Herpes simplex infections are confirmed in several ways. Generally, the appearance of the skin eruption is strongly suggestive. Viral cultures and rapid assays (analysis of the crust and/ or the fluid inside the blister) are available. The type of test for genital herpes that is used depends on what the lesion looks like, or the morphology of the lesion.
Acute vesicular lesions, or fluid-filled blisters that are newly formed, are more likely to react positively to the rapid assay. Older, crusted patches are better diagnosed with viral culture. A viral culture is the most specific method of finding a genital herpes infection.
Another test for genital herpes is the herpes virus antigen detection test. Cells from a fresh sore are scraped off and then smeared onto a microscope slide. This test finds markers (called antigens) on the surface of cells infected with the herpes virus. This test may be done with or in place of a viral culture.
Herpes simplex virus (HSV) infections can also be diagnosed between outbreaks by the use of a blood test.
Blood tests, which detect antibodies to HSV-1 or HSV-2 infection, can be helpful, although the results are not always clear-cut. Blood tests can find antibodies that are made by the immune system to fight a herpes infection. Antibody tests are occasionally done but are not as accurate as a viral culture at finding the cause of a specific sore or ulcer. Antibody tests cannot tell the difference between a current active herpes infection and a herpes infection that occurred in the past. Because antibodies take time to develop after the first infection, a person may not have a positive antibody test if the infection is recent. Some blood tests can tell the difference between HSV-1 and HSV-2.
A Polymerase Chain Reaction (PCR) test for genital herpes may also be ordered to diagnose HSV. A PCR test can be done on cells or fluid from a sore or on blood or on other fluid, such as spinal fluid. PCR finds the genetic material (DNA) of the HSV virus. This test can tell the difference between HSV-1 and HSV-2. The PCR test is not often done on skin sores, but it is best for testing spinal fluid, for those rare cases in which herpes may cause an infection in or around the brain.
In all cases, it is imperative to obtain enough viral cells for testing, and careful collection methods are therefore important. All crusts should be gently removed or vesicles gently unroofed. A sterile cotton swab pre-moistened in viral culture preservative is used to swab the base of the vesicle to obtain a specimen for analysis.