Testing for sexually transmitted diseases (STDs) is one of the best things you can do to protect yourself and your partner(s) from infection. However, STD tests aren’t perfect. It is possible to get a negative test result and still have an STD.
Given the potential impact of some STDs—from genital warts to fertility issues to cancer and more—it may seem unlikely that you could have one and not know it. But very often, that’s the case.
According to an analysis published in the journal Sexually Transmitted Diseases, an estimated 1 in 5 people in the U.S. had a sexually transmitted infection (STI) on any given day in 2018. Some knew about their diagnosis, while some did not.
There are several potential reasons why STD tests aren’t infallible.
You May Not Have Actually Been Tested
A lot of people think that their healthcare provider screens them for STDs as part of their annual exam. This is, unfortunately, untrue. Many providers don’t regularly screen their clients for STDs, even when practice guidelines say they should.
The only way to be certain you’re getting tested for STDs is to ask your provider to test you. If you have a known exposure, mention it. Be open about your sexual history (past and present), number of partners, how you have sex, and what (if any) preventive measures you take. All of this can highlight the need for specific tests.
You May Have Been Tested Too Soon
Some STD tests are not effective for a newly acquired infection. For example, a study published in 2014 showed that the standard blood test for syphilis is ineffective at detecting early cases of the disease.
Some STD tests, like HIV tests, look for an antibody response (instead of the sexually-transmitted bacterium or virus itself). Antibodies are specific proteins your immune system produces in response to infection.
These tests may be particularly susceptible to errors when given too soon, as it takes time for an antibody response to develop.
The Test Gave an Inaccurate Result
When designing a diagnostic test, there is always a trade-off between sensitivity and specificity.
Almost no test is going to be perfectly able to determine whether or not someone is infected. The ability of an STD test to predict your health is dependent, in part, on the population that test is being used in.
Most tests are designed to be pretty good, and there are almost always ways to make their results more accurate. Still, both false positives and false negatives can be a problem. Which problem you need to worry about depends on the disease in question and the test that is being used to detect it.
You Were Given the Wrong Test
There isn’t always a right test, but sometimes there is a wrong one. As mentioned above, every diagnostic test has trade-offs. There are often tests that are more or less accurate depending on the circumstance and the population.
The problem is that the best test isn’t always available or practical. Therefore, healthcare providers will sometimes end up having to use a less accurate method of diagnosis.
Your Healthcare Provider Didn’t Test for the STD You Have
There are some diseases for which there are no commercial tests or that healthcare providers simply don’t bother testing for because they’re unlikely to cause serious problems if left untreated.
For example, healthcare providers don’t test for molluscum contagiosum because they assume that anyone infected will have symptoms and because the infection will usually run its course without any serious side effects.
In addition, healthcare providers are probably unlikely to test for rectal chlamydia, anal cancer, and other rectal STDs for other reasons. They may not offer the tests because of the relative rarity of these conditions. They might also be uncomfortable asking sexual history questions that would help them determine that you’re at risk.