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Diagnostic Tests For Genital Herpes

Until recently, viral culture was the "gold standard" for herpes diagnosis. Culture allows definitive diagnosis and permits distinction of HSV-1 from HSV-2, which is important for prognosis and counseling. Cultures are most sensitive while lesions are in the vesicular-pustular stage. Sensitivity rapidly declines as lesions ulcerate and crust.

PCR is three to five times more sensitive than viral culture, is offered by most reference laboratories, and is quickly replacing viral culture as the "gold standard." It is important to note that a negative culture for HSV does not rule out genital herpes.

Type-specific antibody testing based on HSV glycoprotein G is the most important and reliable diagnostic tool for chronic HSV infection. Antibody tests based on complement fixation, indirect immunofluorescence, or neutralization technologies do not reliably distinguish antibodies to HSV-1 from those to HSV-2. A negative result on an antibody test can be reassuring in that it excludes the diagnosis in a patient who has symptoms suggestive of long-standing or recurrent herpes. A positive result on a test that is not HSV glycoprotein G based is of little diagnostic value, because these serologic assays do not reliably distinguish between type 1 and type 2 infections. Because more then half of US adults are seropositive for HSV-1 as a result of herpes labialis infection, a positive test for HSV-1 antibody does not provide useful diagnostic information for the evaluation of genital ulcer disease. IgM antibody is often present during recurrent HSV outbreaks and does not indicate recent infection.

The newer type-specific serologic assays have specificities of more than 98% for the detection of HSV-2 antibody and sensitivities of more than 90%. One assay is a rapid, office-based test that can be run on serum or a whole blood specimen collected by a fingerstick and can provide results in less then 10 minutes. It is imperative to specify and request a glycoprotein G–based test when ordering an HSV serologic test. The following type-specific assays have been cleared by the Food and Drug Administration (FDA): Western immunoblot, HerpeSelect HSV-1 and HSV-2 ELISA (Focus Diagnostics, Cypress, CA), HerpeSelect HSV-1 and HSV-2 immunoblot (Focus Diagnostics, Cypress, CA), BioKit HSV-2 Rapid Assay (Biokit USA, Lexington, MA), and Captia HSV-1 and HSV-2 (Trinity Biotech, Wicklow, Ireland).

Other diagnostic tests for possible herpetic lesions include the following:

1. Direct immunofluorescent antibody testing for HSV antigen. Although more rapid (4–6 hours) than culture, this test may not differentiate HSV-1 and HSV-2.
2. Enzyme-linked immunosorbent assay (ELISA) testing for HSV antigens in clinical specimens. This test offers a rapid alternative to culture (results are available in 3–4 hours), but its use is generally confined to large reference laboratories and teaching institutions.
3. Microscopy of Papanicolaou smears or Giemsa staining (Tzanck test). These tests, although inexpensive, are insensitive, nonspecific, and not recommended.