Can You Get a False Positive HSV-2 Blood Test?

False-positive HSV-2 blood tests are surprisingly common, especially when the result falls in the low-positive range. Depending on which commercial test your lab uses, up to roughly one in three positive results may not hold up under confirmatory testing. The problem stems from how these tests work, how antibodies from the closely related HSV-1 virus can muddy the picture, and the basic math of screening in a population where most people do not carry HSV-2. Understanding when to trust a result and when to push for a second opinion can save you from weeks or months of unnecessary distress.

How HSV-2 Blood Tests Work and Where They Go Wrong

The most widely used HSV-2 blood tests are enzyme-linked immunoassays, commonly known as ELISAs, or related automated platforms. They look for IgG antibodies your immune system produces against glycoprotein G-2, a protein found on the surface of herpes simplex virus type 2. Because HSV-1 and HSV-2 share a great deal of genetic material, older blood tests could not tell the two apart. Modern “type-specific” tests use glycoprotein G to distinguish them, since each virus type has its own version of this protein.¹

The test does not report a simple yes or no. It produces an index value, a number that reflects how strongly the sample reacted. For most commercial assays, an index above 1.1 is called positive, between 0.9 and 1.1 is equivocal, and below 0.9 is negative. The trouble is that a result of, say, 1.3 is technically positive but barely clears the threshold. And a large share of false positives cluster in this low-positive zone between 1.1 and 3.5.

The Low-Positive Zone

If your test came back with an index value between 1.1 and about 3.5, there is a real chance it does not reflect an actual HSV-2 infection. One study using the widely available Focus HerpeSelect ELISA found that only about 35% of low-positive results in that range were confirmed as true positives when checked against a more reliable assay, compared with 92% confirmation for results above 3.5.² That means roughly two out of three people with a low-positive HerpeSelect result were not actually infected.

A 2024 study comparing three automated platforms found similar patterns. The DiaSorin assay produced false positives in about 21% of HSV-2 results with index values below 3.0. Based on estimated U.S. seroprevalence around 12%, the researchers calculated that nearly one of every three positive DiaSorin HSV-2 results would be falsely positive. The Roche and Bio-Rad platforms performed better, with positive predictive values of about 96% and 87%, respectively.³ This tells you something important: which brand of test your lab uses matters, and you probably were not told which one it was.

Even results well above 3.5 are not bulletproof. Among samples confirmed as false positives in one analysis, about 20% to 39% had index values above 3.5, depending on the assay used.⁴ A high index makes a true positive more likely, but it does not guarantee one.

How HSV-1 Antibodies Cause Cross-Reactivity

One of the main drivers of false-positive HSV-2 results is prior infection with HSV-1, which causes most oral cold sores. Because the two viruses are close relatives, antibodies against HSV-1 can sometimes bind to the HSV-2 test antigens just enough to push the index value above the positive cutoff. This is not a flaw anyone has fully solved; it is built into the biology of trying to distinguish two viruses that share substantial protein structure.

Research using inhibition assays, which block HSV-1 antibodies before running the HSV-2 test, has shown that people with very high levels of HSV-1 IgG are particularly vulnerable to this interference. In one study, people whose results were “indeterminate” after inhibition testing had significantly higher HSV-1 antibody levels than either the true-positive or the false-positive group, suggesting that high concentrations of HSV-1 antibodies create complexes that interfere with the test’s ability to read HSV-2 correctly.⁵

A separate study of men attending an STD clinic found that those who also carried HSV-1 antibodies were significantly less likely to have their HSV-2 ELISA result confirmed by Western blot: 76% confirmed, versus 93% among men without HSV-1 antibodies. Raising the ELISA index cutoff to 3.0 or above for HSV-1-positive men boosted the positive predictive value to 93% or higher.⁶ If you know you carry HSV-1, and your HSV-2 result is in the low-positive range, cross-reactivity becomes the leading suspect.

Why IgM Tests Are Even Worse

Some providers order an HSV IgM test instead of, or alongside, an IgG test. IgM is the class of antibody the body typically makes first during a new infection, so the idea is that a positive IgM result means a recent acquisition. In practice, HSV IgM testing is widely regarded as unreliable by infectious disease specialists, and many guidelines recommend against using it at all for herpes diagnosis.

