Even if herpes tests aren't perfect, they are often the only way to know if you are infected. The problem is that many herpes blood tests look for antibodies to the virus - and those are not produced right away. The question is this: How long should you wait before you can be reasonably certain a genital herpes blood test will be effective?
Blood tests for HSV-2, the virus that usually causes genital herpes, generally look for the presence of antibodies to the virus rather than the virus itself. Unfortunately, it takes some time for the body to mount a detectable antibody response after infection. That's why you can't simply go get tested the day after you've been exposed to genital herpes to find out if you have the disease - even if you were infected, at that point there's no way for the test to pick it up. So, how long does it take for a genital herpes blood test to become positive after exposure? The answer is that it depends on the test.
Various studies have investigated how long it takes from when genital herpes symptoms show up to when an individual tests positive on a blood test for HSV-2. As it turns out, the range varies a great deal, both between studies and between tests. For example the median time from symptoms to a positive HSV-2 blood test was:
- HerpeSelect ELISA - 21 days for people who were HSV-1 negative and 23 days for people who were HSV-1 positive. (Ashley-Morrow et al. 2003)
- Western Blot - 40 days for people who were HSV-1 negative and 47 days for those who were HSV-1 positive. (Ashley-Morrow et al. 2003)
- Western Blot - 68 days. (Morrow et al. 2003)
- Kalon ELISA - 120 days. (Morrow et al 2003)
- Focus ELISA - 21 days. (Morrow et al 2003)
In other words, since it usually takes around 2 weeks for symptoms to show up, it's probably a good idea to wait at least a month or two before even considering getting an HSV-2 test after a potential exposure -- and you might want to consider getting retested after six months. Remember, after all, that those are only median times and that they were highly variable. Some infected people will seroconvert quickly, while others will take even longer before a herpes blood test accurately shows them to be positive.
Note: There is no practical way for researchers to directly answer the question of how long it takes for a positive test to show up in asymptomatic people, since you'd have to know both that they had been infected and when they had become infected. If a person is asymptomatic, neither piece of information is available -- since there is no way to tell they are infected until they test positive. The assumption is that the time course to a positive genital herpes blood test is similar to that seen in symptomatic individuals, although that assumption is difficult to verify since most asymptomatic individuals who eventually test positive do not know exactly when they were exposed.
Sources:
Ashley-Morrow R, Krantz E, Wald A. Time course of seroconversion by
HerpeSelect ELISA after acquisition of genital herpes simplex virus type 1
(HSV-1) or HSV-2. Sex Transm Dis. 2003 Apr;30(4):310-4.
Morrow RA, Friedrich D, Krantz E. Performance of the focus and Kalon
enzyme-linked immunosorbent assays for antibodies to herpes simplex virus type 2
glycoprotein G in culture-documented cases of genital herpes. J Clin Microbiol.
2003 Nov;41(11):5212-4.
