A herpes blood test is a screening for the herpes simplex virus. A blood test screening for herpes will detect the presence of herpes simplex virus antibodies. Most blood tests for herpes are performed to screen for herpes simplex 2, the strain of herpes simplex most likely to cause genital herpes. The benefit of using a blood test to screen for a herpes infection is that no outbreak of symptoms is required to make an accurate diagnosis. A herpes blood test is an extremely reliable method of herpes testing. Ten weeks must pass between a suspected herpes infection and blood testing for herpes, ten weeks being the period of time it takes for herpes antibodies to appear.