Can a Herpes Test Be Wrong? False Positives & Negatives

Herpes tests can absolutely be wrong, and the error rate is higher than most people expect. Blood tests for herpes simplex virus type 2 (HSV-2) are the most common source of inaccurate results, with one large systematic review estimating that at the manufacturer’s standard cutoff, roughly half of all positive HSV-2 blood test results could be false positives in a general population with typical infection rates.¹ That startling number is one reason the U.S. Preventive Services Task Force recommends against routine herpes screening in people without symptoms. The story is different for swab-based tests, which perform well when a visible sore is present, but they come with their own blind spots.

Why Blood Tests Produce So Many False Positives

The blood tests most commonly used for herpes detect antibodies your immune system produces in response to infection, not the virus itself. Two widely used commercial HSV-2 IgG tests are the HerpeSelect ELISA (by Bio-Rad/Focus) and the DiaSorin assay. Both target a viral protein called glycoprotein G, which differs between HSV-1 and HSV-2, making type-specific testing possible in theory. In practice, these tests are far from perfect at distinguishing a true positive from a false alarm.

The JAMA systematic review pooled data from ten studies involving over 6,500 people and found that the most widely used IgG test had a sensitivity of about 99% but a specificity of only 81% at the manufacturer’s recommended cutoff. That specificity gap matters enormously: it means roughly one in five uninfected people will test positive. When the review modeled what would happen if you screened 100,000 people at a typical U.S. seroprevalence of 16%, the result was about 15,840 true positives and nearly 16,000 false positives, giving a positive predictive value of just 50%.¹ In plain terms, a coin flip.

A more recent study comparing three automated HSV-2 IgG platforms confirmed how much the test brand matters. Based on an estimated 12% HSV-2 seroprevalence in the U.S., the Roche assay achieved a positive predictive value of about 96%, Bio-Rad came in around 87%, and the DiaSorin test managed only about 69%, meaning nearly one of every three DiaSorin-positive results was wrong.² If your test was run on one of the less accurate platforms, the chances of a false positive climb substantially.

The Index Value Gray Zone

Most HSV-2 IgG tests do not simply report “positive” or “negative.” They produce a numerical index value, and results above a certain threshold (often 1.10) are called positive. But research consistently shows that low-positive index values, roughly in the 1.10 to 3.50 range, are the danger zone for false positives. In low-risk populations, up to half of results in that range may be falsely positive.²

A study that examined over 2,300 consecutive DiaSorin HSV-2 IgG positive samples found that about 18% of them were negative on the HerpeSelect ELISA, and among those discordant results, 83% had DiaSorin index values between 1.10 and 3.00. When a subset of these samples was tested against a composite reference standard, 59 out of 60 discordant samples turned out to be DiaSorin false positives.³ So if your result comes back with a low index value and you have no history of symptoms, the odds that it is a false positive are high enough to warrant confirmatory testing before drawing any conclusions.

How Infection Rates in Your Community Affect Your Result

The accuracy of any screening test depends not just on how the test performs in a lab but also on how common the condition is in the group being tested. When you screen a population where relatively few people are actually infected, even a small false positive rate generates a flood of incorrect results that can outnumber the true ones. This is exactly what happens with herpes blood testing in low-prevalence groups.

A study of university students with no history of genital herpes found that the positive predictive value of the Focus HSV-2 ELISA was low in this setting, leading the authors to caution against recommending HSV screening in low-prevalence or mixed populations.⁴ The takeaway is personal: if you are young, have never had symptoms, and belong to a demographic group with low HSV-2 rates, a positive blood test is more likely to be wrong than it would be for someone in a higher-prevalence group. Context changes what the same test result means.

Cross-Reactivity Between HSV-1 and HSV-2

HSV-1 (the type most associated with oral cold sores) and HSV-2 (the type most associated with genital herpes) are closely related viruses that share substantial genetic overlap. The glycoprotein G region that type-specific blood tests target has areas of strong similarity between the two types.⁵ That overlap creates a practical problem: if you carry high levels of HSV-1 antibodies, those antibodies can sometimes interfere with the test’s ability to correctly assess HSV-2 status.

