The main test for herpes depends on whether you have visible sores. If you have active blisters or lesions, a provider will swab the sore and send the sample for a PCR test (also called nucleic acid amplification testing, or NAAT). If you have no symptoms, a blood test can check for antibodies, though blood tests have significant limitations and are not recommended for routine screening.
Swab Tests for Active Sores
When you have a blister, sore, or ulcer, the most reliable way to confirm herpes is a swab test. A provider uses a swab to collect fluid or cells from the lesion, then sends the sample to a lab. There are two main types of swab tests: PCR and viral culture.
PCR is the preferred method. It works by detecting the virus’s genetic material in the sample and is significantly more sensitive than culture. In one study comparing the two, PCR detected the virus in 86% of confirmed cases while culture caught only 43%. PCR is also much faster: results typically come back within a day, compared to about a week for culture. Both tests are highly specific, meaning a positive result is very reliable.
Viral culture was the traditional gold standard for years and is still used in some settings. A lab takes the swab sample and tries to grow the virus in a controlled environment. It works best on fresh, fluid-filled blisters. As lesions begin to crust over and heal, both PCR and culture become less reliable because the amount of virus on the skin surface drops. A negative swab result when you have active sores does not rule out herpes, since viral shedding is intermittent.
Both PCR and culture can tell you whether the infection is HSV-1 or HSV-2, which matters for understanding your likely pattern of outbreaks and transmission risk.
Timing Your Swab Test
If you develop sores and want the most accurate result, get swabbed as early as possible. The ideal time is when a blister is new and still filled with fluid. Once a sore starts drying out, scabbing, or healing, the chance of a false negative rises. Recurrent outbreaks also tend to shed less virus than a first episode, which can further reduce sensitivity. If you notice what might be a herpes sore, getting to a clinic within the first 48 hours gives you the best shot at a clear answer.
Blood Tests for Herpes
Blood tests look for antibodies your immune system produces in response to HSV infection. They don’t detect the virus itself. Instead, they tell you whether your body has ever been exposed to HSV-1, HSV-2, or both. This makes them useful when you have no active sores to swab but want to know your status.
The most common blood tests are type-specific IgG tests, which can distinguish between HSV-1 and HSV-2. However, these tests have well-documented accuracy problems. For HSV-2, the standard IgG test has a sensitivity of about 92% (meaning it catches most true infections) but a specificity of only 57% when compared to the gold-standard Western blot. That low specificity means a substantial number of positive results are actually false positives, especially in people at low risk of infection. For HSV-1, sensitivity drops to around 70%, with specificity at 92%.
The CDC does not recommend herpes blood testing for people without symptoms in most situations, specifically because of the high chance of incorrect results. A false positive can cause real emotional harm for something you may not actually have.
The Window Period Problem
After a new herpes infection, your body needs time to build detectable antibodies. If you get a blood test too soon after exposure, the result may come back negative even though you’re infected. Most people develop detectable antibodies within 2 to 12 weeks, but for some it can take longer. If you suspect a recent exposure and your blood test is negative, a repeat test several weeks later gives a more reliable picture.
What “Low-Positive” Results Mean
IgG blood tests report results as an index value rather than a simple positive or negative. Previous research has found that index values below 3.0 are associated with a high rate of false positives. If your HSV-2 IgG result falls in this low-positive range, it should not be taken at face value. A confirmatory test is the appropriate next step.
The gold-standard confirmatory blood test is the Western blot, developed at the University of Washington. This test separates viral proteins and checks which ones your antibodies react to, providing a much more detailed and accurate picture. It reports results for HSV-1 and HSV-2 separately as positive, negative, or indeterminate. The Western blot is not FDA-cleared and is only available through the University of Washington lab, so it requires a specific order from your provider and can take longer to process. But for anyone with a low-positive IgG result, it’s the most definitive blood test available.
Which Test You’ll Actually Get
In practice, the test you receive depends entirely on your situation. If you walk into a clinic with visible sores, you’ll get a swab, ideally a PCR test. If you have no symptoms but want to know your status, perhaps because a partner disclosed their diagnosis, a blood test is the only option. Some providers will order both a swab and a blood test during a first outbreak to confirm the diagnosis and establish which type you have.
Herpes is not included in standard STI panels. If you’ve been tested for “everything,” herpes was almost certainly not part of that screening. You need to specifically ask for it. And if a blood test does come back positive with a low index value, requesting Western blot confirmation before drawing conclusions is a reasonable step that can save you from acting on inaccurate information.