IgM assays in general suffer disproportionately from false-positive results across many infectious diseases, leading to misdiagnoses and unnecessary treatment.⁷ For HSV specifically, most commercial IgM kits use whole-virus antigen rather than the type-specific glycoprotein G, which means they cannot reliably tell HSV-1 apart from HSV-2. Cross-reactions between the two virus types have been documented: in one study, patients who were clearly positive for only HSV-1 IgM also showed positive HSV-2 IgM results, consistent with cross-reactivity rather than dual infection.⁸

Making matters worse, IgM can reappear during reactivation of a longstanding herpes infection, not just during initial acquisition. A positive IgM result does not necessarily mean you were recently infected, and it does not necessarily mean HSV-2 is the virus you are carrying. If your diagnosis rests solely on an IgM test, it rests on very thin ground.

Other Factors That Can Interfere With Results

Cross-reactivity with HSV-1 is the best-studied cause of false positives, but it is not the only one. Certain immune conditions and acute illnesses can produce antibodies that stick nonspecifically to the assay components.

Rheumatoid factor, an autoantibody present in rheumatoid arthritis and some other autoimmune conditions, has been identified as a cause of false-positive herpes serology. In one early analysis, all twelve sera that were falsely positive by indirect EIA contained rheumatoid factor, and autoantibodies caused inconclusive results in twenty additional samples.⁹

More recently, researchers found that 45% of hospitalized COVID-19 patients tested falsely positive for HSV IgM on a commonly used automated platform. No positives were detected when the same samples were run on a different assay, pointing to immunoassay interference rather than actual herpes infection. The false positives correlated with the presence of SARS-CoV-2 antibodies, and in about a fifth of cases the interference could be traced to nonspecific IgM binding to the test’s solid-phase component.¹⁰ This is a vivid example of how an unrelated illness can generate a completely spurious herpes result.

Confirmatory Tests and What to Ask For

If you have a positive HSV-2 screening test and any reason to question it, such as a low index value, no history of symptoms, or known HSV-1 infection, confirmatory testing is the logical next step. The two main options are Western blot and inhibition-based assays.

The Western blot, particularly the University of Washington’s assay, has long been considered the reference standard. It separates viral proteins by size and looks for antibody binding to multiple specific bands, which makes it far less likely to mistake HSV-1 antibodies for HSV-2. However, it is not commercially available everywhere, can be expensive, and turnaround time is often measured in weeks rather than days.

Inhibition assays work by first neutralizing HSV-1 antibodies in the sample, then re-running the HSV-2 test. If the positive signal disappears after HSV-1 antibodies are blocked, the original result was likely a cross-reaction. These are available through some reference laboratories and can be a practical alternative to Western blot.

That said, no confirmatory test is perfect. In one study in Kenya, using the Western blot as a reference, the HerpeSelect ELISA had 100% sensitivity but only 40% specificity, meaning it caught every true positive but also flagged many people who were not infected. The Kalon ELISA performed somewhat better on specificity at 79%. Even the Western blot’s sensitivity appeared to vary depending on which reference standard was used.¹¹ These numbers come from a high-prevalence population in East Africa, so the dynamics differ from a low-prevalence U.S. setting, but they illustrate that even gold-standard tests have their limits.

A 2025 study reinforced the message by showing that both of the ELISA assays evaluated had high false-positive rates and low positive predictive values in a low-prevalence population, and that switching to automated chemiluminescent immunoassay platforms or adding Western blot confirmation substantially improved accuracy.¹²

Why Prevalence Matters More Than You Might Expect

You do not need to understand the formal statistics to grasp the core issue: when a disease is uncommon in the population being tested, even a fairly accurate test will produce a surprising number of false positives. Think of it this way. If only 12 out of every 100 people carry HSV-2 (a reasonable estimate for the general U.S. adult population), and the test correctly identifies 97 of 100 true positives but also incorrectly flags 5 of 100 uninfected people, those 5 false alarms come from a pool of 88 uninfected people while the 12 true positives come from a much smaller group. The math tilts the proportion of all positive results that are actually wrong higher than most people would guess.

This is exactly why the predictive value of a positive test is so much lower than its raw accuracy statistics suggest in low-prevalence settings, and why the one-in-three false-positive rate for certain assays described above is not an exaggeration. If you are in a group with a higher likelihood of HSV-2 infection, say because you have a partner with known genital herpes or a history of typical symptoms, the same test result is much more likely to be correct. Context reshapes how much a positive result means.

