Herpes Blood Test Accuracy — IgG, IgM, False Positives Explained

Herpes blood tests are uniquely confusing — and uniquely error-prone — compared to other STI tests. Low positive results in particular have surprisingly high false-positive rates. The CDC explicitly does NOT recommend routine HSV blood screening for asymptomatic people for this reason.

If you've been told you're positive for HSV-2 on a blood test and you don't have symptoms or known exposure, this guide is for you.

The short answer

• Two main HSV blood tests: IgG (long-term antibody, accurate weeks after exposure) and IgM (early antibody, but unreliable for HSV)
• IgG index values matter: Low positive (1.1-3.5) has high false-positive rates; high positive (>3.5) is more likely true
• IgM should generally not be ordered: Cross-reacts with HSV-1 and other viruses
• Confirmatory testing exists: Western blot is the gold standard but limited availability
• CDC does NOT recommend HSV blood screening for asymptomatic people without specific risk

The two main blood tests

IgG (Immunoglobulin G)

• Detects long-term antibodies the body produces against HSV
• Develops over weeks to months after first infection
• Stays positive for life
• Can be type-specific (HSV-1 vs HSV-2 distinguishable)
• Most commonly ordered HSV blood test

IgM (Immunoglobulin M)

• Detects early antibodies, produced shortly after infection
• Theoretically useful for recent infection
• In practice: cross-reacts heavily with HSV-1, can stay positive long-term (not really "acute")
• CDC explicitly does NOT recommend IgM testing for HSV

What IgG index values mean

When you get an HSV-1 or HSV-2 IgG result, it comes with a number — the "index value" or "ELISA index." This is a ratio of how strong the test signal is.

Why low positives are a problem

At low index values (1.1-3.5), the HerpeSelect HSV-2 IgG ELISA — the most common commercial test — has a false-positive rate of around 50% based on multiple studies. This isn't a flaw of the test; it's a function of how antibodies cross-react at low concentrations.

If your HSV-2 IgG is between 1.1 and 3.5, you should ask about confirmatory testing.

Why high positives are more reliable

Above 3.5, the test correlates strongly with true infection. The higher the index, the more confident you can be in the result.

Confirmatory testing options

If you have a low-positive IgG result and want certainty:

Western blot (UW HSV Western Blot)

• The gold-standard confirmatory test
• Done by the University of Washington
• Detects antibodies against multiple HSV proteins
• Very accurate but not widely available
• Requires special order; mailed sample
• Costs $200-300+; often not covered by insurance

Re-test after a few months

• IgG should rise over time if real infection
• Retesting at 4-6 months can clarify

PCR swab of a lesion

• If you have any lesion at all, swab it
• PCR directly detects the virus
• Definitive if positive

Type-specific clinical evaluation

• Sometimes the most useful test is a careful exam by a clinician familiar with herpes

Why blood tests are mostly NOT screening tests

CDC recommends against routine HSV blood testing in asymptomatic people without a specific clinical reason because:

1. False positives are common — leads to unnecessary diagnoses, psychological distress, relationship effects
2. Knowing doesn't change much — there's no cure to start; suppression is for symptomatic people
3. Stigma harms — a false positive can carry significant social and psychological cost
4. Prevention is the same regardless of result — condoms, antivirals during outbreaks, U=U-style guidance

When IS HSV blood testing reasonable?

• Pregnancy with unknown status and partner with HSV
• Symptoms but no active lesion to swab
• Known exposure to HSV-2 with clinical concern
• Some HIV care contexts
• Patient-requested with informed counseling about false positives

How HSV-1 and HSV-2 IgG differ

The tests are type-specific — they can usually tell HSV-1 from HSV-2.

• HSV-1 IgG: Very common to be positive. Most adults have HSV-1 (oral) by middle age. A positive HSV-1 IgG mostly tells you about old oral exposure.
• HSV-2 IgG: Less common. Around 12% of US adults have HSV-2 antibodies. A positive HSV-2 IgG is more likely to indicate genital infection.

Important: HSV-1 can cause genital infection (and increasingly often does). A positive HSV-1 IgG doesn't rule out genital herpes from HSV-1.

Common false-positive scenarios

The HerpeSelect ELISA can show false positives in:

• HIV-positive individuals
• Recent infections with other viruses
• Some autoimmune conditions
• Pregnancy (rare)
• Vaccinations (rare)
• Cross-reactivity with HSV-1 antibodies (at low values)

What to ask your doctor

If you get a positive HSV blood test:

"What was the index value? Was it low positive (1.1-3.5) or high positive (>3.5)?"

"Did the lab order HSV-1 IgG, HSV-2 IgG, or both?"

"Was an IgM ordered? (If yes, I'd push back — CDC doesn't recommend it.)"

"Given the index value and my clinical history, should I get a Western blot confirmatory test?"

"Was this part of a routine screening or did I have specific symptoms / exposure?"

If you get a negative HSV blood test:

"When was my last possible exposure? Am I within the window period?" (HSV IgG can take 3-6 months to develop fully)

"Should I be retested in 3-6 months if exposure was recent?"

How to think about your result

High-positive IgG, history of symptoms or known exposure

• Very likely true positive
• Standard management: antivirals during outbreaks, partner discussions, U=U-style guidance

High-positive IgG, no symptoms, no known exposure

• Possible asymptomatic infection (~80% of HSV is asymptomatic at any given time)
• Some chance of false positive even at high values, but much lower
• Counseling about transmission, suppression options

Low-positive IgG (1.1-3.5), no symptoms, no known exposure

• Substantial chance of false positive — confirm with Western blot if possible
• Don't accept this as a definitive diagnosis without confirmation
• Don't change relationships or self-image until confirmed

Equivocal (0.9-1.1)

• Retest in 4-8 weeks
• Could be developing antibody or background noise

What this all means for screening

The honest summary:

• Don't screen for HSV with blood tests if asymptomatic and no specific concern
• Do test if you have an active lesion — swab it for PCR
• Don't accept "low positive" HSV-2 IgG as a definitive diagnosis — confirm with Western blot
• Don't order IgM tests — they don't tell you what they claim to

What a positive result does NOT mean

• You will have outbreaks (most people with HSV-2 antibodies have few or no recognized outbreaks)
• You can't have safe relationships (transmission risk is manageable)
• You can't have children (vertical transmission risk is low and preventable)
• Your life is ruined (it really isn't)

See newly diagnosed with herpes — what to do next for context.

Bottom line

HSV blood tests are useful in specific contexts but problematic when used as routine screening:

• IgG tests with high positive index values are reliable
• IgG tests with low positive index values have ~50% false-positive rates — confirm with Western blot
• IgM tests should generally not be ordered
• Don't screen asymptomatic people unless there's a specific reason

If you've been told you have HSV-2 based on a blood test, ask for your index value and ask whether confirmatory testing is appropriate. A "positive" you don't need to live with is one of the most consequential medical conversations to get right.

For more on herpes — testing, daily life, treatment — see our herpes pillar guide, HSV test results — what your number actually means, and newly diagnosed with herpes.