CMV IgG (Cytomegalovirus — IgG antibodies)

What this test measures

• CMV IgG: Immunoglobulin G antibodies specifically directed against cytomegalovirus proteins. They appear 2 to 4 weeks after primary infection, peak at 6 to 12 weeks, and then persist at stable, detectable levels for life. Their presence indicates past exposure to CMV (recent or old infection) and acquired humoral immunity. They alone do not distinguish between an old infection and a recent primary infection without measurement of avidity.
• CMV IgM (often measured as a complement): immunoglobulin M antibodies appearing early during primary infection (as early as 1-2 weeks) and during reactivations; they usually persist for 3 to 6 months but can last up to 12 months or longer in some cases — their isolated presence therefore does not necessarily indicate a recent primary infection
• CMV IgG Avidity Index: a measure of the binding strength (avidity) of anti-CMV IgG antibodies to their antigens — low avidity (< 30–40 %, depending on the laboratory) at the onset of infection, gradually increasing over 3 to 6 months to high avidity (> 60 %) in established infection; a key parameter for dating primary infection, particularly during pregnancy (high avidity in the first trimester = infection prior to pregnancy, therefore no risk of current primary infection)
• Quantitative PCR for Cytomegalovirus (CMV viral load): Detection and quantification of CMV DNA in blood (plasma or leukocytes), urine, bronchoalveolar lavage fluid (BALF), cerebrospinal fluid (CSF), or stool, depending on the context. Essential for diagnosing active infection and monitoring treatment in immunocompromised individuals. Not part of the standard serological workup, but the reference test for at-risk situations.

Results Interpretation

Clinical indications for CMV IgG testing

• Pre-nuptial or pre-conception check-up: determination of CMV serological status in women of childbearing age to identify seronegative women at risk of primary infection during pregnancy; information and preventive measures if seronegative (hand washing after contact with secretions of young children - main source of contamination for pregnant women, avoid sharing utensils and mouth-to-mouth kissing with young children).
• Surveillance during pregnancy: CMV serology (IgG + IgM) is indicated if the pregnant woman presents with mononucleosis-like symptoms, if fetal anomalies suggestive of congenital CMV are detected on ultrasound (ventriculomegaly, bowel hyperechogenicity, intracranial calcifications, intrauterine growth restriction, microcephaly), or as part of a voluntary prenatal diagnostic approach. If seroconversion is confirmed: amniocentesis with CMV PCR on amniotic fluid after 21 weeks of amenorrhea to assess fetal transmission.
• Pre-transplant assessment (solid organ and hematopoietic stem cells): systematic donor and recipient CMV serology — CMV D+/R− status (donor seropositive, recipient seronegative) is the situation with the highest risk of post-transplant CMV disease; it will guide prophylactic strategy (oral valganiclovir for 3–6 months post-transplantation) or preemptive strategy (weekly PCR surveillance with treatment if threshold is exceeded)
• Immunodeficiency assessment (HIV, hematological disorders, chemotherapy): CMV status determination to evaluate reactivation risk; active CMV disease (retinitis, colitis, pneumopathy, encephalitis) occurs mainly when CD4 count is < 50 cells/mm³ in HIV; preventive treatment with valganciclovir in high-risk situations
• Clinical presentation of CMV mononucleosis (heterophile-negative mononucleosis): prolonged fever, marked asthenia, mononucleosis syndrome (atypical lymphocytes on complete blood count) with a negative Monospot test - suspect CMV and EBV; CMV serology (IgG + IgM + avidity) and EBV; PCR if immunocompromised
• Evaluation of prolonged unexplained fever: CMV can cause a persistent febrile syndrome even in the immunocompetent; this should be considered in the evaluation of prolonged fever without an identified source.

Congenital Cytomegalovirus (CMV) — Specific Challenges

• Epidemiology: Congenital CMV is the most common congenital infection in developed countries, affecting 0.5 to 1% of all live-born infants; in Canada, approximately 3,000 to 4,000 newborns are infected each year; 10–15% of infected newborns are symptomatic at birth (hepatosplenomegaly, microcephaly, petechiae, jaundice, chorioretinitis); 85–90% are asymptomatic, but 10–15% of these will develop late sequelae (sensorineural hearing loss, cognitive impairments)
• Risk of fetal transmission: 30–40% % in cases of maternal primary infection during the first trimester (highest risk of severe complications); 1–2 % in cases of reactivation or reinfection in the presence of pre-existing immunity; maternal primary infection during the first trimester is the situation posing the highest risk
• Neonatal diagnostics: CMV PCR on urine or saliva within the first 3 weeks of life (gold standard) — after this period, a positive PCR may reflect postnatal infection acquired through breast milk (benign) rather than congenital infection
• Treatment: oral valganciclovir for 6 months in symptomatic newborns with central nervous system involvement — demonstrated reduction in auditory and neurodevelopmental sequelae at 2 years in the CASG 112 trial
• Prevention: no vaccine currently available; strict hygiene measures for pregnant women who are seronegative: careful hand washing after diaper changes or contact with young children's nasal secretions, avoid sharing glasses, cutlery, toothbrushes with children under 3 years old, and avoid mouth-to-mouth kisses on their lips

Reference values and units

• CMV IgG: expressed in arbitrary units per milliliter (AU/mL) or as an index depending on the reagent used; positivity thresholds vary by analytical platform — usually negative if 1.0 AU/mL; refer to the reference values of the laboratory performing the analysis
• CMV IgM: positive / negative / indeterminate according to signal/cut-off ratio; IgM are subject to false positives (rheumatoid factor, cross-reactions with other herpesviruses) — always interpret in clinical context
• CMV IgG Avidity Index: low avidity < 30–40 % (depending on the laboratory) = probable recent infection (< 3 months); high avidity > 60 % = past infection; gray zone 30–60 % — interpretation based on context
• PCR CMV (viral load): expressed in copies/mL or IU/mL (international units per milliliter); treatment thresholds in transplant recipients are defined by each transplant center according to the type of graft and individual risk; detectable PCR in the context of symptoms in an immunocompromised patient requires treatment without waiting for an arbitrary threshold

Consult at Clinique Omicron

Clinique Omicron physicians prescribe and interpret CMV serology (IgG, IgM, avidity) as part of pre-conception, prenatal, pre-transplant, and immunodeficiency evaluations. They refer patients to appropriate specialists (infectious disease specialist, maternal-fetal medicine specialist, hematologist, transplant nephrologist) based on results and clinical context. Consultations are available at our Quebec locations and via telemedicine throughout the province. To book an appointment, visit cliniqueomicron.ca.

The content of this page is provided for informational purposes only and does not substitute for the advice of a qualified healthcare professional. CMV serology interpretation should always be performed in a clinical context by a physician, particularly in pregnant women and immunocompromised patients.