Why Routine Screening Is Not Recommended

Given how often these tests get it wrong, you might wonder why they are ordered at all in people without symptoms. Most major guidelines have reached the same conclusion: they should not be, at least not as routine screening. International guidelines advise against routine population-level serologic screening for HSV-2 in asymptomatic people, citing poor test specificity, the lack of a cure or effective therapeutic vaccine, no demonstrated population-level benefit, and psychosocial harms associated with diagnosis.¹³

The U.S. Preventive Services Task Force has also recommended against screening asymptomatic adolescents and adults for genital herpes, largely because the harms from false positives and psychological distress outweigh the uncertain benefits in people who have no reason to suspect they are infected.¹⁴

Despite this, many people end up with an HSV-2 blood test because they asked for a “full STI panel” and assumed herpes was included by default, or because a provider added it without a detailed discussion of the test’s limitations. If you were tested without symptoms and without a specific clinical reason, and the result is positive in the low range, the odds of a false positive are meaningfully higher than in someone who was tested because of suspicious sores.

When a Blood Test Is the Right Choice

Serology is not always the wrong call. When someone has an active outbreak with visible sores, the preferred test is a direct swab, either viral culture or, increasingly, PCR. PCR detects viral DNA far more reliably than culture and is becoming the preferred method for diagnosing active lesions.¹⁵ But when no lesion is present, there is nothing to swab. That is where type-specific IgG blood tests fill a genuine gap: for people with a partner who has genital herpes, for pregnant women who need to know their status, or for anyone trying to understand their risk profile when they have no active symptoms to test directly.

The key is treating a screening blood test as the beginning of the diagnostic process rather than the final word. A low-positive result in someone with no symptoms and no known exposure is a question mark, not a diagnosis.

The Emotional Toll of a Positive Result

Getting a positive HSV-2 result, even a questionable one, can be devastating. Research conducted for the USPSTF found that people receiving a serologic HSV-2 diagnosis reported a range of short-term emotional responses including surprise, denial, distress, and sadness, along with fears about telling partners, feeling sexually undesirable, and feeling “like damaged goods.”¹⁴

That said, the longer-term picture is somewhat more reassuring. A systematic review found that in seven of nine studies, an HSV-2 diagnosis by blood test did not produce a persistent negative impact on mental health or sexual satisfaction.¹⁶ A study in an urban HIV clinic similarly found that participants coped well with a new HSV-2 serological diagnosis, with no significant differences in psychosocial or quality-of-life scores compared with those who tested negative.¹⁷

These studies generally examined people who received results in a clinical setting with counseling. The experience of someone who gets a positive result through an anonymous online test or a rushed clinic visit without explanation is likely different, and the emotional damage from a false positive that goes unchallenged can persist much longer than it should. If you are struggling after a result, pursuing confirmatory testing is not just medically reasonable; it is psychologically important.

Practical Steps If You Have a Positive Result You Doubt

Knowing that false positives exist is only helpful if you know what to do about it. Here is a straightforward path forward:

• Get your index value. Call the lab or your provider and ask for the actual number, not just “positive” or “negative.” If it is between 1.1 and 3.5, a false positive is a serious possibility.
• Ask about the assay brand. Not all tests perform equally. Some platforms have positive predictive values in the 70% range, while others are above 95%. Your provider may need to contact the lab to find out which kit was used.
• Request confirmatory testing. A Western blot through a reference laboratory, or an HSV-2 inhibition assay, can clarify the result. Western blot availability varies by region, so you may need to advocate for it.
• Consider your HSV-1 status. If you carry HSV-1 antibodies, cross-reactivity becomes a plausible explanation for a low-positive HSV-2 result.
• Weigh your clinical context. Have you ever had sores that could be herpes? Does a sexual partner have a confirmed HSV-2 diagnosis? A positive result in someone with a clear exposure history is far more likely to be accurate than the same result in someone tested “just because.”

Many people receive positive results and are simply told they have herpes, with no discussion of index values, no mention of false-positive rates, and no offer of confirmatory testing. The burden often falls on the patient to ask the right questions.

Pregnancy and the Stakes of Getting It Right

One situation where HSV-2 status has immediate clinical consequences is late pregnancy. A mother who acquires genital herpes near the time of delivery carries a much higher risk of transmitting the virus to her newborn than someone with a longstanding infection. For this reason, some providers consider type-specific serology for pregnant women, particularly if their partner has known genital herpes.

But the false-positive problem does not disappear in pregnancy; if anything, it becomes more consequential. A false-positive HSV-2 result late in pregnancy could lead to an unnecessary cesarean delivery or antiviral treatment that was never needed. Patient materials have acknowledged that a small chance of a false positive exists and that confirmatory testing is available.¹⁸ Given the stakes, any positive screening result in pregnancy warrants careful interpretation, repeat testing, or confirmatory assay rather than immediate action based on a single number.