False Positive STD Tests: Why They Happen

A positive result can be frightening, and it is natural to wonder whether the test got it wrong. Sometimes a test does. That is why many STD tests are done in two steps, and why a first positive is usually followed by a second test.

Can an STD test be a false positive?

Yes, with some tests and in some situations. A false positive is a positive result when the infection isn’t there. Screening tests can produce some false alarms, especially at borderline levels. A second, different test sorts the real positives from the false ones.

A positive should never be waved away. Don’t assume it is false, and don’t ignore it. Ask for the confirming test.

What can cause a false positive herpes test?

Herpes blood tests look for antibodies. The CDC says commercial type-specific tests have weak specificity, especially at low index values, and that one commonly used HSV-2 test often is falsely positive at low index values (1.1 to 3.0). Because of this, the CDC says a confirmatory test with a second method should be performed before the result is interpreted.

The CDC doesn’t recommend routine herpes screening for people without symptoms. A herpes blood test is most useful when your clinician has a reason to order it, so talk through the result with them. The herpes blood test accuracy page covers this in more detail.

Can a syphilis test be a false positive?

Syphilis is tested in two steps for this reason. The CDC says false-positive nontreponemal test results can be associated with multiple medical conditions unrelated to syphilis, including other infections such as HIV, autoimmune conditions, vaccinations, injecting drug use, pregnancy and older age. A person with a reactive nontreponemal test should always receive a treponemal test to confirm.

MedlinePlus says the same screening antibodies can be made in response to syphilis, some autoimmune diseases and other infections, so a positive screen needs confirmation.

Can an HIV test be a false positive?

HIV screening is also done in steps. MedlinePlus says a positive screening result means signs of HIV were found in your sample, and a follow-up test is needed to confirm it unless you had a NAT test. The CDC says all reactive rapid tests should be assessed with a laboratory-based antigen/antibody test.

Home tests work the same way. The FDA says that even when used correctly, the OraQuick in-home test will give some false negatives and a small number of false positives. A positive result needs more testing in a medical setting.

Can chlamydia, gonorrhea or trichomoniasis tests be false positives?

NAATs are the usual tests for these infections. The CDC mentions specific situations in which a positive can mislead:

• Retesting too soon after treatment. The CDC says chlamydia NAATs less than 4 weeks after completing therapy are not recommended, and that trichomonas NAATs shouldn’t be done until at least 3 weeks after treatment, because leftover genetic material can be detected.
• Throat samples for gonorrhea. The CDC says some NAATs may have low specificity for throat specimens because of related bacteria that normally live there. If a NAAT is positive, a confirmatory culture should be attempted before retreatment, especially if no culture was collected.

What should I do after a positive result?

1. Ask which test it was and what the confirming test is.
2. Get the confirming test. Your clinician or the lab will usually arrange it.
3. Don’t start or stop medicine on your own. Treatment, when needed, is prescribed by a clinician.
4. Follow your clinician’s advice about partners and sex while you wait.

If the confirming test is negative, ask whether you need another test later, since timing can also affect results. If it is positive, the next-steps guide covers what comes after.