Can a Syphilis Test Be a False Positive?

A positive syphilis result can come as a shock, especially if you feel fine or have no reason to expect it. The good news is that syphilis testing is designed with a safety net: one result is never meant to be the whole story.

Can a syphilis test be a false positive?

Yes. The CDC says that using only one type of blood test is not enough to diagnose syphilis. It can cause false-negative results in people tested during the first stage, and false-positive results in people without syphilis or in people who were treated before.

That is why syphilis testing normally involves two different kinds of test, and why a single reactive result should lead to a second test rather than to conclusions.

How does two-step syphilis testing work?

The CDC says a presumptive diagnosis needs two laboratory blood tests:

• A nontreponemal test, such as RPR or VDRL. Results come as a number (a titer), which might correlate with disease activity and is used to monitor how treatment is working.
• A treponemal test, such as TP-PA, EIA, CIA, an immunoblot or a rapid test. These are specific to the syphilis bacterium, and are often used to confirm the first result.

Labs use one of two orders. In the traditional order, the nontreponemal test comes first and a reactive result is followed by a treponemal test. In the reverse order, a treponemal screening test comes first, followed by a nontreponemal test.

What does a reactive result mean?

A reactive result means the test picked up a signal. On its own it doesn’t tell you whether you have syphilis now, had it before, or got a false alarm.

If the first test is treponemal and the nontreponemal follow-up is negative, the CDC says the lab should run a second, different treponemal test. The CDC describes what each outcome means:

• Second treponemal test positive: if you were treated before, no further management is needed unless you may have been re-exposed. If you were never treated, you should be offered treatment.
• Second treponemal test negative, with low risk: further evaluation or treatment is not indicated. The first result may have been a false positive.

What can cause a false-positive syphilis test?

The CDC says false-positive nontreponemal results can be linked to other infections (such as HIV), autoimmune conditions, vaccinations, injecting drug use, pregnancy and older age. That’s why a reactive nontreponemal test should always be followed by a treponemal test.

Treponemal screening can also give false positives when someone has a low likelihood of infection.

Why might my test stay positive after treatment?

Most people with a reactive treponemal test stay reactive for life, whether or not they were adequately treated. The CDC notes that 15% to 25% of people treated in the primary stage become nonreactive after 2 to 3 years.

Nontreponemal titers, by contrast, usually go down after treatment and can become nonreactive over time. So your clinician compares those numbers across repeat tests to see how treatment worked.

What about a false positive in pregnancy?

Pregnancy is one situation where nontreponemal tests can be falsely positive. But congenital syphilis is serious, so a reactive screening test needs prompt attention. The CDC says a positive EIA or CIA is more likely a false positive in people living in communities with low syphilis rates, with an uninfected partner and no history of treated syphilis.

In selected low-risk cases, the CDC says repeating the tests within 4 weeks can be considered. If both remain negative, no treatment is needed. This is a decision for your prenatal provider, not something to wait out alone.

What should I do next?

Ask your clinician which tests were done and what the second result shows. Don’t start or stop any medicine on your own, and avoid sex until the picture is clear. If you do have syphilis, it can be cured with the right antibiotics, and your partners may need testing too. They can order an online STD test.