Testing for herpes simplex virus types 1 and 2 depends on whether you have visible sores or not. If sores are present, a provider swabs the lesion and sends the sample for analysis. If there are no sores, a blood test checks for antibodies your immune system has built against the virus. Each approach has different strengths, timing requirements, and limitations worth understanding before you get tested.
Swab Tests for Active Sores
When blisters or open sores are present, the most direct way to test is a swab. A provider collects fluid from the lesion, and that sample is analyzed one of two ways: viral culture or PCR.
A viral culture grows the collected cells in a lab to see if the virus is present. It’s been used for decades but misses a significant number of infections. In comparative studies, culture detected HSV in roughly half the cases that PCR caught. PCR (polymerase chain reaction) looks for the virus’s genetic material directly. It’s faster, more sensitive, and now considered the preferred swab method at most labs.
Timing matters with swab tests. They work best on fresh, fluid-filled blisters. Once a sore has started crusting over or healing, the amount of virus drops sharply and the chance of a false negative climbs. If you suspect an outbreak, getting swabbed within the first 48 hours gives you the most reliable result. A swab test also tells you the type, HSV-1 or HSV-2, which is useful for understanding your likely outbreak pattern and transmission risk.
Blood Tests When There Are No Symptoms
If you don’t have sores, blood testing is the only option. These tests don’t look for the virus itself. Instead, they detect IgG antibodies, proteins your immune system produces in response to HSV infection. Because your body makes slightly different antibodies for each type, a type-specific blood test can distinguish between HSV-1 and HSV-2. The assays used in most labs rely on a surface protein called glycoprotein G, which differs between the two virus types and allows the lab to report each result separately.
Results come back as an index value. Below 0.90 is negative. Between 0.90 and 1.09 is equivocal, meaning the test can’t say either way. At 1.10 or above, the result is considered positive. These cutoffs apply to both HSV-1 and HSV-2 on the most widely used platforms.
The Window Period
After a new infection, your body needs time to produce enough antibodies for the test to detect. This window period is 3 to 6 months. If you were recently exposed and test negative, that result may not reflect your actual status. Retesting after the full window has passed gives a more reliable answer. Antiviral medication taken early can further delay antibody development, stretching the window even longer.
The False Positive Problem
Blood tests for HSV-2 have a well-documented issue with false positives, particularly when the index value falls in the low positive range, just above the 1.10 cutoff. The FDA has specifically warned that the chance of a false result increases when values land near the threshold. Many clinicians and sexual health experts consider index values between 1.10 and 3.5 unreliable enough to warrant confirmatory testing before accepting the diagnosis.
HSV-1 blood tests are somewhat less prone to this issue, but cross-reactivity between the two virus types can still muddy results. If your HSV-2 index is low positive and you have no history of genital symptoms, a confirmatory test is worth pursuing before drawing conclusions.
Confirming Uncertain Results
The gold standard for confirmation is the HSV Western Blot, developed and performed at the University of Washington’s Clinical Virology Lab. This test separates viral proteins and checks your blood against them in a more detailed way than standard assays. It’s the most reliable method available for resolving ambiguous or low-positive results.
The Western Blot is a lab-developed test, not FDA-cleared, which means most commercial labs don’t offer it. You or your provider need to contact the University of Washington lab directly to arrange testing. A blood sample is drawn locally and shipped to Seattle. It’s not covered by all insurance plans, but for people stuck in diagnostic limbo with a low-positive index value, it often provides a definitive answer.
Why IgM Testing Is Unreliable
Some labs still run IgM antibody tests for herpes, but most sexual health guidelines discourage them. IgM antibodies appear early in an infection, so the idea is to catch new infections sooner. In practice, IgM tests can’t reliably distinguish between a new infection and a reactivation of an old one. They also can’t differentiate HSV-1 from HSV-2, and they produce a high rate of false positives. If your provider orders herpes bloodwork, IgG type-specific testing is what you want.
Who Should Get Tested
The CDC does not recommend routine HSV-2 blood screening for the general population, including asymptomatic pregnant women. The reasoning comes down to the false positive issue: in people with no symptoms and no known exposure, the chance that a positive blood test is wrong is high enough to cause more harm than good.
Blood testing is recommended in specific situations. If you have recurring genital symptoms that keep testing negative on swabs, serology can help clarify whether herpes is the cause. It’s also useful if a sexual partner has been diagnosed with genital herpes and you want to know your own status, or if you’ve had a clinical diagnosis of herpes that was never confirmed in a lab. Providers may also consider testing for people being evaluated for STIs who have multiple sexual partners, and it’s more strongly recommended for people living with HIV.
At-Home Test Kits
Several companies sell herpes test kits that let you collect a sample at home and mail it to a lab. Most use a finger-prick blood sample for antibody testing. The lab analysis itself is generally comparable to what a clinic uses, but the weak link is sample collection. An inadequate blood spot or a poorly handled sample can lead to inaccurate results. Labs affiliated with hospital systems or state health departments tend to have more oversight of the analysis process.
Home kits also typically don’t include pre-test or post-test counseling, which matters for herpes more than many other infections. A low-positive HSV-2 result delivered through an app, without context about false positive rates or confirmatory testing, can cause unnecessary distress. If you go the home testing route, knowing that a positive result may need follow-up with a more specific test gives you a more complete picture.
Choosing the Right Test
If you have an active sore, get it swabbed with a PCR test. This gives you the most accurate, type-specific result and doesn’t depend on your antibody levels. If you have no symptoms but want to know your status, a type-specific IgG blood test is the appropriate choice, keeping in mind the 3 to 6 month window after potential exposure. If your blood test comes back with a low positive index value for HSV-2, ask about confirmatory testing through the Western Blot before treating the result as definitive.