Research looking at people whose HSV-2 results were ambiguous on confirmatory testing found that those with indeterminate results had significantly higher HSV-1 antibody levels than people whose results were clearly false positive or clearly true positive. The likely explanation is that at very high concentrations, HSV-1 antibodies physically interfere with the binding reaction the test relies on to detect HSV-2 antibodies.⁶ So if you already know you have HSV-1, which a large portion of the population does, a low-positive HSV-2 IgG result deserves extra skepticism. Cross-reactivity with other herpesviruses in the same family, including varicella-zoster (the chickenpox virus) and Epstein-Barr virus, has also been documented as a potential source of false positives.

Why IgM Testing Is Unreliable

Some clinicians order an IgM blood test for herpes, sometimes under the assumption that IgM antibodies indicate a recent or “new” infection. In practice, herpes IgM tests are poorly regarded by infectious disease specialists and are discouraged by major guidelines. The tests cannot reliably distinguish between HSV-1 and HSV-2, cannot reliably differentiate new infection from reactivation, and are prone to false positives from several unrelated causes.

An early study that investigated false-positive IgM reactions found that among sera initially positive for HSV and/or varicella-zoster IgM, all twelve samples that were positive only on the indirect test (and negative on a more specific assay) contained rheumatoid factor, a type of autoantibody associated with inflammatory conditions. Another twenty samples gave inconclusive results because of autoantibodies.⁷ Rheumatoid factor is common in people with autoimmune conditions and even shows up in a fraction of healthy people, making this a real-world concern rather than a laboratory curiosity. If your provider ordered an IgM test and it came back positive, consider requesting a type-specific IgG test or confirmatory testing instead.

When Swab Tests Get It Wrong

Swab-based tests work differently from blood tests. Instead of looking for your immune response, they look for the virus directly in fluid or tissue collected from a sore. There are two main approaches: viral culture, which tries to grow the virus in a lab, and PCR (polymerase chain reaction), which detects viral DNA. Both are highly specific, meaning false positives are rare. The vulnerability of swab tests lies on the other side: false negatives.

Viral culture, once considered the standard, misses a meaningful fraction of infections. In a comparison study, culture detected about 88% of confirmed HSV-positive samples, while a laboratory-developed PCR test caught about 98%.⁸ In a separate study of neonatal surface swabs, limiting the analysis to samples positive by only one method showed PCR at 100% sensitivity versus just 50% for culture, with both methods achieving 100% specificity.⁹

The practical implication: if your sore was swabbed and the culture came back negative, that does not rule out herpes. Cultures can fail for several reasons. The lesion might have been sampled too late, after the active viral shedding period ended. Blisters that have already crusted over yield far less virus. The sample might not have been transported properly, since the virus is fragile outside the body. And if you are already taking antiviral medication like valacyclovir, the amount of virus present in the sore drops, making detection harder. PCR is the more reliable swab test, but even PCR can miss the virus if the lesion is at the wrong stage.

Situations That Can Distort Results

Beyond the inherent limitations of the tests themselves, certain clinical situations make herpes testing less reliable.

IVIG therapy: Intravenous immunoglobulin contains pooled antibodies from thousands of donors. After infusion, recipients can suddenly test positive for infections they do not have. A prospective study found that six of seven patients receiving IVIG had at least one serology turn positive within 72 hours of infusion, including antibodies for HSV-1 and HSV-2.¹⁰ The effect appeared to increase with higher cumulative IVIG doses.¹¹ If you receive IVIG for an immune condition, any subsequent herpes blood test should be interpreted with extreme caution.
Weakened immune systems: People who are immunocompromised, whether from HIV, chemotherapy, organ transplant medications, or other causes, may have a delayed or blunted antibody response. This can lead to false negative blood tests because the immune system simply has not produced detectable antibody levels, even though the virus is present.
Early infection: After initial exposure, it takes time for antibodies to develop to a level the blood test can detect. This seroconversion window is typically several weeks but can stretch longer in some individuals. Testing too early after a potential exposure often produces a false negative. If you test negative soon after a specific concern, repeating the test after a few months gives a more reliable answer.

