The short answer: A negative swab and a positive blood test don't necessarily contradict each other, because the two tests look for different things. A swab (PCR or culture) looks for the virus in a sore at that moment, and the CDC says a negative swab, especially from an older or healing sore, does not rule out herpes. A blood test looks for antibodies from an infection at any time, but the most common HSV-2 blood test is often falsely positive at low index values (1.1 to 3.0), so the CDC calls for a second, different test (Biokit or Western blot) before that kind of result is interpreted.

If your lab results seem to say "yes" and "no" at the same time, you're not alone; it's one of the most common questions on herpes forums. Below, we walk through what the CDC's STI Treatment Guidelines on genital herpes say about why results disagree, what the numbers mean, why a partner can test negative, and what to ask a clinician. For a side-by-side look at the blood tests themselves, see our guide to herpes blood tests and which one actually works.

Swab vs. blood test: what does each one measure?

Herpes tests fall into two groups, and they answer different questions:

• Virologic tests (swabs): a sample is taken from a sore and tested for the virus itself, either by NAAT (nucleic acid amplification, such as PCR) or by viral culture. A positive result shows HSV is in that sore, and the lab can tell whether it's HSV-1 or HSV-2. The CDC calls NAAT the most sensitive test for sores.
• Type-specific serologic tests (blood tests): these look for IgG antibodies to HSV-1 or HSV-2. According to the CDC, these antibodies develop during the first weeks after infection and last indefinitely. A blood test can suggest that someone has the infection, but it doesn't show where on the body the virus is or whether a particular sore is herpes.

The CDC's genital herpes fact sheet adds a point that matters a lot for people with confusing results: a herpes blood test "cannot tell you who gave you the infection or when you got the infection."

Why can a herpes swab come back negative?

A negative swab is only a snapshot. The CDC guidelines list several reasons a swab can miss herpes:

• Timing of the sore. The guidelines say that failing to find HSV by NAAT or culture, "especially in the presence of older lesions or the absence of active lesions, does not indicate an absence of HSV infection because viral shedding is intermittent."
• The kind of test. Viral culture has low sensitivity, especially for recurrent sores, and it drops quickly as sores begin to heal. NAAT is more sensitive, but FDA-cleared assays still vary in sensitivity from 90.9% to 100%.
• No sore to test. The CDC says random or "blind" genital swabs taken when no sore is present should not be used to diagnose genital herpes, because sensitivity is low and a negative result doesn't exclude infection.

The CDC also says HSV PCR on blood should not be used to diagnose genital herpes, except when a rare, widespread (disseminated) infection is suspected.

A negative swab can also simply be right about that particular sore, since genital bumps have other causes. Because a blood test can't say where an infection is, the two results together can mean the sore wasn't herpes but the person has HSV antibodies from an infection elsewhere or at another time, such as oral HSV-1 picked up in childhood.

What does a positive swab with a negative blood test mean?

The reverse mismatch has a common explanation, too: timing. The CDC says false-negative blood tests "might be more frequent at early stages of infection," because antibodies take weeks to develop. Someone with a new infection can have a positive swab from a first sore before their blood test turns positive. The CDC also puts the sensitivity of type-specific HSV-2 blood tests at 80% to 98%, so some people with HSV-2 test negative on a given blood test.

Why isn't a herpes IgM test recommended?

Some labs still offer IgM antibody tests, and a positive IgM result causes a lot of worry. The CDC is direct about it: IgM testing for HSV-1 or HSV-2 "is not useful because IgM tests are not type specific and might be positive during recurrent genital or oral episodes of herpes." Its conclusion: "HSV IgM testing is not recommended."

In practice, a positive IgM can't show which virus type is present and doesn't reliably mean an infection is new. The CDC recommends type-specific IgG tests, most of which are based on glycoprotein G (gG2 for HSV-2, gG1 for HSV-1); older type-common tests can't tell HSV-1 and HSV-2 apart.

What does a HerpeSelect index value of 1.1 to 3.0 mean?

Many lab reports for the HerpeSelect HSV-2 enzyme immunoassay (EIA), the most commonly used HSV-2 blood test, show an index value, not just "positive" or "negative." Here's what the CDC guidelines say about it:

• The HerpeSelect HSV-2 EIA "often is falsely positive at low index values (1.1–3.0)."
• One study the CDC cites found an overall specificity of 57.4%, and a specificity of only 39.8% for index values of 1.1 to 2.9. Specificity is how well a test avoids false positives. Lower specificity means more false positives.
• Because of this, "particularly with low index values (<3.0)," the CDC says a confirmatory test using a second method should be done before the result is interpreted.

