HSV antibody testing has the highest false-positive rate of any common STI test. Understanding IgG vs IgM, index values, and low-positives matters more for herpes than for any other test.

HSV Test Results — What Your Number Actually Means

If you have just received an HSV (herpes) blood test result and you are trying to figure out what the numbers mean, you have run into one of the most frustrating tests in modern medicine. HSV antibody testing has notoriously poor specificity — meaning false positives are common — and the way labs report results adds confusion most patients never get explained.

Here is the honest, practical explanation.

The two HSV tests you may have had

IgM antibodies

Skip this section if your test was just IgG. IgM tests are NOT recommended for routine herpes diagnosis — they have high false-positive rates and cannot reliably distinguish new from old infections.

If your provider ordered an HSV IgM test, the result is often unreliable in either direction. Many false positives during other viral illnesses. Recommended next step: ignore the IgM and run a type-specific IgG test.

IgG antibodies — type-specific

This is the standard test. It looks for antibodies your immune system has developed against HSV-1 or HSV-2 specifically. Results come back as:

HSV-1 IgG: positive / negative (with an index value, see below)
HSV-2 IgG: positive / negative (with an index value, see below)

The two are reported separately because they are different viruses with different transmission and outcome patterns.

What the index value means

This is the part most patients are not told. The HSV-2 IgG result is not just positive or negative — it comes with an index value, usually a number.

How to interpret the HSV-2 IgG index

< 0.90: Negative
0.90 - 1.10: Equivocal (borderline; retest in 3-4 weeks)
1.10 - 3.49: Low positive — high false-positive risk, see below
≥ 3.50: Strong positive — almost certainly true infection

The CDC and the American Sexually Transmitted Diseases Association (ASTDA) both acknowledge that index values between 1.10 and 3.50 have a meaningful false-positive rate.

Multiple studies have shown that:

In low-prevalence populations (e.g., asymptomatic screening of general population), more than half of low-positive HSV-2 results are false positives.
In high-prevalence populations or symptomatic patients, low-positives are more likely to be true positives.

This means: a "positive" HSV-2 IgG test with an index of 1.5 is genuinely uncertain. A "positive" with index 4.5 is almost certainly real.

How to interpret HSV-1 IgG

The false-positive issue is less severe for HSV-1, partly because the test performs better and partly because the disease is so common (~67% of US adults are positive). HSV-1 IgG results are usually treated as binary positive/negative.

Why is the HSV-2 test so messy?

A combination of:

The most common HSV-2 IgG assay (HerpeSelect) has cross-reactivity with non-HSV viruses, especially in low-positive ranges.
HSV-1 antibodies can cross-react with HSV-2 assays in some patients.
The asymptomatic prevalence is high (about 11% of US adults have HSV-2 antibodies, most without ever having had a recognized outbreak), so screening positives are common even in healthy people.
No symptoms means no clinical context to distinguish true from false positive.

This is why CDC does NOT recommend routine HSV-2 antibody screening in asymptomatic people. It causes more harm (anxiety, relationship strain) than good (changed management).

What if my result was low-positive?

If your HSV-2 IgG came back positive with an index between 1.10 and 3.50:

Option 1: Confirmatory testing (recommended)

Ask for a Western blot — the gold-standard HSV antibody test, available through the University of Washington's lab. It is more specific and resolves most low-positive ambiguity. Your provider can order it; you may need to push a bit because it is not the default lab assay.

Option 2: Retesting at a later date

Wait 8-12 weeks and re-test with the same assay. If still positive at a similar low-positive value, more likely to be true. If converted to higher index, true positive. If now negative or equivocal, probably false positive.

Option 3: Type-specific symptom history

A clinician comfortable with HSV can take a careful history of any genital symptoms you have ever had — even years ago, even mild — and decide if a clinical diagnosis is more or less likely.

What if I have a clear true positive?

A high-positive HSV-2 IgG (index 3.5+), or any positive in someone with a history of genital outbreaks, is a real diagnosis. From there:

See our newly diagnosed with herpes guide
See our herpes pillar guide for full transmission, treatment, and management info
Consider daily suppressive antivirals if outbreaks are frequent

When NOT to get HSV blood tested

The CDC explicitly recommends AGAINST routine HSV blood testing for:

Asymptomatic adults in the general population
People with no known exposure
Pregnant women (some exceptions)
"Just to know" anxiety-driven testing

The reason: the test has a meaningful false-positive rate, a positive result without clinical context changes very little about how you manage your sex life, and the anxiety load of an uncertain result is real.

When testing IS reasonable

Active genital lesion — swab the lesion (NAAT/PCR) rather than blood-test; far more specific
Recent exposure to a known HSV-positive partner — blood test at 3 months
Clinical suspicion based on symptom history
Pregnancy planning where partner has HSV
Before starting daily suppressive antivirals (sometimes)

A note on at-home HSV testing

Several at-home test kits offer HSV antibody testing. They use the same HerpeSelect IgG assay with the same false-positive issues. Cheap, convenient, but the result interpretation problem is identical.

Result from an at-home kit + low index value = exactly the same ambiguity as the same test done at a doctor's office. Confirmatory Western blot still requires a clinical order.

The bottom line

HSV-2 IgG positive at index 3.5+ → almost certainly real HSV-2 IgG positive at index 1.10-3.50 → uncertain; consider Western blot or retest in 8-12 weeks HSV-1 IgG positive → almost certainly real; mostly informational (very common) IgM test → ignore; not useful

If you received a low-positive HSV-2 result and are spiraling, take a breath. Get the Western blot. Resolving the ambiguity is the right move before letting an uncertain test result change how you live.

For everything else on herpes — transmission, treatment, outbreak management, cure research — see our complete herpes pillar guide.

Sources:

CDC. Sexually Transmitted Infections Treatment Guidelines, 2021.
ASTDA. Position statement on HSV serology.
Ashley-Morrow et al. Performance of focus HerpeSelect ELISA. Sex Transm Infect 2003.
Agyemang et al. Performance of commercial enzyme-linked immunoassays for diagnosis of HSV-2 infection. Sex Transm Dis 2017.