What does a positive herpes simplex virus (HSV) immunoglobulin M (IgM) test mean and what steps should be taken for confirmation and treatment?

HSV IgM Positive Result: Interpretation and Management

A positive HSV IgM test is unreliable and should NOT be used for diagnosis—you need confirmatory testing with either direct viral detection (PCR/culture from active lesions) or type-specific IgG serology after the appropriate window period. 1

Why HSV IgM Testing is Problematic

HSV IgM serology has severe limitations that make it unsuitable as a standalone diagnostic test:

• High false-positive rate: IgM tests frequently produce false positives, particularly in patients with other viral infections. Recent data shows COVID-19 patients had a 45% false-positive rate for HSV IgM due to immunoassay interference 2
• Poor specificity: IgM can be positive during both primary and recurrent infections, making it impossible to distinguish new from reactivated infection
• Non-type-specific: Most IgM tests cannot differentiate between HSV-1 and HSV-2, which is clinically important for counseling and prognosis

Recommended Diagnostic Approach

If You Have Active Lesions:

• PCR or viral culture from the lesion base is the gold standard for diagnosis
• This provides definitive diagnosis AND type-specific information (HSV-1 vs HSV-2)
• Do not rely on serology when active lesions are present

If You Have NO Active Lesions:

Wait 12 weeks from potential exposure, then obtain type-specific IgG serology 1:

• Use glycoprotein G-based assays (HerpeSelect or similar)
• Critical caveat: If your index value is 1.1-2.9, the specificity is only 39.8% 1
• Index values ≥3.0 have 78.6% specificity and may be sufficient without confirmation 1
• Index values <3.0 require confirmatory testing with a second assay (Biokit rapid test) or Western blot to avoid false-positive diagnosis 1

Common Pitfalls to Avoid

1. Do not make treatment decisions based on IgM alone—the false-positive rate is too high
2. Do not test during the window period (first 12 weeks after exposure)—you'll get false negatives 1
3. Patients with HSV-1 are more likely to have false-positive HSV-2 tests with low index values 1
4. Do not repeat serology before 12 weeks if initially negative 1

What to Do Now

Immediate steps:

• Assess whether you have or recently had genital/oral lesions
• If yes → obtain PCR/culture from active lesions
• If no → disregard the IgM result and wait 12 weeks from last potential exposure for type-specific IgG testing

Do not start antiviral treatment based solely on a positive IgM without clinical symptoms or confirmatory testing, as this is a chronic infection diagnosis with lifelong implications 1, 3.