What is the initial test for suspected Cytomegalovirus (CMV) infection in a pregnant woman?

Diagnostic Testing for Suspected CMV Infection in Pregnant Women

For suspected CMV infection in pregnant women, the initial diagnostic test should be CMV immunoglobulin G (IgG) and IgM antibody testing with IgG avidity testing when applicable. 1, 2

Initial Serologic Testing Algorithm

1. First-line testing:

   • CMV IgG and IgM antibody testing
   • IgG avidity testing (if IgG positive)

2. Interpretation of results:

   • IgM negative and IgG negative: No evidence of CMV infection
   • IgM positive and IgG positive with low avidity: Suggests primary CMV infection within the past 3-4 months
   • IgM positive and IgG positive with high avidity: Suggests non-primary infection (reactivation or reinfection)
   • IgG positive, IgM negative with high avidity: Past infection, low risk of congenital transmission

Follow-up Testing Based on Initial Results

• If primary infection is suspected (IgM positive and IgG positive with low avidity, or IgG seroconversion):

  • Amniocentesis with PCR for CMV DNA is the best confirmatory test for fetal infection 1
  • Optimal timing: after 21 weeks gestation and >6 weeks from maternal infection 1

• If results are inconclusive:

  • Serial serologic testing may be helpful
  • High CMV IgM antibody titer is a strong predictor of low IgG avidity, suggesting primary infection 3

Ultrasound Monitoring

• If CMV infection is confirmed or strongly suspected, ultrasound monitoring every 2-3 weeks is recommended to detect:
  • Echogenic fetal bowel
  • Cerebral calcifications
  • Microcephaly
  • Ventriculomegaly
  • Hydrops fetalis
  • Intrauterine growth restriction 4, 5

Important Considerations and Pitfalls

• False negatives can occur: A negative maternal CMV IgM in the second trimester cannot definitively exclude congenital CMV infection 6

• Timing matters: IgM antibodies may become negative within weeks to months after primary infection, potentially missing infections that occurred early in pregnancy 7, 6

• Avidity testing is time-sensitive:

  • Low avidity (<40%) indicates primary infection within the last 20 weeks with high reliability 7
  • Intermediate avidity typically excludes primary infection within the last 12 weeks 7

• Risk assessment: Primary CMV infection in early pregnancy carries the highest risk for congenital CMV and adverse fetal outcomes 5

• Testing limitations: CMV IgG avidity testing may not be widely available in all laboratories in the United States 3

Prevention During Pregnancy

While not directly related to testing, all pregnant women should be counseled on hygiene-based preventive measures, especially those in regular contact with young children:

• Thorough handwashing after contact with young children
• Avoiding sharing food, drinks, or utensils with young children
• Avoiding contact with saliva and urine of young children 4, 5

Remember that early detection is crucial, as the risk of harm to the fetus is greatest following primary CMV infection in early pregnancy, and antiviral treatment may be considered in confirmed cases of fetal infection 5.