So a "positive" HSV-2 result with an index of, say, 1.8 isn't a diagnosis on its own under CDC guidance. It's a result that needs confirming.

What about a high index value?

The CDC's warning about false positives is aimed mainly at low index values (below 3.0). The guidelines don't set a cutoff above which a result is automatically certain, and they describe the commercial EIAs as having "poor specificity" in general. A higher value that is also positive on a second-method confirmatory test means two different kinds of tests agree. What that means for an individual is a question for a clinician who knows their history.

What are confirmatory tests like the Biokit and Western blot?

The CDC names two confirmatory options for HSV-2: the Biokit test and the Western blot. Both use a different method from the HerpeSelect EIA, and the guidelines say using them for confirmation has been reported to improve the accuracy of HSV-2 blood testing.

Two details are easy to miss:

• Not every "second test" counts. The CDC says the HerpeSelect HSV-2 immunoblot should not be used for confirmation, because it uses the same antigen as the HerpeSelect EIA.
• If confirmation isn't available, the CDC says people should be counseled about the limits of testing before blood is drawn, and that clinicians should be aware that false positives happen.

What if the Western blot is negative or indeterminate?

A low-positive EIA that isn't confirmed by a second method is exactly the situation the CDC's specificity warning describes: false positives at low index values are common, and the CDC doesn't treat an unconfirmed low-positive EIA as a result to interpret on its own. Some labs report a confirmatory result as "indeterminate," meaning the test didn't give a clear answer either way. The CDC guidelines don't set out a single next step for that case, so it's worth going over with a clinician, including whether and when repeat testing makes sense.

How long after exposure should a herpes blood test be repeated?

The CDC's guidance is specific here. Because false negatives are more common early in infection, "in cases of recent suspected HSV-2 acquisition, repeat type-specific antibody testing 12 weeks after the presumed time of acquisition is indicated." A negative HSV-2 blood test taken a few weeks after a possible exposure may simply be too early.

Our post STD Testing 101: Why Your Results Might Miss Herpes, HPV & More explains why herpes often isn't on a standard STI panel.

What does a positive HSV-1 blood test mean?

HSV-1 results are harder to read than HSV-2 results, according to the CDC:

• Because nearly all HSV-2 infections are sexually acquired, HSV-2 antibodies imply an anogenital infection.
• HSV-1 antibodies alone are "more difficult to interpret." The blood test doesn't distinguish oral from genital infection, and many people with HSV-1 antibodies picked up oral herpes in childhood, often without symptoms.
• HSV-1 blood testing also has low sensitivity, so it can miss HSV-1 antibodies.
• The CDC says genital HSV-1 is confirmed by a virologic test (a swab) from a genital sore.

If my blood test is positive, why did my partner test negative?

This happens often, and it doesn't by itself mean either result is wrong. Several facts from the research help explain it:

• Transmission doesn't happen every time. In a large trial published in the New England Journal of Medicine (Corey et al., 2004), researchers followed 1,484 monogamous heterosexual couples in which one partner had symptomatic genital HSV-2 and the other didn't. Over 8 months, 27 of 741 susceptible partners (3.6%) in the placebo group acquired HSV-2, compared with 14 of 743 (1.9%) when the partner with HSV-2 took daily valacyclovir. All couples were counseled on safer sex and offered condoms.
• Shedding varies. The CDC says people with HSV-2 antibodies who have never had symptoms have a 50% lower risk of genital viral shedding than people with symptomatic genital herpes.
• The partner may have been tested too early. If an exposure was recent, the CDC's 12-week retest window applies to the partner's result, too.
• No test is perfect. HSV-2 blood tests range from 80% to 98% sensitivity, so a partner's negative result can occasionally be a false negative.
• The infection may predate the relationship. A blood test can't show when someone was infected. Most people with HSV-2 have never been diagnosed, and the CDC estimates 11.9% of people aged 14 to 49 in the US have HSV-2.

The CDC also recommends that partners of people with symptomatic genital herpes be offered type-specific HSV-2 blood testing, to learn whether they already have HSV or are at risk of acquiring it. A negative partner result is useful information for both people. If you're in that situation, our guide on how to date when one of you has an STI and one doesn't covers the relationship side.