Confirmatory Testing and the Western Blot

Given the well-known limitations of standard IgG screening tests, confirmatory testing matters. The gold standard has long been the HSV Western blot, developed at the University of Washington. It detects antibodies against multiple viral proteins rather than just one, giving it much better ability to distinguish true positives from false ones.

A study examining the HerpeSelect ELISA against the Western blot reference in a Kenyan population found that while the HerpeSelect had 100% sensitivity, its specificity was just 40%, meaning it called more than half of truly negative people positive. The Kalon ELISA performed better, at 92% sensitivity and 79% specificity, but still fell short of the Western blot’s accuracy.¹² The Western blot is not widely available through standard commercial labs, which is a major practical barrier. It has historically been offered through the University of Washington’s clinical virology laboratory, though availability may vary. An inhibition assay is another confirmatory option that some labs can perform; it works by blocking HSV-2-specific antibodies and measuring whether the test signal drops, which helps separate true from false results.

If you have a low-positive IgG result and no history of symptoms, the most reasonable path is to ask your provider about confirmatory testing before accepting the diagnosis. Newer, more accurate automated platforms are entering the market, and some studies suggest that raising the index value threshold for calling a result positive (for instance, using 3.5 instead of 1.1) substantially improves specificity with only a modest loss in sensitivity.¹

Why Routine Herpes Screening Is Not Recommended

The U.S. Preventive Services Task Force issued a recommendation against routine serologic screening for genital herpes in asymptomatic adolescents and adults, including pregnant women.¹³ The reasoning rests on several factors: the high false positive rate in the general population, the absence of evidence that screening reduces transmission or improves health outcomes, and the documented psychological harm that accompanies a positive result.

That last point deserves emphasis. Serologic screening was specifically associated with psychosocial harms, including distress and anxiety.¹ The USPSTF also noted that no studies have examined the accuracy of serologic HSV tests specifically in pregnant women, leaving a gap in the evidence for a group that might seem like an obvious candidate for screening.¹³

This does not mean herpes testing is never appropriate. Testing makes sense when you have symptoms and a sore can be swabbed. It makes sense when a partner has been diagnosed and you want to know your status. It makes sense in certain clinical scenarios where knowing HSV status changes management decisions. What the recommendation pushes back against is blanket screening of the general population, where the math on false positives simply does not work in the patient’s favor.

The Emotional Toll of an Uncertain Result

A herpes diagnosis carries social stigma that is disproportionate to its medical significance for most people. Research looking at psychosocial outcomes found that people newly diagnosed with genital herpes experienced increased psychological distress compared to those who had known about their diagnosis for some time, while long-term studies showed that HSV-2 positive and HSV-2 negative individuals did not differ significantly in psychological well-being over time.¹⁴ The initial shock and shame of a diagnosis appeared to be a bigger factor than the infection itself.

Now consider what happens when that diagnosis is wrong. A person who receives a false positive herpes test may experience all the distress of a real diagnosis, alter their sexual behavior, disclose to partners, or even begin antiviral treatment they do not need. They might end a relationship or avoid new ones. The emotional damage from a false positive can persist even after the error is corrected, because the stigma is hard to unlearn. This is exactly why experts push back against ordering herpes blood tests without a clear clinical reason, and why understanding the limits of these tests is not just an academic exercise.

