Herpes is tested in two main ways: swabbing an active sore or taking a blood sample to check for antibodies. The best option depends on whether you currently have symptoms. A swab test during an active outbreak is the most reliable method, while blood tests can detect past exposure even when no sores are present.
Swab Tests During an Active Outbreak
If you have a visible sore, blister, or lesion, a swab test is the most accurate way to confirm herpes. A clinician will open the blister and scrape the base of the lesion with a swab to collect fluid and cells. This matters because the virus concentrates at the base of the sore, not just in the fluid on top.
Timing is important. Specimens collected during the acute or vesicular (blister) stage contain the highest amount of virus. Ideally, you should get swabbed within three to four days of symptoms appearing, and no later than seven days. After that, the lesion begins healing and viral shedding drops, which increases the chance of a false negative. If you use creams, ointments, or antiseptics on the area before your appointment, these can interfere with the test, so leave the sore untreated before collection.
There are two types of swab tests, and they are not equally reliable:
- PCR (polymerase chain reaction): This test detects the virus’s DNA and is now the standard method. In head-to-head comparisons, PCR has 100% sensitivity, meaning it catches virtually every positive case from a swab.
- Viral culture: The older method, which tries to grow the virus in a lab. Its sensitivity is only about 50%, so it misses roughly half of true infections. Cultures also take longer to produce results. Most labs have moved to PCR, but some still use cultures.
Both methods can identify whether the infection is HSV-1 or HSV-2, which is useful information since the two types behave differently over time. If you’re getting a swab test, it’s worth confirming that your provider is ordering the PCR version.
Blood Tests When No Symptoms Are Present
Blood tests don’t detect the virus itself. They detect antibodies your immune system produces in response to herpes. This means a blood test can tell you whether you’ve ever been exposed to HSV-1 or HSV-2, but it cannot tell you when you were infected or where on your body the infection is located.
The standard blood test looks for IgG antibodies, which your body produces days to weeks after initial infection and then maintains for life. After exposure, it takes 2 to 12 weeks for these antibodies to reach detectable levels. If you test too early, within the first few weeks after a potential exposure, you can get a false negative simply because your body hasn’t built up enough antibodies yet.
Modern type-specific blood tests work by targeting a protein called glycoprotein G, which differs between HSV-1 and HSV-2. This allows the test to distinguish between the two types on a single blood draw. Older, non-type-specific tests that lump both types together are less clinically useful and generally not recommended.
Why IgM Tests Are Not Recommended
You may see IgM antibody tests offered, sometimes marketed as a way to detect “new” infections. Major health organizations advise against using IgM tests for herpes. IgM antibodies can reappear during recurrences (not just new infections), cross-react with other viruses, and produce unreliable results. If your provider orders an IgM test instead of an IgG test, it’s reasonable to ask why.
Understanding Your Blood Test Results
IgG blood test results come back as an index value, not a simple positive or negative. Understanding where your number falls matters, because results in a certain range are frequently wrong.
For the most commonly used screening test (HerpeSelect), an index value below 0.90 is negative, 0.90 to 1.09 is equivocal, and 1.10 or above is considered positive. Here’s the catch: low-positive results, those falling between 1.10 and 3.50, have a significant false-positive rate. The CDC recommends that any result in this low-positive range be confirmed with a second, more specific test. One study found that up to 38.7% of low-positive HSV-2 results and as many as 76.1% of low-positive HSV-1 results on certain platforms were falsely positive.
If your index value is above 3.50, a false positive is much less likely, and the result is generally considered reliable without further testing. If your number falls in that 1.10 to 3.50 zone, don’t panic. Ask for confirmatory testing before drawing conclusions.
Confirmatory Testing for Uncertain Results
The gold standard for confirming a questionable blood test result is the Western Blot, a more detailed antibody test developed at the University of Washington. This test is not widely available through standard commercial labs. It is performed at UW Medicine’s virology lab in Renton, Washington, and your provider needs to specifically order it and arrange for the sample to be sent there. It runs on Mondays and Thursdays.
The Western Blot has not gone through FDA clearance (it’s classified as a laboratory-developed test), but it is widely regarded in the clinical community as the most accurate serologic test for herpes. It can confirm or rule out both HSV-1 and HSV-2. If you’ve received a low-positive screening result and want certainty, this is the test to request. Some providers may also use an HSV-2 IgG inhibition assay as a confirmatory option, which is more widely available through reference labs.
Why Routine Screening Isn’t Standard
If herpes is so common, you might wonder why it isn’t part of a standard STI panel. The CDC explicitly does not recommend routine HSV-2 blood screening for the general population, including asymptomatic pregnant women. The reasoning comes down to the limitations of current blood tests. The high false-positive rate in low-positive ranges means screening large numbers of people without symptoms would produce many incorrect diagnoses, causing unnecessary anxiety and follow-up testing without clear clinical benefit.
Testing is recommended when you have symptoms, when a sexual partner has herpes, or when you want a complete picture of your STI status and understand the limitations of the results. If you specifically request a herpes blood test, most providers will order one, but it typically won’t be included automatically alongside tests for chlamydia, gonorrhea, or HIV.
Which Test to Ask For
Your situation determines the best test. If you have an active sore, blister, or any suspicious lesion, get a PCR swab as soon as possible, ideally within the first few days. This gives you the most accurate result and identifies the virus type. If you have no symptoms but want to know your status, ask for a type-specific HSV IgG blood test, and make sure it’s been at least 12 weeks since your last potential exposure to avoid false negatives during the antibody window period.
If your blood test comes back with a low-positive index value between 1.10 and 3.50, request confirmatory testing before accepting the diagnosis. And if you’re told you’re being tested for herpes as part of a routine panel, verify which test is actually being run. A type-specific IgG test and a PCR swab give you actionable information. A non-type-specific antibody test or an IgM test generally does not.