What this test measures
CMV IgM is the immunoglobulin-M antibody against cytomegalovirus. It typically appears 2–3 weeks after primary infection, peaks at 4–8 weeks, and gradually declines over months. Unlike many other viruses, CMV IgM can also reappear during reactivation or reinfection, making interpretation more complex.
For accurate timing in pregnancy, CMV IgG avidity testing (low avidity = recent primary infection within last 3 months; high avidity = older infection) is added when IgM is positive. CMV DNA PCR confirms active replication.
Why it matters
In India, CMV IgM is most commonly ordered as part of TORCH screening during pregnancy (Toxoplasma, Rubella, CMV, HSV). A positive maternal IgM raises concern about congenital CMV — the leading non-genetic cause of sensorineural hearing loss and developmental delay in newborns.
However, IgM interpretation is tricky: it can be present in primary infection, reactivation, or reinfection — and the risk of congenital CMV is highest only with primary maternal infection. Avidity testing and consultation with a fetal medicine specialist are needed before any intervention. In immunosuppressed patients (transplant, HIV with low CD4, chemotherapy), CMV IgM helps identify active disease, supported by CMV DNA PCR and clinical signs.
How to prepare
No fasting required. Mention pregnancy status, gestational age, immunosuppression, or transplant status — these change interpretation significantly.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Anti-CMV IgM (AU/mL or Index / Reactive-Non-reactive)[1][2] | Non-reactive (Negative) | Non-reactive — no recent CMV activity detected. In pregnancy, lower risk of primary infection in the current period. If clinical suspicion remains high, repeat IgM and IgG in 2–3 weeks. | Reactive — recent primary infection, reactivation, or reinfection. In pregnancy, add CMV IgG avidity to distinguish primary (low avidity) from old infection. Add CMV DNA PCR to confirm active replication. |
CMV IgM + IgG + avidity — pregnancy interpretation
| IgM | IgG | IgG Avidity | Interpretation |
|---|---|---|---|
| Negative | Negative | — | Susceptible — strict hygiene to avoid primary infection. |
| Positive | Negative | — | Very early primary infection — repeat in 2 weeks for IgG. |
| Positive | Positive | Low | Recent primary infection (within 3 months) — fetal medicine input needed. |
| Positive | Positive | High | Old infection or reactivation — lower congenital risk. |
| Negative | Positive | — | Past CMV infection. |
Frequently asked questions
My CMV IgM is positive — does that mean my baby will be affected?
Not necessarily. IgM can indicate primary infection, reactivation, or reinfection. Only primary maternal infection carries the highest risk of congenital CMV. CMV IgG avidity and possibly amniocentesis (PCR on amniotic fluid) are needed for accurate risk assessment.
What is CMV avidity testing?
Avidity measures the strength of antibody binding. Low avidity = recent primary infection (within ~3 months). High avidity = older infection (>3 months). Used to date a CMV infection in pregnancy.
How long does CMV IgM stay positive?
Typically several months, occasionally longer. Persistent IgM does not necessarily mean ongoing active infection.
I am immunocompromised and CMV IgM is positive. What now?
Add CMV DNA PCR to confirm active replication. If symptoms (fever, colitis, retinitis, pneumonitis) match, antiviral therapy with ganciclovir or valganciclovir may be needed.
Can CMV IgM be a false positive?
Yes — non-specific reactivity from other infections (EBV, autoimmune disease, rheumatoid factor) can cause false positives. Combined IgM + IgG + avidity testing improves accuracy.
What if both IgM and IgG are negative in a pregnant woman?
You are susceptible. Strict hygiene (especially after handling toddler saliva, nappies) and possibly repeat testing in later trimesters per your obstetrician.
Is there treatment for CMV in pregnancy?
Antenatal valaciclovir has shown some benefit in selected cases, and hyperimmune globulin has been studied — these are specialist decisions. Most management focuses on monitoring and counselling.
Related Infectious Disease tests
Tests commonly ordered alongside CYTOMEGALO VIRUS (CMV) - IgM, or that help interpret an unexpected result.
Sources & references
- CDC — Cytomegalovirus · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — CMV Test · accessed 2026-05-30T00:00:00.000Z
- NCBI StatPearls — Cytomegalovirus · accessed 2026-05-30T00:00:00.000Z
- IAP — Congenital CMV Guidance · accessed 2026-05-30T00:00:00.000Z
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