False Positive Tests for STDs

STD testing, and diagnostic testing in general, isn't a perfect science. Sometimes people test positive for diseases they do not have. This is known as a false positive test. False positives occur because no diagnostic test is perfect. As scientists design diagnostic tests that can find smaller and smaller evidence of a disease, they also open the door to accidentally detecting something that isn't really there. It's a tough balancing act. 

Test quality is always a matter of balancing the need to try to catch as many cases as possible (sensitivity) with the need to not diagnose people who aren't actually sick (specificity.) Unfortunately, it's difficult to design tests that are good at both. Therefore, scientists try and figure out which outcome is worse in any given situation - a false positive or false negative and weigh things accordingly.

In general, if missing a diagnosis could cause long term harm, and the treatments for a condition aren't particularly dangerous or unpleasant, doctors would rather risk false positive tests. In such situations, it's better to over diagnose and over treat. However, if being diagnosed improperly could lead to serious harm -- either physically through the use of dangerous treatments or emotionally because of the stigma associated with an infection -- then it's better to under diagnose. If you miss a case, because of a false negative, you can still try to catch the disease at a later stage when the condition is more clear.

Doctors are very worried about false positive herpes tests. Because herpes is very common, and many people never have symptoms, doctors don't consider missing a diagnosis to be a very big deal. In contrast, doctors know that the disease is so stigmatized that a false positive test could be life-changing in a very negative way. In other words, this is a situation where doctors think it is more to avoid false positives than to miss a chance to diagnose a herpes infection.

Because of concerns about false positive tests, doctors are often reluctant to test for the virus in the absence of either symptoms or a known exposure, even though blood tests do exist. It's a legitimate concern. The positive predictive value of a test is how likely it is that a person actually has a disease when they've tested positive for it. The biggest factor involved in that calculation is how many people in the tested population are expected to really have the disease.

What does that mean in the case of genital herpes? Research suggests that approximately 16 percent of U.S. adults with no symptoms are infected with genital herpes. While that's a lot, it's not quite high enough to make up for the limitations of herpes blood tests for screening purposes. A 2016 article did some calculations and found that, in people without symptoms, a positive herpes test would only be correct half the time.