The Accuracy of a Herpes Blood Test

Wondering whether you have herpes can be stressful. If you think you have herpes because you have symptoms or suspect that you may have been exposed to the virus through sex, a healthcare provider can offer you a blood test to confirm whether you have been infected with the herpes simplex virus (HSV).

Herpes blood testing is a two-step process in which an initial positive result is followed by a second confirmatory test. A positive result from both tests can be considered a definitive diagnosis of herpes.

While herpes blood tests offer a high level of accuracy, they are not infallible. Moreover, if you have symptoms, they may not be as useful or informative as an HSV viral culture or PCR test (both of which can detect HSV in a swab of fluid from a herpes sore).

A blood sample being held with a row of human samples for analytical testing including blood, urine, chemistry, proteins, anticoagulants and HIV in lab
Rafe Swan / Getty Images

This article explores the accuracy of herpes blood tests and outlines the current recommendations for herpes testing in the United States.

How Test Accuracy Is Measure

The accuracy of all medical tests, including herpes tests, is measured based on two values: sensitivity and specificity.

Sensitivity is how often a test correctly identifies someone with a disease. A test that is 90% sensitive will correctly identify 90 people out of 100 who have the disease. Ten people will have a false-negative result.

Specificity, on the other hand, refers to a test's ability to correctly identify someone who doesn't have the disease. If a test is 90% specific, that means that 90 people out of 100 will be correctly diagnosed as not having a disease and that 10 people will have a false-positive result.

The higher the sensitivity and specificity, the lower the chances of a false (incorrect) result.

Accuracy by Test Type

Two blood tests are used to screen for herpes. To ensure that an initial positive result is correct, a second test using a different method of detection is used to confirm the results.

The first test is the enzyme-linked immunosorbent assay (ELISA). An ELISA looks for immune proteins, called antibodies, that are produced by the immune system in response to the HSV. HSV antibodies are present even when there are no symptoms.

If the ELISA result is positive, your healthcare provider may administer a Western blot assay. This second test looks for proteins on the surface of the virus, called antigens, which serve as the virus' "ID tag." HSV antibodies produced in response to these antigens help the immune system target its attack.

According to the U.S. Preventive Services Task Force, the accuracy of the two blood used to diagnose herpes breaks down as follows:

 Test Sensitivity  Specificity
HerpeSelect ELISA Kit 96% to 100% 87% to 100%
Herpes Western Blot Assay Over 99% Over 99%

Due to its high level of sensitivity and specificity, the Western blot is considered the gold standard of herpes blood testing.

With that said, the accuracy of the tests can be affected by the timing of the tests. For an ELISA to return an accurate result, the immune system needs to produce enough antibodies to reach detectable levels.

Testing too early during the so-called window period can end up causing a false-negative result (in which the test says you don't have HSV even if you do).

Window Period for Genital Herpes

The window period for genital herpes can range from three to 12 weeks. While most cases can be detected by week three, some people may take far longer to produce detectable antibodies.

Other Testing Options

When an ELISA is confirmed with a Western blot, it is unlikely for a herpes diagnosis to be wrong. But, other tests may be more useful and/or reliable in certain situations. Both tests rely on the direct detection of HSV using fluids obtained from a swab of a herpes sore.

Polymerase Chain Reaction (PCR)

A polymerase chain reaction (PCR) test can detect HSV by making copies of the virus' genetic material in a process known as nucleic amplification. Even if there is only a small amount of virus, the genetic material can be amplified enough to return an accurate positive diagnosis.

PCR is considered the gold standard of testing in cases where herpes has invaded the brain or spinal cord (typically in people with advanced HIV infection).

Viral Culture

An HSV viral culture is a process in which the virus is "grown" in the lab using a swab of fluid. Compared to all other methods of HSV detection, viral culture is considered the gold standard of testing.

Even so, an HSV viral culture is more time-consuming, taking between three to eight days to return results. Moreover, user error can affect the results. For instance, the test is likely to be less accurate if the healthcare provider swabs the scab rather than the sore itself. Delays in transport or improper refrigeration can also weaken the accuracy of the test.

Limitations of Testing

As accurate as herpes tests are, they are not used for routine screening. This is because, in the absence of symptoms, a positive result doesn't mean you will ever have symptoms or require treatment. On the other hand, knowing your HSV status may reinforce safer sex practices if you are in a relationship.

Herpes Screening Recommendations

Currently, the U.S. Preventive Service Task Force advises against herpes screening in people without herpes symptoms, including those who are pregnant.

This doesn't mean that you shouldn't get tested under certain circumstances. According to the Centers for Disease Control and Prevention, HSV testing can be used in asymptomatic (symptom-free) people who:

  • Had sexual contact with someone with genital herpes
  • Is getting tested for other sexually transmitted infections
  • Has HIV and a history of genital herpes symptoms
  • Is at higher risk of infection for whatever reason (including having multiple sex partners)

Summary

Herpes can be diagnosed with blood tests. This includes the ELISA test that detects herpes antibodies and a Western blot test used to confirm the results. Herpes testing is generally recommended for people with herpes symptoms only.

Although the ELISA and Western blot tests are highly accurate when used together, other direct testing methods may be used, particularly if you have symptoms. This includes a PCR test that can detect the genetic material of the virus and a viral culture that can "grow" the virus in the lab. Both rely on a swab of fluid from a herpes sore.

A Word From Verywell

Although routine herpes screening is not recommended for the general public, it has its place even if you don't have symptoms. If you suspect you may have herpes for any reason, speak with your healthcare provider to assess your risk and determine if a herpes test is appropriate based on your risks and circumstance.

Frequently Asked Questions

  • What tests are used to diagnose herpes simplex?

    There are two blood tests used to diagnose herpes infections:

    • ELISA test, used for initial testing, detects herpes-specific antibodies.
    • Western blot, used to confirm a diagnosis, detects herpes antigens.
  • How accurate are herpes blood tests?

    The accuracy of the newer-generation herpes blood tests is exceptionally high. The sensitivity and specificity vary by the type of test used:

    • ELISA: Sensitivity of 96% to 100% and specificity of 97% to 100%
    • Western blot: Sensitivity and specificity of over 99%
  • Can a herpes test return a false-positive or false-negative results?

    The likelihood of a false-positive result is low but can occur due to user error or improper storage or handling. By contrast, a false-negative result is likely if your body has not produced enough antibodies to reach detectable levels. For this reason, people are advised to wait at least three weeks from the time of a suspected exposure before getting tested. This waiting period is commonly known as the "window period."

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5 Sources
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