When does Cytomegalovirus (CMV) Immunoglobulin G (IgG) become positive after initial infection?

CMV IgG Positivity Timeline After Initial Infection

CMV IgG antibodies typically become positive within 2-3 weeks after initial infection, with full seroconversion usually complete by 4-6 weeks post-infection. 1

Diagnostic Timeline of CMV Infection

Early Phase (0-2 weeks post-infection)

• CMV IgG is typically negative
• CMV IgM may become detectable
• PCR for CMV DNA may be positive in blood or other specimens before antibody response

Seroconversion Phase (2-6 weeks post-infection)

• CMV IgG becomes detectable around 2-3 weeks post-infection
• Initial IgG antibodies have low avidity (weak binding strength)
• CMV IgM is usually strongly positive
• PCR for CMV DNA may remain positive

Maturation Phase (1-6 months post-infection)

• CMV IgG levels increase and remain positive
• IgG avidity gradually increases over 3-6 months 2
• Low IgG avidity (<40%) indicates primary infection within the previous 3-4 months 2
• CMV IgM may persist for several months in some individuals

Clinical Significance of CMV IgG Testing

In Pregnancy

• CMV IgG positivity with low avidity indicates recent primary infection (within 3-4 months) 2
• This is critical as primary CMV infection during pregnancy carries a higher risk of congenital transmission (approximately 40%) 3
• When evaluating suspected CMV infection in pregnancy, both IgG and IgM should be tested along with IgG avidity 1

In Immunocompromised Patients

• CMV IgG positivity indicates past infection and risk for reactivation 4
• In transplant recipients, CMV serostatus (IgG) of both donor and recipient is crucial for risk stratification 1
• CMV-seropositive recipients are at risk for CMV reactivation after transplantation 1

Avidity Testing to Determine Timing of Infection

• CMV IgG avidity testing helps distinguish recent from past infection
• Low avidity (<40%) indicates infection within the previous 3-4 months 2
• High avidity excludes primary infection within the preceding 3 months 2
• The rate of avidity increase (ΔAI) may correlate with risk of congenital transmission 3

Common Pitfalls in CMV IgG Interpretation

• False negative results may occur in early infection (test too soon)
• CMV IgG positivity alone cannot distinguish between recent and past infection
• CMV IgM can persist for months after primary infection or may be positive during reactivation
• IgG avidity testing should be used to determine timing of infection when needed
• Seroreversion (loss of detectable antibodies) can rarely occur in advanced immunosuppression 4

Special Considerations

• In transplant recipients, CMV IgG status determines prophylaxis and monitoring strategies 1
• For pregnant women, timing of CMV infection is critical for assessing fetal risk 1, 2
• In immunocompetent individuals, isolated CMV IgG positivity without IgM generally indicates past infection and requires no specific intervention 4

Remember that while CMV IgG becomes positive within 2-3 weeks after initial infection, the complete diagnostic picture should include IgM testing and avidity assessment when the timing of infection is clinically important.