Laboratory Tests

The clinical diagnosis of genital herpes is both insensitive and non-specific. Therefore the clinical diagnosis of genital herpes should be confirmed by laboratory testing. Clinical laboratories should offer both virus detection tests and antibody detection tests.

Virus Detection

Virus detection tests begin by vigorously swabbing lesion sites to collect a viral sample. Viral detection tests include:

Culture (viable virus)
Isolation of HSV in cell culture is the preferred virologic test of most healthcare providers. Cell culture requires the collection of live virus samples that require special care in transport to the laboratory to retain viability. When viable samples are used, culture can be highly specific (if typing is performed) and positive results are generally reliable. The sensitivity of culture declines rapidly as lesions begin to heal and for this reason frequently non-positive results are falsely negative. Type-specific serology tests should be used in these cases to confirm a clinical diagnosis of genital herpes.

Antigen Detection (virus antigen)
The common methods used for antigen detection are immunofluorescence or enzyme immunoassays. These tests are inexpensive and rapid, but have relatively low specificity and lack usefulness in asymptomatic patients.

Polymerase Chain Reaction or PCR
PCR assays for HSV DNA are highly sensitive but expensive to perform and therefore, not suited for routine use.

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Antibody Detection

Antibody detection or serology tests involve the detection of antibodies to HSV in the blood. Both type-specific and nonspecific antibodies to HSV develop during the first several weeks following infection and remain in the blood indefinitely. Serology tests an important adjunct to virus detection method in patients with no obvious symptoms. The following tests are in this category:

Type-common (crude antigen) serology tests measure HSV antibodies in the blood but are unable to distinguish between HSV type-1 and HSV type-2 infection. Detection of antibodies can be useful to diagnose HSV infection when culture, virus antigen detection or PCR yield negative results, but are not helpful in making a definitive diagnosis of genital herpes in a patient with prior oral HSV type-1 infection.

Type-specific serology tests are those that accurately distinguish between HSV type-1 and HSV type-2 antibodies.

The Western Blot assay for HSV is highly accurate in differentiating HSV type-1 and HSV type-2 antibodies when used with a step to cross-adsorb antibodies to HSV type-1 and HSV type-2 antigens. However, it is expensive to perform, takes several days, and is not commercially available. The provider of Western Blot testing services is the University of Washington.

There are several type-specific ELISA (enzyme immunoassay) tests commercially available that are relatively easy to perform in the laboratory and give results quickly. Type-specific assays for HSV antibodies must be based on the HSV-specific glycoprotein G2 to accurately diagnose infection with HSV type-2 and with glycoprotein G1 to accurately diagnose HSV type-1.

HerpeSelect, from Focus Diagnostics offers two FDA-cleared lab-based assay formats for detecting type-specific HSV IgG antibodies: HerpeSelect immunoblot uses purified recombinant type-specific gG-1 and gG-2 antigens, and native HSV common antigens immobilized on nitrocellulose membranes. HerpeSelect ELISA consists of two micro-plate assays, one for detecting antibodies to HSV type-1 (gG1), and the other for antibodies to HSV type-2 (gG2). Both assay formats had sensitivity and specificity values ranging from 91-100% in clinical trials with sexually active adults and pregnant women.

Other FDA-cleared gG-based type-specific assays are available. Trinity Biotech manufactures HSV type-1 and type-2 kits, which have recently received FDA clearance. A type-specific HSV-2 assay, formerly available from Diagnology (POCkit^®), is expected to be reintroduced soon by another manufacturer.

Serological tests for HSV only indicate prior infection and do not identify the site of infection. These tests may give false-negative results in the early stages of infection because antibodies may not reach detectable levels for 8 weeks or longer. False-positive results can also occur in patients with low likelihood of HSV infection. Repeat testing or a confirmatory test may be indicated in some cases.

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