The Accuracy of a Herpes Blood Test

A blood sample being held with a row of human samples for analytical testing including blood, urine, chemistry, proteins, anticoagulants and HIV in lab
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It can be incredibly stressful to wonder if you have herpes. That's true whether you're concerned because of symptoms because you know you've just been exposed, or for any other reasons. One way to determine whether or not you have genital herpes or oral herpes infection is to get a herpes blood test.

That said, many people have questions about how accurate herpes blood tests are. The truth is that herpes blood tests work reasonably well, but no test is perfect. That's one reason doctors may be reluctant to test for herpes. They worry about balancing the patient's desire to know with the potential emotional damage caused by a positive herpes blood test result. Due to the stigma associated with herpes, that worry can be in place whether the result is true or false. 

How Accurate Is a Herpes Blood Test?

It's always possible for a test to give inaccurate results.

The accuracy of a herpes blood test depends on two things: how many people in the tested population have herpes and which specific test was used.

The sensitivity/specificity of two different, relatively standard, herpes blood tests are as follows:

  • ELISA:
    • 91% sensitivity and 92% specificity for HSV-1
    • 96% sensitivity and 97% specificity for HSV-2
  • Immunoblot:
    • 99% sensitivity and 95% specificity for HSV-1
    • 97% sensitivity and 98% specificity for HSV-2

What does that mean for you? How common herpes comes into the calculation. It affects how likely positive tests and negative tests are to be correct. In fact, it can make a bigger difference than herpes blood test accuracy!

Let us make the reasonable assumption that around 50% of the population are infected with HSV1. That's the virus primarily associated with oral herpes and cold sores. It's also associated with a growing number of genital herpes infections. Then assume that 25% of people are infected with HSV2. That's the virus primarily associated with genital herpes. In that scenario, the positive predictive value and negative predictive value are as follows:

  • ELISA:
    • HSV-1: Approximately 92% of positive tests give the correct result.
    • HSV-2: Approximately 92% of positive tests are correct, and 98% of negative tests are correct.
  • Immunoblot:
    • HSV1: Approximately 95% of positive and 99% of negative tests are correct.
    • HSV-2: Approximately 94% of positive and 99% of negative tests are correct.

The Upshot

Herpes blood tests are actually pretty accurate.

That's particularly true for the type-specific tests that are most often recommended! In a relatively high prevalence population, they give accurate results the vast majority of the time. It's worth noting, though, that if my prevalence estimates were off, it would make a big difference. What if we worked from the assumption that only 10% of the population was infected with either virus? Then although almost all negative tests would still be accurate, positive tests would only be correct 55% to 85% of the time. In other words, there would be a lot of false-positive tests.

The possibility of false-positive tests in populations where herpes isn't common is a big concern. In fact, it is one of the reasons that screening for herpes is not widely recommended. Doctors are concerned that the stress of a false positive test may outweigh the benefits of early detection of the virus in someone who is asymptomatic.

Still, since herpes can be transmitted in the absence of symptoms and suppressive therapy can help prevent transmission, I don't necessarily agree. I personally believe that people who know they may be at risk can make an informed decision to undergo a herpes blood test to be screened for the virus. That is particularly true if they are in a situation where they could be exposing new sexual partners to the virus. It is, however, important to understand that false-positive tests can happen. It's also critical to know that, even if you are infected with a herpes virus, living with herpes is not the end of the world.

For the record? The Centers for Disease Control and Prevention currently estimate that the prevalence of herpes is pretty high. They estimate that by the age of 50, between 20 and 60 percent of adults are infected with HSV-2. There are big differences in prevalence that depend on race and sex. 

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Article Sources

  • Geretti AM. Genital herpes. In: Ross J, Ison C, Carder C, Lewis D, Mercey D, Young H. Sexually transmitted infections: UK national screening and testing guidelines. London (UK): British Association for Sexual Health and HIV (BASHH); 2006 Aug. p. 76-84.(Accessed online 12/28/08)

  • Xu, F. et al. (2006) "Trends in Herpes Simplex Virus Type 1 and Type 2 Seroprevalence in the United States" JAMA, 296:964-973