Herpes blood tests look for the body's reaction to a herpes infection rather than searching directly for the virus. Depending on the type of test used, they can take up to four months to become positive after the time of infection. If and when you have symptoms, however, the lesions can be tested as soon as they show up.
When a person becomes infected, with herpes or any other pathogen, the immune system tries to fight off the infection. Part of that process involves the production of antibodies against the infecting organism. There are several kinds of antibodies, but the two types that herpes blood tests look for are IgG and IgM. Herpes IgM antibodies usually are detectable by herpes blood tests within 7-10 days after initial infection, and levels stay high for approximately two weeks. Herpes IgG antibodies do not show up until slightly later after initial infection.
A positive herpes IgG test, if the test result is accurate, means that your body has been infected with the herpes simplex virus. Type specific herpes IgG tests can often distinguish between HSV-1 and HSV-2, but they can not detect whether a particular infection is oral or genital in the absence of symptoms. HSV-1 usually infects the mouth, causing oral herpes, and HSV-2 usually infects the genitals, but either virus can infect either location. Herpes IgG and IgM tests can only tell you that you have been infected, not where.
If you test positive for herpes IgG but not IgM, then your herpes infection is probably not recent - i.e. from the past 2 months. Individuals with recent infections are more likely to test positive for both herpes IgG and IgM or herpes IgM alone. The converse, however, isn't true. Positive herpes IgG and IgM results together do not necessarily mean you were infected recently. Between 30 and 70 percent of patients with recurrent herpes infections will test positive for herpes IgM
It is possible to have a false positive or false negative result - on either a herpes IgG or IgM test. Therefore, if your herpes blood test results do not agree with your known risk factors and sexual history, talk to your doctor about possible issues with testing. Diagnostic testing isn't perfect, but you may not be accurately assessing your risk. Many people do not understand that herpes can be transmitted even when their partner has no symptoms and does not know they are infected.
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