Diagnosis

The herpes simplex virus is usually identifiable by its characteristic lesion: A thin-walled blister on an inflamed base of skin. If the diagnosis is uncertain, more tests will be needed. Patients diagnosed with genital herpes should be tested for Chlamydiatrachomatis and other sexually transmitted diseases.

Microscopic Examination of Tissue Scrapings (Tzanck Test)

The Tzanck test uses scrapings from herpes lesions and is useful for identifying the presence of herpes simplex. The scrapings are stained and microscopically examined. Findings of specific giant cells with many nuclei or distinctive particles that carry the virus (called inclusion bodies) indicate HSV infection. The test is quick but accurate in only 50 - 70% of cases. It cannot distinguish between the HSV types or between herpes simplex and herpes zoster.

Viral Cultures

An accurate diagnosis of HSV is best made by taking a fluid sample, or culture, from the lesions as early as possible, ideally within the first 3 days of appearance. The viruses, if present, will reproduce in this fluid sample and can usually be detected after a few days, although if infection is severe, technology exists that can shorten this period to 24 hours. Viral cultures are almost 100% accurate if lesions are still in the clear blister stage. Such tests are not as effective for older ulcerated sores, recurrent lesions, or latency. At these stages the virus may not be active enough to reproduce sufficiently to produce a visible culture.

Immunologic Tests

To confirm results of a Tzanck smear and viral cultures, blood tests are available that can identify antibodies that are specific to the herpes virus and its type. (Antibodies are selective in their attack on viruses, so detecting high levels of an antibody to a specific virus is evidence of infection.)

Immunologic tests are most accurate when administered 12 - 16 weeks after exposure to the virus. The three standard tests are:

  • Western Blot Test. This is the gold standard for researchers with accuracy rates of 99%. It is costly and time consuming, however.
  • HerpeSelect. This includes two tests: ELISA (enzyme-linked immunosorbent assay) and Immunoblot. They are both highly accurate in detecting HSV-1, -2, or both. Results take 1 - 2 weeks.
  • POCkit. This test detects HSV-2 only. Its major advantages are that it requires only a finger prick and results are provided in less than 10 minutes. It is very accurate, although slightly less so than the other tests. It is also less expensive. It is not usually used for pregnant women.

Because HSV-1 recurring infections tend to have a milder course than those due to HSV-2, some doctors now regularly test all infected patients for HSV type. These tests may be specifically beneficial for women who are pregnant or wish to conceive and for the partners of such women. They have limitations, however. For example, it is not clear if these tests are highly accurate in children. They also give no information on the location or duration of the infection.

Immunologic tests using urine or saliva, which would be particularly useful for testing children, are under investigation.

Tests for HSV Encephalitis

Diagnosis of HSV encephalitis may require a number of tests.

Imaging Tests. Electroencephalography traces brain waves and can identify about 80% of cases. Computed tomography (CT) or magnetic resonance imaging (MRI) scans may be used to differentiate encephalitis from other conditions.

Brain Biopsy. Brain biopsy is the most reliable method of diagnosing HSV encephalitis, but it is also the most invasive and is generally performed only if the diagnosis is uncertain.

Polymerase Chain Reaction (PCR). The polymerase chain reaction (PCR) assay looks for tiny pieces of the DNA of the virus, and then replicates them millions of times until the virus is detectable. This test can identify specific strains of the virus and asymptomatic viral shedding. PCR identifies HSV in cerebrospinal fluid and gives a rapid diagnosis of HSV encephalitis in most cases, eliminating the need for biopsies. Sensitivity is almost equal to viral culture and results are also much quicker. (An automatic PCR assay -- the LightCycler -- provides results in 2 hours.)