Are STD tests always accurate?

More than two million new cases of chlamydia, gonorrhea, and syphilis were reported in the U.S. in 2016 alone, according to recent data from The Centers for Disease Control and Prevention (CDC).

That’s the highest number ever in the U.S.

With more people being diagnosed, it’s fair to ask: How accurate are the tests that spot these diseases?

Anne Rompalo, M.D., a professor at Johns Hopkins University School of Medicine who studies sexually transmitted diseases, walked us through your options.


Chlamydia and Gonorrhea

If you go into a clinic or a doctor’s office to get tested for chlamydia (the most common bacterial STI in the United States) or gonorrhea (another common bacterial STI), it’ll likely be done via a urine sample or a swab from the rectum or throat.

The test itself is called a nucleic acid amplification test (NAAT). “It looks for the unique DNA or RNA of the bacteria or virus, and it amplify it exponentially,” Rompalo explains.

That makes it easy for doctors to spot. Cultures (a past technique of testing) took longer and required that more of the virus be present in order to pick it up.

“Nucleic acid amplification tests are very good, and they have moved diagnostics forward,” says Rompalo. “These are incredibly accurate compared to our old diagnostic techniques.”


Data from the CDC suggest that for both STIs, a false positive is incredibly rare (99 percent of the time tests that come back negative are accurate). And if you do have the STI, it’ll pick it up more than 90 percent of the time. In fact, research confirms these new tests blow old ones out of the water: NAATs typically pick up 20 to 50 percent more chlamydia infections than cultures and other earlier tests did, studies show.


Trichomoniasis is a parasite that typically infects people without causing any symptoms. The best test to pick it up is, again, a NAAT, says Rampalo. Research finds the test can be 76 to 100 percent accurate, though the parasite can also be picked up in a culture. Trichomoniasis is difficult to find in a urine sample because urine can be toxic to the parasite, she notes.


The CDC doesn’t recommend testing for herpes in people without symptoms-unless you have symptoms that could be related to herpes, have had sex with a partner who has herpes, or you have multiple partners.


If you have a sore (or what you suspect to be a sore), the best test is what’s called a polymerase chain reaction (PCR) test, says Rompalo. Data from a recent study finds these tests pick up the virus more than 96 percent of the time.

Your doc would take a sample from the sore, and a lab test would look for genes from the herpes virus. This is a very effective test, she notes. (A culture-taking a scrape from a sore and checking it for the herpes virus-is also an option.)

There are also highly-accurate blood tests that look for antibodies that fight the herpes virus. If you don’t have a sore but think you may have been exposed to the virus from someone who has it, the blood test could be a good option, she notes.


Doctors can’t send the bacterium that causes syphilis, spirochetes Treponema pallidum, for culturing, Rompalo notes.


For now, physicians are stuck with imperfect blood tests that try to identify antibodies in the blood. (One study published in JAMA found the tests to be at least 85 percent accurate.) As she puts it: “These tests don’t always do what we want them to do.

They are imperfect measures. We have been struggling with this for decades and are trying to come up with a better diagnostic but are not quite there.”


Most HPV that causes cancer-strains 16 and 18, for the most part-don’t cause symptoms. That means if you notice warts, they’re likely from HPV strains 6 or 11 (which-with the exception of rare cases-are not linked to cancer). Doctors usually identify these upon exam, an effective technique.

There’s currently no FDA-approved HPV test for men, nor an FDA-approved test to find the virus anywhere besides the cervix, according to


That could be because 85 to 90 percent of people who are sexually active have had HPV, Rampalo says, and most fight off the infection on their own.


The main tests for HIV are blood tests. “The tests that are out there are great,” Rampalo says. “They detect HIV earlier than we ever have before.”

Research finds blood tests pick up the disease 99.5 percent of the time. There are three types of tests available: nucleic acid tests (which look for the virus in the blood), antigen/antibody tests (which look for both HIV antibodies produced by your immune system and antigens that are produced even before antibodies develop), and antibody tests (which look for antibodies in your blood or saliva).

Generally, blood tests from a vein detect HIV sooner than pricks or fluid swabs, the CDC notes.


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