Several conditions can cause a false-positive syphilis test result. However, healthcare professionals often use two different tests to diagnose a person with syphilis.

Syphilis is a sexually transmitted infection (STI) carried by bacteria. People with syphilis may have symptoms that come and go as the condition progresses over time. Untreated, they may pass syphilis onto other people despite having no symptoms. Syphilis can also pass from a pregnant person to their baby.

Tests can help tell if a person has syphilis. Syphilis testing usually involves two tests, where the first test screens for syphilis. Since these tests may detect signs of other conditions as well, people may require a second test to confirm and diagnose syphilis.

This article discusses if false-positive syphilis tests are possible. It also discusses what false-positive tests mean, what causes them, and what other infections can be mistaken for syphilis.

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Some syphilis tests may produce a false-positive result. False-positive tests for syphilis can be common.

Syphilis tests usually involve two steps and two different kinds of tests. Both types of tests have different limitations and costs. This is why healthcare professionals typically perform both tests together to provide confirmation.

The first testing step is a syphilis screening test. These often include a nontreponemal test, such as a rapid plasma reagin (RPR) test and a venereal disease research laboratory (VDRL) test.

These detect syphilis by looking for antibodies linked to having syphilis bacteria in a person’s blood. A person’s immune system makes antibodies if it finds harmful substances in the body.

These tests are often simple and inexpensive.

However, a person’s immune system may make these antibodies in response to other conditions. Therefore, using only one kind of test cannot tell if a person has syphilis for sure. They may also produce a false-positive test result for people without syphilis and for people previously treated for syphilis.

People with a positive nontreponemal test result generally require another type of test. This second test looks for antibodies their immune system only makes to fight off syphilis.

These tests are called treponemal or confirmatory tests, including:

  • chemiluminescence immunoassays (CLIA)
  • fluorescent treponemal antibody absorption (FTA-ABS)
  • microhemagglutination assay for antibodies to Treponema pallidum (MHA-TP)
  • T. pallidum enzyme immunoassay (TP-EIA)
  • T. pallidum hemagglutination assay (TPHA)
  • T. pallidum particle agglutination assay (TP-PA)

If any of these tests show a person has these antibodies, they have a reactive treponemal test. This means they have syphilis currently or have had a previously treated syphilis infection.

Healthcare professionals may sometimes perform treponemal tests first. They do this to identify people with treated, untreated, or incompletely treated syphilis. This is called reverse sequence screening, or a reverse algorithm.

However, this method may lead to higher rates of false-positive tests.

Home syphilis testing kits may only test for syphilis bacteria antibodies. They normally need a treponemal lab test to confirm if a person has syphilis.

Learn about syphilis home testing kits.

False-positive test results indicate a person has a specific condition when they do not.

Healthcare professionals and scientists use the terms “reactive,” for tests that indicate antibodies, and “nonreactive,” for those that do not.

If a person has a reactive nontreponemal result and a nonreactive treponemal test result, they likely have a false-positive test. This means it is unlikely they have syphilis.

Several different medical conditions and other factors can cause a person to have a false-positive syphilis test.

These include:

People may mistake several conditions and symptoms for syphilis.

These include:

Infections that can cause false-positive syphilis tests include:

  • malaria
  • HIV
  • hepatitis C
  • other treponemal infections, or infections from similar bacteria that cause syphilis:
    • yaws, which causes long-term skin conditions
    • endemic syphilis, a bacterial infection that is not sexually transmitted
    • pinta, a skin condition

Some frequently asked questions about syphilis tests are:

How accurate are syphilis tests?

Research from 2019 found that 11% of reactive nontreponemal tests were false positives. According to the Centers for Disease Control and Prevention (CDC), 14–40% of treponemal tests using the reverse algorithm are false positives.

However, healthcare professionals can use both tests together to confirm if a person has syphilis.

What if syphilis is reactive, but the RPR is negative?

If a person has a reactive treponemal test but a negative screening nontreponemal test, then they most likely have had syphilis in the past. They may also have the early stages of syphilis.

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Syphilis is a bacterial STI that people may contract through sexual contact with someone who has it. It can also pass from a pregnant person to their baby.

Syphilis tests can screen for and diagnose syphilis. Healthcare professionals normally use two kinds of syphilis tests.

They may use a nontreponemal test first to detect certain kinds of antibodies. Although this test is simple and inexpensive, other conditions can cause the antibodies the test detects.

If a person has a positive — or reactive — nontreponemal test result, they need another test to confirm they have syphilis — a treponemal test. It specifically detects antibodies a person’s body creates to fight syphilis.

Healthcare professionals may sometimes perform a treponemal test and then a nontreponemal test. This is a reverse algorithm test method.

If a person has reactive nontreponemal and treponemal tests, they have syphilis currently or have had syphilis in the past. If they have a reactive nontreponemal test, but a nonreactive treponemal test, they likely do not have syphilis presently. This is called a false-positive test.