Viral Serologies: IgG vs. IgM for EBV, CMV, Parvovirus, and When to Suspect Each Infection
IgM means acute. IgG means past. Except when it doesn't. Here's how to interpret the viral serology panel without getting fooled.
The Universal Principle: IgM vs. IgG
- IgM: The first antibody produced during acute infection. Appears within days to 1–2 weeks of infection. Generally indicates acute or recent infection. Declines over weeks to months.
- IgG: Appears later (2–4 weeks), rises during convalescence, and persists for years to lifetime. Indicates past exposure/immunity.
- IgM positive + IgG negative = early acute infection (seroconversion in progress)
- IgM positive + IgG positive = recent or acute infection (IgG just appearing)
- IgM negative + IgG positive = past infection/immunity
- Both negative = no exposure (susceptible)
IgM is NOT always reliable for timing. False-positive IgM can occur with: polyclonal immune activation (autoimmune diseases, other concurrent infections), rheumatoid factor interference (RF is an IgM that can cross-react in serologic assays), and persistent IgM (some infections like EBV can have detectable IgM for months). Always interpret IgM in clinical context, not as a standalone timestamp.
Virus-by-Virus Guide
EBV (Epstein-Barr Virus) — Infectious Mononucleosis
When to suspect: Adolescents/young adults with fever, severe pharyngitis, posterior cervical lymphadenopathy, fatigue, splenomegaly. Also consider in any age with prolonged febrile illness, atypical lymphocytes on CBC, or hepatitis.
| Test | What It Means |
|---|---|
| VCA IgM (Viral Capsid Antigen) | Positive in acute infection. Appears early, disappears in 4–8 weeks. The best marker for acute mono. |
| VCA IgG | Appears during acute infection and persists for life. Positive alone = past infection. |
| EA IgG (Early Antigen) | Appears in acute infection, disappears in 3–6 months. Supports recent infection but present in only ~70% of acute cases. |
| EBNA IgG (EBV Nuclear Antigen) | Appears 6–12 weeks after onset. Negative in acute mono, positive in past infection. The key test that distinguishes acute from past. |
| Heterophile antibody (Monospot) | Rapid test. ~85% sensitive in adults/adolescents. Can be negative in children <4 years (up to 50% false-negative rate). If monospot is negative but EBV is suspected, order the full EBV panel. |
VCA IgM positive + VCA IgG positive + EA IgG positive + EBNA IgG negative = acute infectious mononucleosis. Once EBNA becomes positive, the infection is past the acute phase. VCA IgG positive + EBNA positive + everything else negative = remote past infection.
In children <4 years, EBV often presents atypically (rash, irritability, mild URI symptoms) and the monospot has a high false-negative rate. Use the EBV-specific antibody panel instead. Also: EBV + amoxicillin = the classic maculopapular rash (occurs in ~70–100% of mono patients given amoxicillin—not a true drug allergy, but an immune-mediated reaction specific to the EBV context).
CMV (Cytomegalovirus)
When to suspect: Mono-like illness with negative monospot/EBV panel ("heterophile-negative mono"). Immunocompromised patients (transplant, HIV) with end-organ disease. Pregnancy screening when primary CMV is suspected (congenital CMV risk).
- CMV IgM positive: Acute or recent infection (but can persist for months and has false positives from RF).
- CMV IgG positive, IgM negative: Past infection. In immunocompetent patients, this is the most common finding and requires no action.
- CMV IgG avidity: High avidity = infection >3–4 months ago. Low avidity = recent infection (<3–4 months). Essential in pregnancy to time the infection relative to conception.
- CMV PCR (DNA viral load): The test for active CMV disease in immunocompromised patients. Serology is less useful in this population—they may not mount an adequate antibody response.
Primary CMV infection during pregnancy (especially first trimester) carries the highest risk of congenital CMV (hearing loss, microcephaly, intellectual disability). If a pregnant patient has CMV IgM positive, CMV IgG avidity is essential to determine if the infection is recent (low avidity = high risk to fetus) or past (high avidity = lower risk). Do not diagnose primary CMV in pregnancy on IgM alone—IgM can persist and give false-positive "acute" results months after the actual infection.
Parvovirus B19
When to suspect: "Slapped cheek" rash (erythema infectiosum/fifth disease) in children. In adults: symmetric polyarthralgia (especially hands/wrists, mimicking RA), reticulocytopenic anemia (aplastic crisis in sickle cell patients), hydrops fetalis in pregnancy.
- Parvovirus IgM: Appears by day 10–12 of infection, persists for 2–3 months. Indicates acute/recent infection.
