Herpes is tested through two main approaches: swab tests that detect the virus directly from a sore, and blood tests that look for antibodies your immune system produces after infection. The best option depends on whether you currently have symptoms. If you have an active sore or blister, a swab test is the most accurate route. If you have no visible symptoms but want to know your status, a blood test is the way to go.
Swab Tests: The Most Reliable Option During an Outbreak
When you have an active sore, blister, or ulcer, a healthcare provider can swab the lesion and send the sample to a lab. This is the preferred method because it detects the virus itself rather than your body’s response to it. There are two types of swab tests, and the difference in accuracy matters.
PCR testing (which detects the virus’s genetic material) is now the standard. It catches roughly 95 to 98% of true infections, compared to about 88% for the older method, viral culture. Both are highly specific, meaning a positive result almost certainly means you have herpes. PCR also reliably tells you whether you have HSV-1 or HSV-2, which is useful information since the two types behave differently over time.
Viral culture, where the lab tries to grow the virus from your sample, was the go-to test for decades. It still works, but it misses more cases, especially when sores are already healing. If your sore has started to crust over or is several days old, the virus may no longer be active enough for culture to detect it. PCR doesn’t have this limitation to the same degree, which is one reason it has largely replaced culture in clinical practice.
Timing matters with any swab test. The best window is within the first 48 hours of a new sore appearing, when viral shedding is at its peak. If you notice a suspicious lesion, getting it swabbed quickly gives you the most reliable result.
Blood Tests: Screening Without Symptoms
Blood tests don’t detect the virus directly. Instead, they look for antibodies, proteins your immune system builds after encountering herpes. Because your body needs time to produce these antibodies, blood tests aren’t useful right after a potential exposure. They’re designed for people who want to know if they’ve been infected at some point in the past, or for confirming a diagnosis when swab testing isn’t possible.
The type of blood test matters enormously. The reliable version is called a type-specific IgG test, which identifies antibodies to a specific protein (glycoprotein G) that differs between HSV-1 and HSV-2. This distinction lets the test tell you which type you carry.
The Window Period
After a new herpes infection, your body doesn’t produce detectable antibodies immediately. This delay, called the window period, means a blood test taken too soon can come back negative even if you’re infected. Experts generally recommend waiting at least 6 to 12 weeks after a suspected exposure before getting a blood test. Even at 6 weeks, only about 60% of new HSV-2 infections will show positive. By 12 weeks, that number rises to around 70%, and most people will seroconvert within a few months. If your first test is negative but you have strong reason to suspect infection, retesting later improves accuracy.
Understanding Low-Positive Blood Test Results
Standard HSV-2 blood tests report results as an index value. A value below 0.90 is negative, 0.90 to 1.09 is equivocal, and 1.10 or above is considered positive. But here’s where it gets complicated: results in the low-positive range of 1.10 to 3.50 have a significant chance of being false positives. The CDC recommends confirmatory testing for anyone whose result falls in this range.
Recent research suggests the problem may extend beyond that cutoff. One study found that 20% of confirmed false-positive results had index values above 3.50 on the most commonly used screening test, meaning they would have been incorrectly accepted as true positives under current guidelines. If your result is low-positive and you have no history of symptoms, confirmatory testing is worth pursuing before accepting the diagnosis.
The Western Blot: Confirming Uncertain Results
The gold standard for herpes antibody testing is the Western Blot, performed at the University of Washington. It’s not a first-line test because it’s expensive and only available through one lab, but it’s the most accurate blood test available for resolving ambiguous results. If you received a low-positive IgG result, an equivocal result, or a positive result that doesn’t match your clinical picture (no symptoms, low-risk history), the Western Blot can provide a definitive answer.
Your provider can order this test through the University of Washington Virology Research Clinic. It typically requires a blood draw at a local lab, with the sample shipped to Seattle for analysis.
Why IgM Tests Should Be Avoided
Some providers still order IgM blood tests for herpes, but major health organizations advise against them. The problems are numerous and serious. IgM tests cannot accurately distinguish between HSV-1 and HSV-2, so a person with only HSV-1 (which the majority of adults carry) can receive a false positive for HSV-2. IgM antibodies also cross-react with related viruses like the ones that cause chickenpox and mono, producing misleading results.
There’s also a common misconception that IgM antibodies indicate a new infection. In reality, IgM can reappear during recurrent outbreaks in up to a third of people who have been infected for years. At the same time, IgM tests can be negative in up to half of people with a confirmed new infection. This combination of poor sensitivity and poor specificity makes IgM testing unreliable for both diagnosis and for determining when someone was infected. If your provider suggests an IgM test for herpes, asking for a type-specific IgG test instead is a reasonable request.
Which Test to Choose and When
Your testing path depends on your situation:
- Active sore or blister: Get a PCR swab test as soon as possible, ideally within 48 hours of the lesion appearing. This gives the clearest, most reliable diagnosis and identifies the virus type.
- No symptoms, recent exposure: Wait at least 12 weeks, then get a type-specific IgG blood test. Testing earlier risks a false negative.
- No symptoms, general screening: A type-specific IgG blood test is appropriate. Be aware that routine STI panels often do not include herpes testing unless you specifically request it.
- Low-positive or equivocal blood result: Request confirmatory testing, either a repeat IgG test or the University of Washington Western Blot.
One detail that surprises many people: the CDC does not recommend universal herpes screening for people without symptoms, largely because of the false-positive issues with available blood tests and the psychological impact of an incorrect diagnosis. Testing is most useful when you have symptoms, a known exposure, or a specific clinical reason to check your status.