Interpretation of Positive CMV IgG Results
A positive CMV IgG test indicates past exposure to cytomegalovirus and the development of immunity against the virus, with antibodies that typically persist for life after the initial infection. 1
Clinical Significance
- A positive CMV IgG alone represents evidence of previous infection and immunity, not active infection 1
- Over 90% of the normal adult population has IgG antibodies to CMV, indicating the high prevalence of this virus 1
- CMV IgG antibodies typically remain detectable for life after the primary infection 1
Clinical Interpretation in Different Populations
General Population
- In immunocompetent individuals, a positive CMV IgG without IgM generally requires no further testing or intervention 1
- False interpretation of positive CMV IgG as active infection should be avoided 1
Pregnant Women
- Pregnant women with positive CMV IgG alone are considered to have immunity and low risk of congenital CMV transmission 1
- If both IgG and IgM are positive during pregnancy, IgG avidity testing is essential to distinguish between recent primary infection and past infection 1, 2
- High CMV IgG avidity (>65%) during the first trimester of pregnancy is a reliable indicator of past infection, even with positive IgM results 3
- Low avidity (<50%) strongly suggests recent primary infection within the previous 3-4 months 4
Transplant Recipients
- CMV IgG status of both donor and recipient is critical for risk assessment in transplant settings 1
- All candidates for allogeneic stem cell transplantation should be tested for anti-CMV IgG antibodies to determine their risk for de novo infection or reactivation 5
- CMV-negative recipients should ideally receive transplants from CMV-negative donors when available 5
When Additional Testing is Needed
- When CMV IgM is also positive, IgG avidity testing should be performed to determine if the infection is recent or past 6
- In immunocompromised patients with suspected CMV disease, CMV viral load testing by nucleic acid amplification test (NAAT) is more clinically relevant than antibody testing 1
- In patients with inflammatory bowel disease resistant to immunosuppressants, CMV testing should be considered as CMV is associated with adverse outcomes 1
Common Pitfalls to Avoid
- Misinterpreting a positive CMV IgG as evidence of active infection 1
- Relying solely on serology for diagnosis of active CMV disease in immunocompromised patients instead of viral load testing 1
- Failing to perform avidity testing when both IgG and IgM are positive, especially in pregnant women 2, 6
- Overlooking the importance of CMV IgG status in transplant risk assessment 5, 1