What this test measures
IgM antibodies against HSV-2 (the main cause of genital herpes). Rises within 1–3 weeks of primary infection and may stay positive for 1–6 months. However, IgM is not very specific — false positives are common, IgM can also rise during HSV-2 reactivation, and cross-reactivity with HSV-1 IgM is significant. For most clinical scenarios, HSV-2-specific IgG (gG2-based ELISA) is preferred, and active lesions should be tested by PCR.
Why it matters
Genital herpes is common worldwide. NACO and IUSTI guidelines recommend HSV-2 IgG as the screening test, with PCR of active lesions for definitive diagnosis. HSV-2 IgM has a limited role — useful to support a clinical picture of recent primary infection, antenatal screening when HSV-2 IgG is negative, or evaluation of neonatal exposure. Order alongside HSV-2 IgG (and HSV-1 IgG / IgM if clinically relevant) to make sense of the result.
How to prepare
No fasting required. Random sample. Disclose any genital lesions / ulcers (PCR of lesion is preferred for active disease), pregnancy, and HIV status (immunocompromise alters serology).
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 |
|---|---|---|---|
| HSV-2 IgM (index / S/CO ratio)[1][2] | Negative (S/CO < 0.9) | Negative — no recent primary HSV-2 infection. Doesn't rule out past infection (check IgG). | Positive — possible recent primary infection (1–6 months window). Confirm with HSV-2 IgG seroconversion (negative → positive); active lesions should also be PCR-tested. False positives are common — don't treat based on IgM alone. |
HSV serology interpretation
| HSV-2 IgM | HSV-2 IgG | Interpretation |
|---|---|---|
| Negative | Negative | No infection (re-test in 3 months if recent exposure) |
| Positive | Negative | Possible recent primary infection — repeat IgG in 8-12 weeks to confirm seroconversion |
| Positive | Positive | Active infection (primary or reactivation) — PCR lesion if available |
| Negative | Positive | Past infection (latent); may reactivate |
Frequently asked questions
Why is HSV-2 IgG preferred for screening?
IgG is more specific and indicates established or past infection. IgM has frequent false positives and cross-reacts with HSV-1, making it unreliable for routine screening.
I have a genital ulcer — what should I order?
PCR of the lesion (swab the ulcer base for HSV PCR) is the gold standard — most sensitive for active infection. Serology supports the clinical picture but doesn't identify the active lesion source.
How long after exposure does IgM appear?
1–3 weeks. Window-period testing within 2 weeks of exposure may miss early infection.
Can HSV-1 cause genital herpes?
Yes — increasingly common (oral-genital transmission). HSV-1 genital herpes tends to have fewer recurrences than HSV-2 genital herpes. Test for both if genital ulcer present.
Is my partner safe if I have HSV-2 IgG?
HSV-2 transmits through skin-to-skin contact, more so during outbreaks but also during asymptomatic shedding. Antiviral suppression (acyclovir, valacyclovir) plus condom use reduces transmission risk by ~50%.
Will pregnancy affect this test?
Pregnancy itself doesn't cause false positives. But primary HSV-2 in late pregnancy carries risk of neonatal herpes — antenatal screening and antiviral suppression in late pregnancy can prevent transmission.
Related Infectious Disease tests
Tests commonly ordered alongside HERPES SIMPLEX VIRUS II (HSV)-IGM, or that help interpret an unexpected result.
Sources & references
- CDC — Genital Herpes Diagnosis · accessed 2026-05-30T00:00:00.000Z
- WHO — HSV-2 Fact Sheet · accessed 2026-05-30T00:00:00.000Z
- NACO India — STI Guidelines · accessed 2026-05-30T00:00:00.000Z
- IUSTI — Genital Herpes Guideline · accessed 2026-05-30T00:00:00.000Z
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