Herpes is a virus that causes ulcers in the oral region (cold sores) and genital region. Herpes type 1 (HSV-1) usually causes oral cold sores, and herpes type 2 (HSV-2) usually causes ulcers in the genital area. However, with increased oral-genital contact, either type can be found in either location, though most genital herpes (85 percent of cases) is caused by HSV-2. Most people who have become infected with HSV-2 never develop ulcers at all, but they are still infectious, and can pass it on to their sexual partners. A variety of tests for herpes are available. If you get a positive test result, you've either had herpes, or you have a false positive test (a positive test result but no infection).
Herpes can be diagnosed most easily and accurately when you have an ulcer. At that time, your health care provider can swab the ulcer and send it to the lab to test for the virus. If you get a positive result from a swab of the ulcer, you have genital herpes.
However, many people visit their health care providers when they do not have active ulcers. They may have a history of ulcers or perhaps they think they could have been infected by a past or current sexual partner. In that case, the diagnosis of herpes is more difficult to make. Blood tests (serology) can be done to check for infection-fighting proteins (antibodies) that are produced by your immune system when you are infected with HSV. A variety of serologic tests are available, and each type has a different rate of false positive results.
Older tests had a higher rate of false-positive results for genital herpes, because they were non-specific; they could test for HSV, but could not distinguish HSV-1 infection from HSV-2 infection. This made the tests difficult to interpret, as many people had positive test results because they'd had cold sores (HSV-1). Newer tests are type-specific, and can test for either HSV-1 or HSV-2. If you have a positive serologic test for HSV-2, you have most likely been infected with HSV-2 at some point in the past. However, false positives can still occur in about 5 percent of cases, depending on the type of test.
If you are given a positive test result for genital herpes, it's important that you talk to your health care provider and ask about the likelihood that you are infected, and if so, how you can avoid passing the infection on to your sexual partners.