urine is used for confirmation (most seropositive mothers shed CMV in breast milk, which can lead to a false-positive saliva test result in a recently breastfed infant). Refer to the CDC website for information on testing for congenital CMV infection in newborns.
For most healthy individuals who acquire CMV after birth, there are very few disease symptoms and no long-term health consequences. Infected individuals may present with mononucleosislike symptoms. CMV-induced mononucleosis can be symptomatically indistinguishable from EBV-induced mononucleosis. EBV is most often the causative agent, but CMV might be considered in the differential diagnosis if a negative heterophil antibody test rules out EBV mononucleosis. In most patients in the symptomatic phase of the disease, CMV IgM antibodies will be positive. However, IgM antibodies may not peak until 4-7 weeks into the infection and may remain elevated for up to 1 year or longer. IgG antibodies should continue to increase at least fourfold during active infection. Therefore, in an otherwise healthy patient in whom EBV has been ruled out, it is not necessary to repeat a CMV IgM test if a previous IgM test result was positive.
PCR quantitative testing is used to monitor individuals at risk of developing CMV after solid organ or hematopoietic stem cell transplant. Quantitation of CMV viral load in plasma correlates with progression of disease. Monitor according to site-specific transplant protocols.