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Herpes Blood Test Accuracy and False Positives

By Richard D. Porter, Patient Advocate Updated

Quick answer

A herpes blood test looks for antibodies, and the CDC says current herpes tests are not as exact as tests for chlamydia and gonorrhea. Results can be falsely positive, especially at low index values (the CDC says one HSV-2 blood test often gives false positives at index values of 1.1 to 3.0), and can be falsely negative if the test is done too early. If you get a low positive, ask whether a confirmatory test with a different method is needed. If you have a sore right now, a swab of the sore is the more sensitive test.

The test you need

Herpes: Type-specific blood testReliable by day 84

Too earlyMay detect; retest if negativeReliable

Counted from your most recent exposure. Source: CDC. Work out your own dates

Test
Type-specific herpes antibody blood test (HSV-1 and HSV-2)
Sample
A blood sample
When
CDC: after exposure, it can take up to 16 weeks or more for current tests to detect infection, and a blood test done too soon can be wrong
Results
STDCheck's herpes page says results are ready in 1 to 2 days

A herpes blood test can feel like it should give a clean yes or no. In practice it is less exact than that. The CDC says current herpes tests “are not as exact as tests for chlamydia and gonorrhea,” so what the result means depends on which test was used, when it was done and what number is printed on the report.

How accurate is a herpes blood test?

A herpes blood test looks for antibodies your body makes in response to the virus, not the virus itself. The CDC says antibodies to HSV develop during the first weeks after infection, so timing matters.

Accuracy also varies by virus type:

  • HSV-2 tests. The CDC says the sensitivity of glycoprotein G type-specific tests for HSV-2 antibody vary from 80% to 98%. That means some infections are missed.
  • HSV-1 tests. The CDC says HSV-1 blood testing has low sensitivity for detecting HSV-1 antibody.
  • Swab tests. If there is a sore, a swab test (NAAT) is the most sensitive herpes test, according to the CDC.

Can a herpes blood test give a false positive?

Yes. The CDC says false-positive results are much more likely with herpes tests than with chlamydia or gonorrhea tests, and wrong results are more likely in people at low risk of infection.

The problem is greatest at low index values. The index value is a number some labs report with a result. The CDC says one HSV-2 blood test “often is falsely positive at low index values (1.1 to 3.0).” The CDC cites one study with a specificity of 39.8% for index values of 1.1 to 2.9.

What is a confirmatory herpes test?

If a first blood test comes back positive, especially with a low index value, the CDC says a confirmatory test using a second method should be done before the result is interpreted. It names the Biokit test and the Western blot as examples.

If you are looking at a lab report with a low number, ask your clinician:

  • Which test was used, and what was my index value?
  • Do I need a confirmatory test with a different method?
  • Should I repeat the test later?

Can a herpes blood test be negative if I have symptoms?

It can. The CDC says a blood test done too soon after infection can give a wrong result, and false negatives may be more frequent at early stages of infection. If you have a sore, the better route is a swab of the sore while it is still fresh, not a blood test. The CDC says a provider can swab a blister or sore that is not already crusted over or healing.

If the swab is not possible and you are worried about a recent exposure, the CDC advises repeating type-specific antibody testing 12 weeks after the presumed time of infection. It also notes that after exposure, current tests can take up to 16 weeks or more to detect infection. Read more in when to get tested for herpes.

Are there herpes blood tests I should avoid?

Yes. The CDC says HSV IgM testing is not recommended. If a lab or an online service offers an IgM herpes test, a type-specific antibody test is the more useful one to ask about.

Should I get a herpes blood test at all?

The CDC does not recommend routine herpes blood screening for the general population. It does say blood tests might be useful in some situations, such as when a partner has herpes. Whether it is right for you is a conversation to have with a clinician, ideally before you order.

What should I do after a positive result?

A positive blood test is not a diagnosis on its own, so avoid drawing conclusions from the first number you see. Ask about confirmation, and ask what the result does and doesn’t tell you. The CDC describes genital herpes as a common STI that can be treated, and a clinician can talk through options if it is relevant to you.

Get care now if

  • You have new painful blisters or sores, particularly if they are spreading or getting worse quickly
  • You have a fever or feel generally unwell along with genital sores
  • You are pregnant and have genital sores, especially close to your due date
  • Your immune system is weakened, for example by HIV or chemotherapy, and you have new sores
  • You have sores near your eye, or eye pain, redness or blurred vision

Call 911 for a medical emergency. Otherwise, an urgent care clinic, sexual health clinic or your doctor can see you.

When to test

  1. If you have a sore or blister nowAsk for a swab of the sore. The CDC says swab tests are the most sensitive, and a sore that is not yet crusted over or healing can be swabbed.
  2. If you are testing after a possible exposure with no soreA blood test done too soon can be wrong. The CDC advises repeating type-specific antibody testing 12 weeks after the presumed time of infection if recent infection is possible.
  3. If you get a low positive resultAsk your clinician whether a confirmatory test using a different method is needed before you decide what the result means.

Common questions

Is a herpes test part of a standard STD panel?

Not always. The CDC says herpes blood tests may or may not be part of the tests your healthcare provider gives you. If you want one, ask for it by name, and ask which infections you are and aren't being tested for.

How long do herpes blood test results take?

It depends on the test. The CDC says point-of-care blood tests for HSV-2 antibodies give results during a clinic visit. STDCheck's herpes page says its results are ready in 1 to 2 days.

Should I get a herpes blood test with no symptoms?

The CDC does not recommend herpes blood screening for the general population, partly because of false results. It can be considered in some situations, such as when a partner has herpes, so talk it through with a clinician.

Does a positive HSV-1 result mean I have oral herpes?

Not always. STDCheck's own herpes page says a positive result for HSV-1 does not always mean a person has oral herpes. Ask a clinician what a result means for the part of your body you are worried about.

Next: Type-specific herpes antibody blood test (HSV-1 and HSV-2).

A blood sample. CDC: after exposure, it can take up to 16 weeks or more for current tests to detect infection, and a blood test done too soon can be wrong

Read next

Sources

This guide is general information written by a patient advocate using public health sources. It is not a diagnosis. Only a clinician or a lab test can tell you whether you have an infection. Read how we write and check our guides.