What questions can you ask a clinician about mixed results?

Bringing the actual lab report, not just "positive" or "negative," helps. Questions people often find useful:

• Which test was this exactly: a swab (PCR or culture) or a blood test? If a blood test, was it type-specific IgG, and was IgM also run?
• What was the index value, and does it fall in the 1.1 to 3.0 range the CDC flags?
• Can a second-method confirmatory test (Biokit or Western blot) be ordered, and how is the sample sent?
• Based on the timing of any possible exposure, would a repeat blood test 12 weeks later be useful?
• If a sore comes back, can it be swabbed early, while it's fresh?
• Should a partner be offered testing, and what would their result tell us?

If you've already been diagnosed, or you're processing a positive result, our post on what to expect after a first herpes outbreak or positive test walks through the next steps.

What do conflicting herpes results mean for dating and disclosure?

Uncertain results are stressful, especially in a new relationship. A few things can help:

• Be honest about what you know and don't know. "My blood test was low-positive and I'm waiting on a confirmatory test" is accurate and gives a partner what they need to make their own choices. The CDC's counseling guidance for people with genital herpes stresses telling current partners and telling future partners before starting a sexual relationship.
• Disclosure expectations and legal duties differ by state. Laws on STI disclosure vary across the US, so a licensed attorney is the right source for legal questions where you live.
• Risk can be lowered, not just avoided. The CDC says condoms used consistently and correctly reduce, but don't eliminate, the risk of genital herpes transmission, and that daily valacyclovir reduces HSV-2 transmission in discordant heterosexual couples where one partner has a history of genital HSV-2. The CDC also notes there are no data on whether suppressive therapy prevents transmission when HSV-2 was found only by a blood test in someone without symptoms. Whether any medicine makes sense is a decision for a clinician.
• Date safely and without pressure. Never pressure a partner, make sure everything is consensual, meet new matches in public and verify who you're talking to.

A confirmed herpes diagnosis is common and manageable. When you're ready, our guide on how to start dating again after a positive test result is a good place to start.

FAQ

Can a herpes swab be negative if I have herpes?

Yes. The CDC says a negative swab, especially from an older or healing sore or when no active sore is present, does not rule out herpes, because the virus is shed on and off. Viral culture is less sensitive than PCR and becomes less sensitive as sores heal.

Is a positive herpes IgM test a diagnosis?

No, not according to the CDC. It says IgM tests are not type specific, can be positive during recurrences, and are not recommended for herpes testing.

Is an HSV-2 index of 1.1 to 3.0 a false positive?

Not always, but often. The CDC says the HerpeSelect HSV-2 EIA is often falsely positive at index values of 1.1 to 3.0 and calls for confirmation with a second method, such as the Biokit or Western blot, before the result is interpreted.

How long should I wait to retest after an exposure?

The CDC says that for a recent suspected HSV-2 exposure, repeat type-specific antibody testing is indicated 12 weeks after the presumed time of acquisition.

Is routine herpes blood screening recommended?

The CDC does not recommend HSV-2 blood screening for the general population. It lists situations where a type-specific blood test is useful, such as recurring or unusual genital symptoms with a negative swab, a clinical diagnosis without lab confirmation, or having a partner with genital herpes.

The bottom line

Herpes tests disagree more often than most people expect, and most of the time there's a clear reason: a swab taken from an older sore, a blood test taken too soon, an IgM test the CDC doesn't recommend, or a low-positive EIA that hasn't been confirmed by a second method. Knowing which test was run, the index value and the timing turns a confusing result into a set of specific questions for a clinician. And whatever your results turn out to be, you don't have to figure out dating alone. Meet people who get it through herpes dating on MeetPositives.

Sources

• Centers for Disease Control and Prevention. Sexually Transmitted Infections Treatment Guidelines, 2021: Genital Herpes (page last reviewed September 21, 2022).
• Centers for Disease Control and Prevention. About Genital Herpes (fact sheet, February 20, 2024).
• Corey L, Wald A, Patel R, et al. Once-daily valacyclovir to reduce the risk of transmission of genital herpes. New England Journal of Medicine. 2004;350(1):11-20.

This article is for general information only and is not medical or legal advice. It does not create a doctor-patient relationship. Talk with a licensed health care provider about your own situation. Laws vary by state; for legal questions, talk with a licensed attorney.