When should I order varicella‑zoster IgG versus IgM serology to assess immunity or acute infection?

Varicella-Zoster Serology: IgG vs IgM Testing

Order varicella-zoster IgG serology to assess immunity status; IgM testing is not recommended for routine immunity screening and has limited utility even for acute infection diagnosis. 1

When to Order Varicella IgG

IgG testing is the appropriate test for determining immunity status in the following clinical scenarios:

• Adults with uncertain or negative varicella history, particularly those who grew up in tropical or subtropical climates where natural infection rates may be lower 1
• Healthcare personnel without documented immunity, as certainty regarding immunity is essential to prevent nosocomial transmission 1
• Pregnant women without documented immunity, due to risk of fetal transmission and congenital varicella syndrome 1
• Immunocompromised patients without documented immunity, given their risk for severe disease 1
• Patients with inflammatory bowel disease or other immune-mediated diseases being considered for immunosuppressive therapy, to determine if vaccination is needed before treatment initiation 1

Interpreting IgG Results

• Positive IgG indicates immunity in both healthy and immunocompromised individuals (except bone marrow transplant recipients, who should be considered non-immune regardless of antibody status) 1, 2
• Negative IgG in previously vaccinated individuals does not necessarily indicate susceptibility, as commercial assays have suboptimal sensitivity for detecting vaccine-induced antibodies 1
• U.S. birth before 1980 is considered evidence of immunity for most adults (except healthcare personnel, pregnant women, and immunocompromised persons), as 96-99% of adults in this age group have serologic evidence of VZV infection 1

Why IgM Testing Should Be Avoided

IgM testing is not recommended for immunity screening and has significant limitations even for acute infection diagnosis:

For Immunity Assessment

• CDC explicitly recommends against serologic testing before or after varicella vaccination due to low test sensitivity 1
• IgM does not indicate immunity status—only IgG provides information about protective immunity 1

For Acute Infection Diagnosis

IgM has poor sensitivity and specificity for diagnosing acute VZV infections:

• In herpes zoster, IgM is positive in only 37-84% of cases, making it an unreliable diagnostic marker 3, 4
• IgM remains positive for only 3.5 weeks after herpes zoster onset (95% CI: 2.8-4.6 weeks), providing a narrow diagnostic window 3
• IgM appears 3-5 days after symptom onset in both varicella and zoster, meaning early testing will be falsely negative 5, 6
• IgM testing accounts for up to 11% of VZV tests despite not being recommended, representing inappropriate utilization 7

Preferred Diagnostic Approach for Acute Infection

For suspected acute varicella or herpes zoster, clinical diagnosis or PCR testing—not serology—is the appropriate approach:

• Varicella and herpes zoster should be diagnosed clinically based on characteristic vesicular rash in dermatomal distribution 1, 8
• PCR testing of vesicular fluid or lesion swabs is the gold standard for laboratory confirmation when needed, with sensitivity and specificity approaching 100% 1
• Serology (IgG or IgM) is not useful for diagnosing acute chickenpox or shingles 1
• Laboratory confirmation is only needed for immunocompromised patients with atypical presentations 8

Clinical Algorithm

For Immunity Assessment:

1. Check vaccination records first—two documented doses of varicella vaccine constitute evidence of immunity 1
2. If vaccination records unavailable, obtain varicella history from healthcare provider (not patient/parent self-report for children) 1
3. If history uncertain or negative, order IgG only (not IgM) 1
4. Do not order serology if patient was born in U.S. before 1980 (except healthcare workers, pregnant women, immunocompromised) 1

For Suspected Acute Infection:

1. Diagnose clinically based on characteristic vesicular rash 1, 8
2. If laboratory confirmation needed (atypical presentation, immunocompromised host), order PCR of lesion material—not serology 1, 8
3. Never order IgM for diagnostic purposes 1, 7

Common Pitfalls to Avoid

• Do not order IgM to assess immunity—it provides no information about protective immunity 1
• Do not rely on negative IgG in vaccinated individuals to determine susceptibility, as tests may miss vaccine-induced antibodies 1
• Do not order serology to diagnose active varicella or herpes zoster—use clinical diagnosis or PCR instead 1
• Do not assume bone marrow transplant recipients are immune based on positive IgG or prior history 1, 2
• Do not order post-vaccination serologic testing to verify immune response, as available assays lack adequate sensitivity 1