A herpes blood test is used to detect the presence of herpes antibodies in the blood. Herpes is a common virus that can cause oral herpes and genital herpes. The herpes blood test works by checking for the presence of antibodies produced by the immune system in response to an infection. It does not directly detect the virus itself but indicates whether a person has been exposed to herpes at some point. The results of the test can help healthcare providers determine if a person has a current or past infection.