What tests can detect current Cytomegalovirus (CMV) and Rubella infections in a pregnant patient with a history of IgG (Immunoglobulin G) CMV and IgG Rubella positivity, and does she require a Polymerase Chain Reaction (PCR) test?

Testing for Current CMV and Rubella Infection in a Pregnant Patient with Positive IgG

In a pregnant patient with positive IgG for both CMV and rubella, you need IgM testing and IgG avidity testing to detect current infection; PCR testing is indicated only if IgM is positive or if fetal abnormalities are detected on ultrasound. 1, 2

Initial Serologic Testing Strategy

For CMV Assessment:

• Order CMV IgM antibody testing immediately to distinguish between past immunity (IgG alone) and potential current/recent infection (IgG + IgM positive) 1, 2
• If both IgG and IgM are positive, IgG avidity testing is essential to determine timing of infection 1
  • Low IgG avidity indicates primary infection within the past 3-4 months 2
  • High IgG avidity suggests infection occurred >3-4 months ago and rules out recent primary infection 2
• Positive IgG alone indicates past exposure with immunity and carries low risk for congenital CMV 1

For Rubella Assessment:

• Order rubella-specific IgM antibody testing between 1-2 weeks after any rash onset or exposure for optimal detection 3
• If drawn earlier than 1 week or later than 4-5 weeks after rash onset, IgM may be falsely negative 3
• Paired sera testing can demonstrate a fourfold rise in IgG titer between acute and convalescent specimens (drawn 10+ days apart) 3
• For exposure without rash, draw acute specimen immediately and convalescent specimen 28+ days after exposure 3

When PCR Testing is Required

CMV PCR Indications:

• Amniocentesis with CMV DNA PCR is indicated when:
  • Maternal primary infection is confirmed (positive IgM with low avidity) 2
  • Perform amniocentesis at least 7-8 weeks after presumed maternal infection AND after 17-21 weeks gestation 4, 2
  • This timing allows adequate viral replication in fetal kidneys and secretion into amniotic fluid 2
• Quantitative CMV DNA in amniotic fluid may assist in predicting fetal outcome 2
• Serial ultrasound every 2-4 weeks should be performed after confirmed fetal infection to detect abnormalities (hyperechogenic bowel, ventriculomegaly, IUGR), though absence of findings doesn't guarantee normal outcome 5, 2

Rubella PCR Considerations:

• PCR testing for rubella is not routinely required if serologic confirmation is adequate 3
• Focus remains on serologic methods (IgM and paired IgG titers) for diagnosis 3

Critical Diagnostic Algorithm

Step 1: Patient has positive IgG for CMV and rubella (known immunity)

Step 2: If symptomatic (rash, flu-like illness) or exposed:

• Order CMV IgM + rubella IgM immediately 1, 2
• If CMV IgM positive → order IgG avidity testing 1

Step 3: Interpret results:

• IgG positive + IgM negative = Past infection only, no current infection, low risk 1
• IgG positive + IgM positive + low avidity = Primary infection, 30-40% transmission risk 2
• IgG positive + IgM positive + high avidity = Secondary/reactivation infection, much lower transmission risk 2

Step 4: If primary CMV infection confirmed:

• Wait minimum 7-8 weeks after infection AND until ≥17-21 weeks gestation 4, 2
• Perform amniocentesis with CMV DNA PCR 2
• Begin serial ultrasounds every 2-4 weeks 2

Common Pitfalls to Avoid

• Do not assume IgG positivity alone rules out current infection - reactivation or reinfection can occur despite pre-existing immunity 1, 4
• Do not perform amniocentesis too early - waiting <7 weeks after maternal infection or performing before 17 weeks gestation yields false-negative results 2
• Do not rely on negative IgM to rule out CMV in pregnancy - fetal infection can occur with maternal reactivation even when IgM is negative 1
• False-positive CMV IgM occurs frequently with Epstein-Barr virus infection or other immune activation 1
• For rubella, timing of specimen collection is critical - IgM drawn too early (<1 week) or too late (>4-5 weeks) after rash may be falsely negative 3
• Do not order routine CMV/rubella screening in asymptomatic pregnant women without exposure history 2

Special Considerations

• Serologic testing may be considered for pregnant women with influenza-like illness or ultrasound findings suggestive of congenital infection 2
• Seronegative healthcare/childcare workers may benefit from serologic monitoring during pregnancy 2
• Retain prenatal screening specimens until delivery to allow retesting if infection is suspected later 3
• Risk of congenital defects with rubella is up to 85% when infection occurs in first 8 weeks of gestation 3