Herpes Testing: IgG vs IgM (and What the Numbers Actually Mean)

If you have ever searched “herpes blood test interpretation,” you have probably seen confusing advice: some pages push IgM tests, others say only IgG matters, and many never explain what a “low positive” really means.

Here is the practical, evidence-based version for 2026:

  • Type-specific IgG is the blood test used for HSV-1/HSV-2 exposure assessment.
  • IgM is not recommended for HSV diagnosis.
  • Low-positive HSV-2 IgG results often need confirmation before being treated as true positives.

This guidance comes directly from CDC herpes testing and treatment guidance, not internet folklore.

Structure diagram of Immunoglobulin G (IgG), the antibody used in type-specific HSV serology
IgG is the antibody class used in standard type-specific HSV blood testing.

What test is preferred when no lesions are present?

When someone does not have an active sore to swab, CDC guidance supports type-specific serologic testing (IgG-based assays that distinguish HSV-1 from HSV-2 antibodies). If lesions are present, virologic testing (typically NAAT/PCR from the lesion) is preferred.

Important limitation: blood tests tell you exposure, not exact site (oral vs genital), especially for HSV-1.

Why HSV IgM is not recommended

CDC STI Treatment Guidelines explicitly state that HSV IgM testing is not useful and is not recommended. Why?

  • IgM assays are not type-specific (cannot reliably separate HSV-1 from HSV-2).
  • IgM can be positive during recurrent episodes, not only new infection.
  • This creates misleading “recent infection” interpretations and unnecessary anxiety.
Structure diagram of Immunoglobulin M (IgM), which CDC does not recommend for HSV diagnosis
For HSV specifically, IgM is not considered a reliable diagnostic strategy.

The “low-positive” HSV-2 IgG problem

This is where many people get burned by overconfident interpretation. CDC notes that commonly used HSV-2 EIAs can be falsely positive at low index values. In the guideline summary, one study found:

  • Overall specificity: 57.4%
  • Specificity for index 1.1–2.9: 39.8%

That means a low-positive result can be wrong often enough that confirmation is important before life-changing conclusions.

Bar chart showing CDC-cited HSV-2 EIA specificity overall vs low index values
Data point from CDC STI Treatment Guidelines discussing poor specificity in low-index HSV-2 EIA ranges and need for confirmatory testing.

When to repeat testing after recent exposure

If exposure is recent, antibodies may not be detectable immediately. CDC notes that in suspected recent acquisition, repeating type-specific testing around 12 weeks can be appropriate. CDC’s patient-facing testing page also notes it can take up to about 16 weeks or more for current tests to detect infection in some people.

So what should you do with a result?

If you have active lesions: ask for swab NAAT/PCR and typing.

If no lesions: use type-specific IgG in the right clinical context (not random panic testing).

If HSV-2 is low-positive: ask for confirmatory testing with a second method (e.g., Biokit or Western blot per CDC guidance) before treating it as definitive.

If someone offers IgM as the key test: that is a red flag for outdated HSV testing practice.

How this connects to HIV risk counseling

WHO and CDC both emphasize that HSV-2 and HIV risk are linked. WHO reports HSV-2 increases HIV acquisition risk by about threefold, and CDC similarly notes a higher HIV risk with genital herpes. That is one reason accurate diagnosis and counseling matter.

Bottom line

The goal is not “test as much as possible.” The goal is test correctly and interpret carefully:

  • Type-specific IgG over IgM
  • Lesion PCR when lesions are present
  • Confirm low-positive HSV-2 results
  • Use timing-aware interpretation after recent exposure

Want to understand what these test outcomes mean in practical risk terms? Use the calculators at HSV-1 risk tool and HSV-2 risk tool for scenario-based estimates.

Sources: CDC STI Treatment Guidelines — Herpes; CDC Herpes Testing FAQ; CDC Genital Herpes Overview; WHO Herpes Simplex Virus Fact Sheet.