Diagnosis

The herpes simplex virus is usually identifiable by its characteristic lesion: A thin-walled blister on an inflamed base of skin. However, other conditions can resemble herpes, and doctors cannot base a herpes diagnosis on visual inspection alone. In addition, many patients who carry the virus do not have visible genital or oral lesions. Laboratory tests are essential for confirming herpes diagnosis. These tests include:

• Virologic tests (viral culture of the lesion)
• Serologic tests (blood tests that detect antibodies)

The U.S. Centers for Disease Control (CDC) recommends that both virologic and serologic tests be used for diagnosing genital herpes. Patients diagnosed with genital herpes should also be tested for other sexually transmitted diseases.

According to the CDC, up to 50% of first-episode cases of genital herpes are now caused by herpes simplex virus 1 (HSV-1). However, recurrences of genital herpes, and viral shedding without overt symptoms, are much less frequent with HSV-1 infection than herpes simplex virus 2 (HSV-2). It is important for doctors to determine whether the genital herpes infection is caused by HSV-1 or HSV-2, as the type of herpes infection influences prognosis and treatment recommendations.

Virologic Tests

Viral culture tests are made by taking a fluid sample, or culture, from the lesions as early as possible, ideally within the first 3 days of the outbreak. The viruses, if present, will reproduce in this fluid sample but may take 1 - 10 days to do so. If infection is severe, testing technology can shorten this period to 24 hours, but speeding up the timeframe during this test may make the results even less accurate. Viral cultures are very accurate if lesions are still in the clear blister stage, but they do not work as well 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.

Polymerase chain reaction (PCR) tests are much more accurate than viral cultures, and the CDC recommends this test for detecting herpes in spinal fluid when diagnosing herpes encephalitis (see below). PCR can make many copies of the virus’ DNA so that even small amounts of DNA in the sample can be detected. PCR is much more expensive than viral cultures and is not FDA-approved for testing genital specimens. However, because PCR is highly accurate, many labs have used it for herpes testing.

An older type of virologic testing, the Tzanck smear test, uses scrapings from herpes lesions. The scrapings are stained and examined under a microscope for the presence of giant cells with many nuclei or distinctive particles that carry the virus (called inclusion bodies). The test is quick but accurate only 50 - 70% of the time. It cannot distinguish between virus types or between herpes simplex and herpes zoster. The Tzanck test is not reliable for providing a conclusive diagnosis of herpes infection and is not recommended by the CDC.

Serologic Tests

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Review Date: 11/05/2010
Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

A.D.A.M., Inc. is accredited by URAC, also known as the American Accreditation HealthCare Commission (www.urac.org)