Laboratory
Tests
The clinical diagnosis of genital herpes is both insensitive and non-specific.
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:
Conventional Culture (viable virus)
Isolation of HSV in cell culture is the preferred HSV 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.
Modified Culture
- Shell Vial: A centifugation enhanced culture technique used to obtain rapid culture results. Generally less sensitive than conventional culture.
- ELVIS® (Enzyme-Linked Virus Inducible System): A technique which combines cell culture amplification with HSV-activated reporter genes. The test produces results which are equal to conventional culture.
Antigen Detection (virus antigen)
The common methods used for antigen detection are:
- Immunofluorescent (IFA) and immunoperoxidase (IPA) assays which detect HSV antigen in smears or tissues. HSV-specific antibodies are labeled with fluorescent dyes or enzymes (peroxidase). The labeled antibodies are incubated with the specimen and bind to HSV antigens in the specimen, if present. The attached fluorescent dye or enzyme can be visualized in appropriate regions of infected cells under a microscope.
- Enzyme Immunoassays, (EIA or ELISA) are performed on fluids or other samples using HSV-specific antibody that is bound to a solid surface. The antibody captures antigen to which anti-HSV antibodies labeled with enzymes are added. These attach to the bound antigen and cause a color change.
Polymerase Chain Reaction
Polymerase Chain Reaction or PCR (virus nucleic acid, DNA) tests are highly sensitive but expensive to perform and therefore, not suited to routine use. PCR amplifies small amounts of virus type-specific DNA to easily detectable levels using pre-prepared genetic sequences unique to HSV-1 or HSV-2. PCR is recommended for use in diagnosing CNS infections of neonates by testing cerebrospinal fluid.
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. During primary infection, the first antibodies produced belong to the IgM class. The presence of IgM antibodies in HSV infections is difficult to interpret and may be confusing. Unlike IgG, IgM antibodies are often not type-specific. The presence of IgM antibodies may indicate a primary infection, a super-infection with the other serotype, or reactivation of a latent HSV infection, (HSV-1 or HSV-2). For this reason, IgM testing is not recommended as a diagnostic tool.
IgG antibody detection, or serology tests, are an important adjunct to virus detection methods. The following tests are in this category:
Type-specific serology tests distinguish between HSV-1 and HSV-2 antibodies.
The advent of recombinant technology has enabled production of purified HSV type-specific antigens gG1 and gG2, and these antigens can be used to detect HSV type-specific IgG antibodies. Sera from individuals infected with only HSV-1 will react only with gG1, sera from individuals infected with only HSV-2 will only react with gG2, and sera from individuals infected with both HSV-1 and HSV-2 will react with both gG1 and gG2.
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 are based on the HSV-specific glycoprotein G2 for HSV -2 and with glycoprotein G1 for HSV -1.
The most frequently used HSV serology tests, HerpeSelect® tests from Focus Diagnostics, are available in four formats for detecting type-specific HSV IgG antibodies.
1. HerpeSelect® Immunoblot uses purified recombinant type-specific gG-1 and gG-2 antigens, and native HSV common antigens immobilized on nitrocellulose membranes.
2. HerpeSelect® ELISA consists of two micro-plate assays, one for detecting antibodies to HSV-1 (gG1), and the other for antibodies to HSV-2 (gG2).
3. Plexus™ HerpeSelect® provides both HSV-1 and HSV-2 from a single specimen using Luminex®xMAP® technology.
4. HerpeSelect® Express™ rapid HSV-2, a point-of-care device providing an HSV-2 serology result from a finger stick in about 15 minutes. (pending CLIA waiver)
Type-common (crude antigen) serology tests measure HSV antibodies in the blood but are unable to distinguish between HSV-1 and HSV-2 infection. Most adults have HSV-1 antibodies, so crude antigen tests ability to detect HSV-2 infections is questionable.
The Western Blot assay for HSV is highly accurate in differentiating
HSV-1 and HSV-2 antibodies when used with a step to cross-adsorb
antibodies to HSV-1 and HSV-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.
Serological tests for HSV only indicate presence or absence of 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. Repeat testing may be indicated in settings with a positive predictive value. 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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