Cytomegalovirus (CMV) Serology

Cytomegalovirus (human herpesvirus 5) is a ubiquitous betaherpesvirus that establishes lifelong latency in monocytes and myeloid progenitors. Serology measures CMV IgM, CMV IgG and IgG avidity. Quantitative PCR for CMV DNA in blood, plasma, urine, saliva or amniotic fluid, pp65 antigenaemia, and interferon gamma release assays for CMV-specific T-cell immunity complement serology. Virus culture and shell vial are largely obsolete.

Seroprevalence rises with age and is about 50 to 60% in adults of high income countries and over 90% in low income countries and in people with lower socioeconomic status.

Congenital CMV is the commonest congenital infection, affecting 0.5 to 1% of live births (higher where seroprevalence is high), with 10 to 15% of infected infants symptomatic at birth and a further 10 to 15% of asymptomatic infants developing sensorineural hearing loss. Primary infection in pregnancy transmits to the foetus in 30 to 40%, and recurrent infection in 1 to 2%.

CMV is the main viral infection after solid organ and stem cell transplantation and a cause of retinitis in advanced HIV.

Indications. Pregnancy (primary infection suspected, or mononucleosis like illness, or foetal ultrasound abnormalities). Newborns with petechiae, jaundice, hepatosplenomegaly, microcephaly, low birth weight for gestation or hearing loss. Transplant donors and recipients (serostatus matching and prophylaxis decisions) and post-transplant surveillance.

Retinitis, colitis, oesophagitis or pneumonitis in an immunocompromised patient. Mononucleosis like syndrome with negative heterophile test. Unexplained hepatitis or fever. Blood donors for CMV-safe components in selected groups.

Specimen. Serum 1 mL, no fasting. Stable for days at 2 to 8 °C. EDTA whole blood or plasma 2 to 3 mL for PCR (analysed within 24 hours). Urine or saliva swab within the first 2 to 3 weeks of life for congenital diagnosis. A dried blood spot can be used retrospectively.

Reference values (assay dependent). CMV IgM and IgG: negative (index under 0.8 or under 6 AU/mL for IgG), equivocal, positive. IgG avidity index: low under 30%, intermediate 30 to 50%, high over 50 to 60% (method dependent). CMV DNA: not detected (quantitative results in IU/mL).

Interpretation.

• In pregnancy, IgG seroconversion, or positive IgM with low avidity IgG, indicates primary infection, with risk of transmission rising from 30% in the first trimester to 70% near term, but with severe sequelae mostly after infection before 20 weeks. Foetal ultrasound, amniocentesis for CMV DNA (after 21 weeks and at least 6 weeks after maternal infection) and magnetic resonance imaging are used.
• High avidity IgG in the first 12 to 16 weeks of pregnancy excludes primary infection in the preceding 3 months.
• Congenital infection is confirmed by CMV DNA in urine, saliva or blood within 3 weeks of birth. Later positive tests may reflect postnatal infection (breast milk, transfusion). Retrospective diagnosis uses the dried blood spot taken at birth.
• Transplant: donor positive and recipient negative (D+/R-) carries the highest risk of primary disease and calls for antiviral prophylaxis or pre-emptive monitoring.
• Viral load thresholds for treatment depend on the organ, the assay and the centre, and the same assay (calibrated in IU/mL) is used for serial monitoring.
• CMV disease is distinguished from infection by organ involvement and histology (owl eye inclusions, immunostaining).

Critical values and findings needing immediate action.

• Rising CMV viral load, fever, cytopenia, colitis or pneumonitis in a transplant recipient: start intravenous ganciclovir (renal dose adjustment), reduce immunosuppression and check for resistance.
• Visual floaters or loss in a patient with CD4 under 50: CMV retinitis, same day ophthalmology and systemic therapy with ganciclovir or valganciclovir.
• Newborn with positive CMV PCR and symptoms (microcephaly, seizures, chorioretinitis): infectious disease and neonatal review, cranial imaging, hearing and eye assessment, and antiviral treatment (valganciclovir) in symptomatic infants.
• Pregnant woman with confirmed primary CMV infection: detailed foetal ultrasound and specialist counselling.
• Bone marrow suppression on valganciclovir or ganciclovir: full blood count monitoring.

Interferences and pitfalls.

• IgM is not specific for primary infection. It persists for months, is positive in reactivation and re-infection, and cross-reacts with Epstein-Barr virus, other herpesviruses and rheumatoid factor. Avidity helps but is not definitive.
• Transplacentally acquired maternal IgG in infants under 12 months does not show infant infection, and IgM is insensitive in the neonate. PCR is the test.
• In immunocompromised patients serology is unreliable, and PCR or antigenaemia assays are used.
• Different PCR assays give different results, and a calibrated international reference material is used. Whole blood loads are about 3 to 5 times higher than plasma loads.
• Viral shedding in urine and saliva of young children is prolonged, and a positive result in an older child is not diagnostic of congenital infection.
• Blood transfusion or immunoglobulin products can cause passive IgG positivity.
• CMV serostatus matters for blood product selection in neonates and seronegative pregnant women.
• Testing for CMV in healthy pregnant women is not routine in many settings because no proven prevention exists, though hygiene advice is given.

Special groups.

• Newborns and infants: hearing screening and follow up are needed even in asymptomatic congenital infection, since late onset deafness occurs.
• Pregnancy: seronegative women are counselled on hygiene (handwashing after nappy changes, avoiding saliva sharing with young children).
• Transplant recipients: prophylaxis (valganciclovir) or pre-emptive monitoring, depending on the organ and risk status.
• HIV: retinitis at CD4 under 50 and gastrointestinal disease, now uncommon with effective antiretroviral therapy.
• Older adults: CMV reactivation contributes to critical illness and immunosenescence.
• Regions: nearly universal infection in early childhood, with congenital infection as a leading cause of childhood deafness, and CMV disease in HIV remains important where antiretroviral therapy is delayed.

Follow up tests. Quantitative CMV DNA (blood, urine, amniotic fluid), IgG avidity, foetal ultrasound and magnetic resonance imaging, newborn hearing test (auditory brainstem response), cranial ultrasound, ophthalmology review, full blood count, liver function tests, creatinine for antiviral dosing, resistance genotyping (UL97, UL54) in non-responders, biopsy with immunohistochemistry for tissue invasive disease, and CD4 count in HIV.