How Accurate is the Herpes Blood Test?

Question: How Accurate is the Herpes Blood Test?

Answer: No test is perfect. It's always possible for a test to give inaccurate results, and the accuracy of a herpes blood test depends on 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

If you make the reasonable assumption that around 50% of the population are infected with HSV1, the virus primarily associated with oral herpes and cold sores, and 25% are infected with HSV2, the virus primarily associated with genital herpes, then 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, the herpes blood tests are actually pretty good! 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 and we worked from the assumption that only 10% of the population is 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.

The possibility of false positive tests in low prevalence populations is one of the reasons that screening for herpes is not widely recommended, since 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 personally believe that screening is worth considering if you know you may be at risk, particularly if you are in a situation where you could be exposing new sexual partners to the virus. It is, however, important to first understand both that false positive tests can happen and that, even if you are infected with a herpes virus, living with herpes is not the end of the world.

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