Herpes Tests: Blood Test vs Swab and Which to Ask For

“Test for herpes” sounds simple, but there are two quite different tests, and picking the wrong one is the most common way to end up with a confusing result. Which one you want depends mostly on one thing: is there a sore right now?

What are the tests for herpes?

There are two main kinds.

• A swab of a sore or blister. A clinician collects fluid from the lesion and the lab runs a NAAT (a nucleic acid amplification test, often called a PCR test) or, less often, a viral culture.
• A blood test for antibodies. This looks for your body’s response to the virus rather than the virus itself.

The CDC says that if sores are not present, a blood test may be used to look for HSV antibodies. MedlinePlus says the same split: swab when there are sores, blood when there are not.

Is a swab or a blood test better?

If you have a sore, the swab. The CDC says HSV NAAT assays are the most sensitive tests because they detect HSV from genital ulcers or other lesions, and that the sensitivity of viral culture is low, especially for recurrent lesions and as lesions begin to heal. MedlinePlus says PCR is faster and more accurate than a viral culture.

Timing matters. The CDC says a provider can take a swab from a blister or sore that is not already crusted over or healing. If you wait until a sore has healed, there is nothing to swab. If you think a sore may be herpes, don’t wait for a convenient appointment.

What does a herpes blood test show?

It shows antibodies to the virus. MedlinePlus says the antibodies could be from an active infection now or a past infection, so it can’t tell you whether a recent sore was herpes or when you were infected. The CDC adds that HSV-1 blood testing does not distinguish between oral and genital infection.

Blood tests need time. MedlinePlus says the body takes up to three months to make HSV antibodies, so a test taken too early can come back negative when infection is present. For recent exposures the CDC says to repeat type-specific antibody testing 12 weeks after the presumed time of acquisition, and another CDC page says it can take up to 16 weeks or more for current tests to detect infection.

Can a herpes blood test be wrong?

Yes, and this is the main caution. The CDC says the chance of a false positive herpes test is much higher than for chlamydia or gonorrhea tests, because current tests aren’t as exact. For one commonly used test, the CDC says results at low index values (1.1 to 3.0) are often falsely positive, and it advises a confirmatory test (Biokit or Western blot) using a second method before interpreting the result.

Two other CDC points: IgM blood tests are not useful for herpes because they are not type-specific, and are not recommended. And a blood test done too soon after infection can be wrong the other way.

Should I get a herpes test if I have no symptoms?

The CDC does not recommend herpes testing for people without symptoms in most situations. It cites the limits of the blood test and a higher chance of wrong results in people at low risk. The CDC’s screening tables say HSV-2 serologic screening in the general population is not recommended.

The CDC says a blood test may be useful if you have genital symptoms, if you have or had a partner with genital herpes, or if a provider found signs of herpes and needs confirmation. MedlinePlus adds that testing without symptoms may be considered for some higher-risk situations, such as multiple partners, other STIs, or sex without condoms. In all of these, a conversation with a clinician comes first.

Where can I get tested for herpes?

A clinician or clinic that can swab a sore is the right route if you have one. Blood tests are drawn at a lab or clinic. STDCheck lets you order a herpes antibody test online and give a blood sample at a testing center. Its herpes page describes a type-specific IgG blood test with results in 1 to 2 days, and doesn’t mention a swab option.

STDCheck’s page also suggests waiting at least four weeks after potential exposure and retesting at three months after a negative. The CDC’s guidance is longer (12 weeks for a repeat, up to 16 weeks or more to detect infection), so plan around the CDC’s timing, and treat an early negative as provisional.

What should I ask for?

• With a sore: “Can you swab this for herpes with a PCR (NAAT) test?”
• Without a sore, with a reason to test: “Is a type-specific blood test appropriate for me, and when should I repeat it?”
• After a low positive: “Does this need a confirmatory test with a different method?”