How Accurate are Herpes Blood Tests?

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Question: How Accurate is the Herpes Blood Test?

Answer: Herpes blood tests work reasonably well, but no test is perfect.

It's always possible for a test to give inaccurate results. The accuracy of a herpes blood test depends on two things. The first is how many people in the tested population have herpes. The second is 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?  That's where how common herpes is comes into the calculation. It effects 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.

In conclusion, 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 disagree. I personally believe that screening is worth considering if you know you may be at risk.

This is particularly true if you are in a situation where you 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 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. 

Fanfair RN, Zaidi A, Taylor LD, Xu F, Gottlieb S, Markowitz L. Trends in seroprevalence of herpes simplex virus type 2 among non-Hispanic blacks and non-Hispanic whites aged 14 to 49 years — United States, 1988 to 2010. Sex Transm Dis 2013;40(11):860–4.

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

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