Point-of-Care and Rapid Tests

Rapid or point-of-care HSV-2 antibody tests, which give a result in minutes rather than days, have been evaluated for use in clinics and potentially for self-testing. An evaluation of one such near-patient test found a significant problem with subjectivity: in a UK study arm, three independent readers disagreed on the result for about 5% of tests, and in an Italian study arm, disagreement rose to about 10%.¹⁵ When one in ten results depends on who is reading the strip, the test has a built-in source of error that sits on top of whatever biological false positive rate the assay itself carries. Rapid tests may have a role in high-prevalence settings where speed and access matter more than precision, but they are not a substitute for laboratory-based testing when accuracy is the priority.

What to Do If You Think Your Result Is Wrong

If you have received a positive HSV-2 blood test and have never had symptoms, a few practical steps can help you figure out whether the result reflects a true infection.

Check the index value: Ask for the numerical result, not just “positive” or “negative.” Low-positive values in the 1.10 to 3.50 range are the most likely to be false positives and warrant confirmatory testing.
Request confirmatory testing: Ask your provider about the HSV Western blot or an inhibition assay. These are not always easy to access, but they are far more accurate than the initial screening test.
Consider retesting on a different platform: If your test was run on the DiaSorin or an older Focus/Bio-Rad platform, a retest on a different assay can sometimes resolve the discrepancy.
Evaluate your risk profile: Your age, sexual history, geographic region, and partner’s HSV status all affect how likely a positive result is to be correct. A clinician who understands the base-rate problem can help you interpret what the number means for you specifically.
Avoid IgM tests: If your positive result came from an IgM test alone, it should not be considered diagnostic. Insist on type-specific IgG testing at minimum.

For negative results you doubt, the key question is timing. If you tested within a few weeks of a potential exposure, a negative blood test does not carry much weight. Retesting after three to four months gives the immune system time to produce detectable antibodies. If a sore was swabbed and the culture was negative, ask whether PCR testing is available, since it is substantially more sensitive. And if you are on suppressive antiviral therapy, that can reduce viral shedding enough to produce a falsely negative swab result, so discuss timing with your clinician.

1
JAMA. Serologic Screening for Genital Herpes: An Updated Evidence Report and Systematic Review for the US Preventive Services Task Force
2
PubMed Central. Performance characteristics of highly automated HSV-1 and HSV-2 IgG testing
3
Sexually Transmitted Diseases. Characterization of Serum Samples With Discordant Results in 2 Herpes Simplex Virus Type 2 IgG Assays
4
PubMed Central. Performance of Focus ELISA Tests for HSV-1 and HSV-2 Antibodies Among University Students With No History of Genital Herpes
5
PubMed Central. Peptide sequences of glycoprotein G-2 discriminate between herpes simplex virus type 2 (HSV-2) and HSV-1 antibodies
6
PubMed Central. Relationship Between HSV-1 Serostatus and HSV-2 IgG Confirmation Results Using an Inhibition Assay
7
Sexually Transmitted Diseases. Causes of false-positive reactions in herpesvirus IgM assays
8
PubMed Central. Comparison of Simplexa HSV 1 & 2 PCR with culture, immunofluorescence, and laboratory-developed TaqMan PCR for detection of herpes simplex virus in swab specimens
9
PubMed Central. Comparison of Herpes Simplex Virus PCR with Culture for Virus Detection in Multisource Surface Swab Specimens from Neonates
10
Open Forum Infectious Diseases. False-positive Serologic Results attributable to IVIG therapy
11
PubMed. Impact of IVIG therapy on serologic testing for infectious diseases
12
PubMed Central. Herpes simplex virus type 2 antibody detection performance in Kisumu, Kenya, using the Herpeselect ELISA, Kalon ELISA, Western blot and inhibition testing
13
JAMA. Serologic Screening for Genital Herpes Infection: US Preventive Services Task Force Recommendation Statement
14
Carolina Digital Repository. Genital Herpes: Psychosocial Outcomes of Labeling Adults and Adolescents with Diagnosis
15
BMJ Journals. An evaluation of near patient tests for detecting herpes simplex virus type-2 antibody