- Parvovirus IgG: Appears by week 3, persists for life. Indicates immunity.
- Parvovirus PCR: Detects viral DNA. Essential in immunocompromised patients (who may not produce IgM) and for diagnosing chronic parvovirus infection (persistent anemia in immunosuppressed patients).
- Parvovirus arthritis mimics early RA: Symmetric small joint polyarthralgia in a young woman with recent viral symptoms and a positive parvovirus IgM. Don't start methotrexate—it's self-limited (weeks to months).
- Aplastic crisis in sickle cell: Parvovirus infects erythroid precursors. In patients with chronic hemolysis (sickle cell, spherocytosis), this causes an abrupt reticulocytopenic anemia. Check parvovirus IgM in any sickle cell patient with sudden worsening anemia.
- Pregnancy: Primary parvovirus in the first/second trimester can cause fetal hydrops and death. If exposure is suspected, check IgM and IgG immediately. IgG positive/IgM negative = immune, no risk. IgM positive = refer to MFM urgently for fetal monitoring.
Other Viral Serologies to Know
| Virus | When to Suspect | Key Tests | Pitfall |
|---|---|---|---|
| Hepatitis A | Acute hepatitis, travel exposure, foodborne outbreak | HAV IgM (acute), HAV total Ab/IgG (immunity) | Total anti-HAV is positive from past infection OR vaccination—can't distinguish without IgM |
| Measles | Fever, cough, coryza, conjunctivitis, maculopapular rash, Koplik's spots; unvaccinated or under-vaccinated | Measles IgM (acute, draw at rash onset), measles IgG (immunity check) | IgM may be negative in first 72 hours of rash; repeat if initially negative. Measles IgG confirms vaccination immunity. |
| Varicella (VZV) | Chickenpox (vesicular rash); immunity check pre-transplant or in healthcare workers | VZV IgG (immunity), VZV IgM (acute—rarely needed, as diagnosis is usually clinical) | VZV IgG confirms immunity from prior infection or vaccination. IgG-negative patients need vaccination (live vaccine—contraindicated in immunosuppressed). |
| HSV (covered in STI post) | Genital/oral lesions | PCR swab of lesion (gold standard); type-specific IgG for remote diagnosis | Don't screen asymptomatic patients. Low-index-value HSV-2 IgG has high false-positive rate. |
| HIV (covered in STI post) | Risk factors, acute retroviral syndrome | 4th-gen Ag/Ab combo (screen); differentiation assay + RNA (confirm) | Window period 2–4 weeks with 4th-gen. Use RNA for very early suspected infection. |
| Mumps | Parotitis, orchitis; outbreaks in vaccinated populations | Mumps IgM (acute), PCR (buccal swab preferred) | IgM may be absent in vaccinated individuals with breakthrough mumps; PCR is more reliable. |
| Rubella | Pregnancy immunity screen; rash illness with arthralgia | Rubella IgG (immunity), IgM (acute); IgG avidity in pregnancy | IgM can persist for months; avidity testing helps time the infection in pregnancy. |
When to Use Serology vs. PCR
- Serology (IgG/IgM): Best for immunocompetent patients with intact antibody responses. Tells you about exposure and timing.
- PCR (viral DNA/RNA): Best for immunocompromised patients (may not produce adequate antibodies), for detecting active viral replication, and for confirming acute infection when serology is ambiguous. Essential for CMV and EBV in transplant patients, parvovirus in immunosuppressed, and HSV in active lesions.
The Pitfalls Summary
- IgM can persist for months (especially EBV, CMV)—a positive IgM alone doesn't prove the infection is happening right now.
- Rheumatoid factor causes false-positive IgM in multiple viral assays—if the patient has RA or other conditions with elevated RF, interpret IgM cautiously.
- Monospot is unreliable in young children—use the full EBV panel for kids <4.
- Serology is unreliable in immunocompromised patients—use PCR for CMV, EBV, and parvovirus in transplant recipients and HIV patients.
- Avidity testing is essential in pregnancy for CMV and rubella when IgM is positive—it's the test that determines fetal risk.
- Parvovirus arthritis mimics RA—check parvovirus IgM before starting DMARDs in a patient with acute-onset symmetric polyarthritis.
Bottom Line
Viral serologies follow a consistent IgM (acute) and IgG (past) framework, but the devil is in the details: persistent IgM, RF interference, monospot limitations in children, and the critical role of avidity testing in pregnancy. Know each virus's unique antibody pattern (especially EBV's four-marker panel), use PCR when serology is unreliable, and never start immunosuppressive therapy for "RA" without ruling out parvovirus first.
Stay sharp out there.
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