Herpes is a common viral infection caused by the herpes simplex virus (HSV). The virus exists as two main types, HSV-1 and HSV-2. Herpes simplex virus testing identifies the presence of the virus in a sample from a blister, sore or fluid to diagnose an acute herpes infection or detects herpes antibodies in the blood to determine previous exposure to herpes.
Both HSV-1 and HSV-2 are contagious and periodically cause small fever blisters (vesicles) that break to form open lesions. HSV-1 primarily causes blisters or “cold sores” around the mouth, while HSV-2 usually causes lesions around the genital area; however, either one can affect the oral or genital areas.
The herpes simplex virus can be passed from person to person through skin contact while the sores are open and healing and sometimes when there are no visible sores. HSV-2 is frequently a sexually transmitted disease, but HSV-1 also may be acquired during oral sex and found in the genital area. According to the American Sexual Health Association, about 50% of adults in the U.S. have HSV-1 and about 17% have HSV-2. Because symptoms may be mild, however, 90% of those who have HSV-2 may be unaware that they have been infected.
When someone is first infected, the person may have obvious and painful blisters at the site of infection, which usually appear within two weeks after the virus is transmitted. The lesions generally heal within two to four weeks. The blisters can appear in the vaginal area, on the penis, around the anus, or on the buttocks or thighs. This primary episode can include a second outbreak of blisters and even flu-like symptoms such as fever and swollen glands. However, not everyone develops blisters, and sometimes symptoms are so mild that they are unnoticeable or mistaken for something else, such as insect bites or a rash.
Once someone is infected and the initial infection resolves, the person will harbor the HSV in a latent form. During periods of stress or illness, the virus may reactivate.
People with conditions that cause their immune system to be suppressed, such as those with HIV/AIDS or those who have had an organ transplant, may have more frequent and serious outbreaks of HSV. While there is no cure for herpes, antiviral medications can suppress outbreaks and shorten the duration of symptoms and active shedding of virus.
Rarely, the virus can cause neonatal herpes when a woman transmits the virus to her baby during a vaginal delivery. Neonatal herpes symptoms appear during the first month of life and, if left untreated, can cause long-term damage to a baby’s health. A pregnant woman who has been diagnosed with herpes may be monitored regularly prior to delivery to identify a reactivation of her infection, which would indicate the necessity for a caesarean section to avoid infecting the baby.The herpes simplex virus can be transmitted to the brain, causing encephalitis. This illness can be fatal or cause serious, permanent neurological problems in those who survive.
Herpes Simplex Virus (HSV) Testing
Herpes simplex virus (HSV) testing is used to diagnose an active herpes infection in those who have genital sores or encephalitis. It is also used for newborns suspected of having neonatal herpes, a rare but serious condition in which herpes is contracted during vaginal birth.
HSV testing may also be used to identify a previous infection. In people with symptoms, testing may be used to distinguish between a primary, active infection and a recurrent infection.
Direct detection of the virus
In people with active genital sores, the preferred methods of testing for the virus are the herpes culture and HSV DNA testing (PCR). PCR testing is preferred if encephalitis or neonatal herpes are suspected.
Herpes culture—a sample of fluid is collected via swab from an open sore and inoculated into cells that allow the virus to grow. Once the virus is detected, it can be further identified as HSV-1 or HSV-2, which may be useful for prognosis. This test is sensitive and specific, but it takes two or more days to complete. False negatives can occur if there is not enough active virus in the sample, which can occur if the lesion is cultured more than 48 hours after the symptoms appear.
HSV DNA PCR—this method detects HSV genetic material in a patient’s sample from the blister, blood, or other fluid, such as spinal fluid. PCR can detect the virus as well as identify the type of herpes virus. Because this test is more sensitive than culture, it is useful in circumstances where the virus is present in low numbers (such as viral encephalitis) or if the lesion is several days old.
HSV antibody testing detects immune proteins that the body produces in response to a herpes infection. The body produces two classes of antibody. It begins to produce the IgM class antibody of several days after a primary (initial) HSV infection and these antibodies may be detectable in the blood for several weeks. It then begins to produce HSV IgG antibody after HSV IgM. IgG levels rise for several weeks, slowly decline, and then stabilize in the blood. Once someone has been infected with HSV, that person will continue to produce small quantities of HSV IgG.
HSV antibody testing can detect both viral types (HSV-1 and HSV-2) and tests are available that can detect the early IgM antibodies as well as the IgG antibodies that remain forever in those who have been exposed.
Although it is not as sensitive as PCR or culture, HSV antibody testing can be used to help diagnose an acute HSV infection if blood samples are collected several weeks apart (acute and convalescent samples). The HSV IgG antibody levels are compared to see if they have risen significantly, indicating a current infection.
What does the test result mean?
Direct detection of the virus
A positive herpes simplex culture or HSV DNA (PCR) test from a vesicle scraping indicates an active HSV-1 or HSV-2 infection.
A negative culture or PCR test result indicates that the herpes simplex virus was not detected but does not definitely rule out the presence of virus. The sample may contain too few copies of the virus to be detected or does not contain actively replicating virus. If suspicion of herpes remains high, repeat testing may be done.
Antibody blood tests
A positive HSV-1 or HSV-2 IgM antibody test indicates an active or recent infection.
A positive HSV-1 or HSV-2 IgG antibody test indicates a previous infection.
A significant increase in HSV IgG antibodies, measured by comparing results from acute and convalescent samples, indicates an active or recent infection.
Negative HSV antibody results mean that it is unlikely that the person has been exposed to HSV or that the body has not had time to begin producing HSV antibodies.