Genital herpes is suspected when multiple painful blisters occur in a sexually exposed area. During the initial outbreak, fluid from the blisters may be sent to the laboratory to try and culture the virus, but cultures only return a positive result in about 50% of those infected. In other words, a negative test result from a blister is not as helpful as a positive test result, because the test may be a false-negative test. However, if a sample of a fluid-filled blister (in the early stage before it dries up and crusts) tests positive for herpes, the test result is very reliable. Cultures taken during an initial outbreak of the condition are more likely to be positive for the presence of HSV than cultures from subsequent outbreaks.

There are also blood tests that can detect antibodies to the herpes viruses that can be useful in some situations. These tests are specific for HSV-1 or HSV-2 and are able to demonstrate that a person has been infected at some point in time with the virus, and they may be useful in identifying infection that does not produce characteristic symptoms. However, because false-positive results can occur and because the test results are not always clear-cut, they are not recommended for routine use in screening low-risk populations for HSV infection.

Other diagnostic tests such as polymerase chain reaction (PCR) to identify the genetic material of the virus and rapid fluorescent antibody screening tests are used to identify HSV in some laboratories.