Herpes is tested in two main ways: swab tests that collect fluid from an active sore, and blood tests that detect antibodies your immune system has built against the virus. The method your provider recommends depends on whether you have visible symptoms at the time of testing. Each approach has meaningful differences in accuracy, and understanding those differences can save you from confusing or misleading results.
Swab Tests: The Most Reliable Option During an Outbreak
If you have an active sore, blister, or ulcer, a swab test is the most accurate way to confirm herpes. A provider uses a small swab to collect fluid or cells directly from the lesion, then sends the sample to a lab. The process takes a few seconds and can be mildly uncomfortable but isn’t painful. Results typically come back within a few days.
Two types of swab tests exist, and they differ significantly in accuracy. PCR (polymerase chain reaction) testing amplifies viral DNA from the sample and is now the preferred method. In studies comparing the two approaches, PCR detected herpes in 86% of confirmed cases, while traditional viral culture caught only 43%. Both methods have perfect specificity, meaning a positive result is highly reliable. But culture misses roughly half of true infections, making PCR the clear winner. If your provider orders a swab test, it’s worth confirming that the lab uses PCR rather than culture.
Timing matters. Swab tests are most accurate when a lesion is fresh, fluid-filled, and hasn’t yet crusted over. Once sores begin healing, the amount of detectable virus drops quickly, and the chance of a false negative rises. If you notice a new sore developing, getting tested as soon as possible gives you the best chance of a definitive answer.
Blood Tests: What They Measure and Where They Fall Short
When no sores are present, the only option is a blood test that looks for antibodies to herpes simplex virus. These tests detect IgG antibodies, which your body produces after infection and retains long-term. Type-specific IgG tests can distinguish between HSV-1 (which most commonly causes oral herpes) and HSV-2 (which most commonly causes genital herpes) by targeting a unique protein on each virus type.
The accuracy of blood tests is lower than most people expect. For HSV-1, IgG blood tests have reported sensitivity ranging from 69% to 99%, with specificity between 77% and 98%. HSV-2 blood tests perform worse, with specificity dropping as low as 57%. A 2016 review of 17 studies found that blood screening tests for HSV-2 produced false positives in roughly 50% of positive results among populations with prevalence similar to U.S. adults. That means if you’re in a lower-risk group and test positive for HSV-2 by blood, there’s a substantial chance the result is wrong.
This is the primary reason the CDC does not recommend routine herpes blood testing for people without symptoms. The risk of a false positive is too high in people who have no reason to suspect infection, and a wrong result can cause significant unnecessary distress.
The “Low Positive” Problem
HSV-2 IgG blood tests report results as an index value rather than a simple positive or negative. Values below 0.90 are negative, and values above 5.0 are considered reliably positive. The trouble zone lies between 0.91 and 5.0, sometimes called “low positive” or “equivocal” results. Per CDC guidelines, results in this range should automatically be sent for supplemental testing because they have a high rate of being false positives.
If you receive an HSV-2 blood test result in this range, ask your provider about confirmatory testing before accepting the diagnosis. Not all labs automatically reflex to a supplemental test, so you may need to request it.
Confirmatory Testing With Western Blot
The gold standard for confirming a herpes blood test result is the Western Blot, a more detailed antibody test developed at the University of Washington. This test separates viral proteins and checks your blood against each one individually, producing far fewer false positives than standard IgG screening tests.
The Western Blot is not widely available. It is performed at UW Medicine’s virology lab in Renton, Washington, and run only twice a week. Your provider can order it and ship your blood sample to the lab. It has not been FDA-cleared, which means insurance coverage varies, but it remains the most accurate blood-based herpes test available. If you’ve received a low-positive or equivocal IgG result and need certainty, this is the test to ask about.
Why IgM Tests Should Be Avoided
Some providers still order IgM antibody tests for herpes, but this test is not recommended by the American Sexual Health Association, the CDC, or most infectious disease specialists. The problems with IgM testing are numerous. It cannot reliably distinguish between HSV-1 and HSV-2, so a person with only oral herpes may receive a misleading positive for genital herpes. It cross-reacts with related viruses like the one that causes chickenpox, producing false positives. It can be negative in up to half of people with a confirmed new infection. And it can turn positive during recurrent outbreaks in people infected long ago, making it useless for determining when someone was infected.
If a provider offers you an IgM test for herpes, ask for a type-specific IgG test or a PCR swab instead.
When Each Test Makes Sense
- You have an active sore: A PCR swab test is the fastest path to an accurate diagnosis. Get tested before the sore starts healing.
- You have no symptoms but a partner tested positive: A type-specific IgG blood test is reasonable, but be aware of the false-positive risk, especially for HSV-2. A result in the low-positive range warrants confirmatory testing.
- You received a low-positive or equivocal blood result: Request supplemental testing or a Western Blot before accepting the result as a true diagnosis.
- You have no symptoms and no known exposure: Routine screening is generally not recommended because the chance of a misleading result outweighs the benefit of testing.
How Long to Wait Before Testing
Blood tests measure your immune response, not the virus itself. After a new infection, your body needs time to produce detectable levels of IgG antibodies. Most experts recommend waiting at least 12 weeks after a possible exposure before taking a blood test. Testing too early can produce a false negative because your body hasn’t generated enough antibodies yet. Swab tests don’t have this limitation since they detect the virus directly, but they require an active lesion to work.