Genital herpes; Fever blisters; Cold sores; HSV-1; HSV-2
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, some patients who carry the virus may not have visible genital lesions. Laboratory tests are essential for confirming herpes diagnosis. These tests include virologic tests (which examine samples of skin taken from the lesion) and serologic tests (blood tests that detect antibodies).
In its 2006 guidelines for sexually transmitted diseases, 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.
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 appearance. 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 microscopically examined for the virus. Findings of specific giant cells with many nuclei or distinctive particles that carry the virus (called inclusion bodies) indicate herpes infection. The test is quick but accurate 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 (blood) tests can identify antibodies that are specific to the virus and its type, herpes virus simplex 1 (HSV-1) or herpes virus simplex 2 (HSV-2). When the herpes virus infects someone, their body‚ ' s immune system produces specific antibodies to fight off the infection. If a blood test detects antibodies to herpes, it‚ ' s evidence that you have been infected with the virus, even if the virus is in a non-active (dormant) state. The presence of antibodies to herpes also indicates that you are a carrier of the virus and might transmit it to others.
Newer ‚Äútype-specific‚ÄĚ assays test for antibodies to two different proteins that are associated with the herpes virus:
Although glycoprotein (gG) type-specific tests have been available since 1999, many of the older nontype-specific tests are still on the market. The CDC recommends only type-specific glycoprotein (gG) tests for herpes diagnosis.
Serologic tests are most accurate when administered 12 - 16 weeks after exposure to the virus. Recommended tests include:
False-negative (testing negative when herpes infection is actually present) results can occur if tests are done in the early stages of infection. False-positive results (testing positive when herpes infection is not actually present) can also occur, although more rarely than false-negative. Your doctor may recommend that you have the test repeated.
Doctors recommend serologic herpes tests especially for:
At this time, doctors do not recommend screening for HSV-1 or HSV-2 in the general population.
It may take a number of tests to diagnose herpes encephalitis.
Imaging Tests. Electroencephalography traces brain waves and can identify about 80% of cases. Computed tomography or magnetic resonance imaging scans may be used to differentiate encephalitis from other conditions.
Brain Biopsy. Brain biopsy is the most reliable method of diagnosing herpes 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 herpes encephalitis in most cases, eliminating the need for biopsies. The CDC recommends PCR for diagnosing herpes central nervous system infections.
Canker Sores (Aphthous Ulcers). Common canker sores (known medically as aphthous ulcers) are often confused with the cold sores of herpes simplex virus 1 (HSV-1). Canker sores frequently crop up singly or in groups on the inside of the mouth or on or under the tongue. They are usually white or grayish crater-like ulcers with a sharp edge and a red rim. They usually heal in 2 weeks without treatment.
Thrush (Candidiasis). Candidiasis is a yeast infection that causes a whitish overgrowth in the mouth. It is most common in infants but can appear in people of all ages, particularly those with impaired immune systems.
Other conditions that may be confused with oral herpes include herpangina (a form of the Coxsackie A virus), sore throat caused by strep or other bacteria, and infectious mononucleosis.
Conditions that may be confused with herpes simplex virus 2 (HSV-2) include bacterial and yeast infections, genital warts, herpes zoster (shingles), molluscum (a virus disease which produces small rounded swellings), scabies, syphilis, and certain cancers.
In a few cases, HSV-2 may occur without lesions and resemble cystitis and urinary tract infections.
Simple corneal scratches can cause the same pain as herpetic infection, but these usually resolve within 24 hours and don't exhibit the corneal lesions characteristic of herpes simplex.
Skin disorders that may mimic herpes simplex include shingles and chicken pox (both caused by varicella-zoster, another herpes virus), impetigo, and Stevens-Johnson syndrome, a serious inflammatory disease usually caused by a drug allergy.
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