Herpes is tested either by swabbing an active sore or by drawing blood to check for antibodies. The right test depends on whether you currently have symptoms. A swab test during an active outbreak is the most reliable way to get a diagnosis, while a blood test can detect a past infection even without visible sores.
Swab Tests During an Active Outbreak
If you have a blister, sore, or any suspicious lesion, the most accurate option is a swab test. A clinician opens the fluid-filled blister with a sterile instrument, then presses a swab firmly against the sore to collect both fluid and cells from the base and edges. The sample goes into a transport vial and is sent to a lab.
Two types of swab tests exist: viral culture and PCR (a DNA-based test). PCR is significantly more sensitive. In one study comparing the two methods in neonatal patients, PCR detected the virus in 86% of confirmed cases while culture caught only 43%. PCR’s sensitivity approached 100% when tested head-to-head against culture on the same specimens. Because of this accuracy gap, PCR has largely replaced culture as the preferred swab test in many labs, though some clinics still use culture.
Timing matters with swab tests. They work best on fresh blisters or sores that haven’t started crusting over. Once a lesion is healing, the amount of virus drops and false negatives become more likely. If your sores have already scabbed, a clinician can still attempt a swab by removing the crust and scraping the base of the lesion, but results are less reliable than from a newer sore.
Blood Tests for Herpes Antibodies
Blood tests detect antibodies your immune system produces in response to herpes, not the virus itself. This makes them useful when you have no active sores but want to know your status. The standard blood test looks for IgG antibodies to a specific protein called glycoprotein G, which differs between HSV-1 (the type that typically causes oral herpes) and HSV-2 (the type more commonly associated with genital herpes). This protein difference is what allows the test to tell you which type you have.
There is a significant catch with timing. After a new exposure, your body can take up to 16 weeks or longer to produce enough antibodies for the test to detect. Testing too early after a potential exposure can produce a false negative. If you think you were recently exposed, waiting at least 12 to 16 weeks before a blood test gives the most reliable result.
Why IgM Tests Are Unreliable
Some providers order an IgM blood test, which is supposed to detect recent infection. This test has serious accuracy problems and most herpes experts advise against using it. IgM antibodies were detected in roughly 30% of people with long-established HSV-2 infections, meaning the test frequently lights up in people who were infected years ago, not recently. IgM only reliably indicates a new infection in people who have no IgG antibodies at all. If your provider orders an IgM test, it’s worth asking for a type-specific IgG test instead.
Understanding Your IgG Results
IgG blood test results come back with a numerical index value, not just a simple positive or negative. A value below 0.90 is typically negative. Values between 0.90 and 1.09 fall into an equivocal (uncertain) range and usually need retesting. A value of 1.10 or higher is considered positive.
Here’s where it gets complicated. Low-positive results, particularly index values between 1.10 and 3.50, have a meaningful rate of being false positives. The CDC recommends confirmatory testing for any result in this low-positive range. If your HSV-2 IgG comes back with an index in the 1.10 to 3.50 zone, a second-level test should be run before you consider the result definitive. Quest Diagnostics, for example, automatically runs a confirmatory inhibition assay on any HSV-2 IgG result with an index between 1.10 and 6.00.
Results with an index value well above 3.50, especially above 5.0 or higher, are much more likely to be true positives and typically don’t need additional confirmation.
The Gold Standard Confirmatory Test
If your results are ambiguous or you need absolute certainty, the HSV Western Blot offered through the University of Washington is considered the gold standard. It’s the most accurate blood test available for herpes and is recognized by the FDA for confirmatory testing. You can order a test kit by calling their virology lab directly. Your provider sends a blood sample to the University of Washington, and the lab runs the analysis. This test is particularly valuable when you’ve received a low-positive IgG result and want to know whether it’s a true infection or a false alarm.
Who Should Get Tested
Routine herpes blood testing for people without symptoms is not universally recommended. The CDC does not include herpes in standard STI screening panels for the general population. The main reasons: the high rate of false positives at low index values, the psychological impact of a diagnosis for a virus that may never cause symptoms, and the limited clinical benefit when someone has no outbreaks.
That said, testing makes clear sense in certain situations. If you have a partner with known herpes and want to check your own status, a type-specific IgG test is reasonable. The same applies if you have recurring sores that haven’t been formally diagnosed, if you’ve had a potential exposure and want clarity after waiting the full antibody window period, or if you’re pregnant and your partner has a history of genital herpes. If you do have an active sore, skip the blood test entirely and go straight for a swab, since it gives you a faster, more definitive answer.
What to Expect at the Appointment
For a swab test, the process takes just a few minutes. The clinician examines the sore, opens it if it’s still a fluid-filled blister, and firmly swabs the area. It can sting briefly, especially on raw or ulcerated skin. Results from a PCR swab typically come back within one to three days, while a viral culture can take up to two weeks.
For a blood test, it’s a standard blood draw from your arm. Results usually return within a few days to a week depending on the lab. If your result falls in the low-positive range and triggers a confirmatory test, that can add additional processing time. The Western Blot from the University of Washington, if needed, may take several weeks from the time the kit is ordered to when results arrive.