Explanation

Syphilis antibodies: what is measured?

The test looks for antibodies against Treponema pallidum, the bacterium that causes syphilis. The laboratory uses the TPHA (Treponema pallidum hemagglutination assay) for this, a so-called treponemal test: the antibodies are specific to the syphilis bacterium and usually remain detectable for life, even after treatment. A positive result is checked with a confirmation test (IgG western blot) and an RPR test, which shows whether the infection is active.

What can it detect?

Syphilis in the first or second stage, from a few weeks after infection
Latent syphilis: an infection without symptoms that can last for years
Syphilis you have had before and that was treated

Result

What does the result mean?

Positive result

A positive result means that antibodies against the syphilis bacterium have been found. This can fit active syphilis, previously treated syphilis or, less often, a false-positive result. Only after a confirmation test and an RPR test is it clear whether there is a syphilis infection and whether it is active. Antibodies against related tropical bacteria (such as yaws) can also make the test positive. Without treatment, syphilis does not go away on its own and after years can damage the brain, eyes, heart and blood vessels, but it can be treated well with antibiotics.

Negative result

A negative result means that no antibodies against the syphilis bacterium have been found. Shortly after infection, antibodies are often not yet detectable. If early syphilis is suspected, such as a painless sore after sex, the LCI guideline (RIVM) advises repeating the blood test after 3, 6 and 12 weeks; with a sore, a test on the wound fluid (PCR) is the first choice. After using antibiotics, the test can stay negative for longer or permanently.

After an abnormal result

With a positive result, a confirmation test and an RPR test follow, and the doctor also tests for HIV and other STIs. With active syphilis, treatment with antibiotics follows and sexual partners are warned, tested and treated if needed. Syphilis is not notifiable for individual cases.

Reliability

How reliable is the test?

According to the LCI guideline (RIVM), the TPHA is one of the standard tests for detecting syphilis. A positive result is not a diagnosis on its own: the guideline requires a confirmation test (IgG western blot) and an RPR test. Because the antibodies remain after treatment, this test is not suitable for showing a new infection in someone who has had syphilis before.

What can affect the result?

In the first weeks after infection the test is often still negative; if early syphilis is suspected, repeating it after 3, 6 and 12 weeks is needed.
Using antibiotics shortly after infection can delay or prevent antibody formation.
Cross-reaction with related tropical treponemal infections gives a positive result that cannot be distinguished from syphilis.
After treated syphilis, the test usually stays positive and then says nothing about a new infection.

Who it is for

Who is the test useful for?

Useful for

Useful after sex without a condom with a new or changing partner, for men who have sex with men, sex workers and people with HIV, if a partner has syphilis, and with symptoms such as a painless sore on or around the penis, vagina, anus or mouth, or a non-itchy skin rash on the trunk, arms and legs. Pregnant women are already tested as standard in the first trimester. If you have had syphilis before, this test is not suitable for showing a new infection.

How common is it?

In 2024, the Sexual Health Centers diagnosed syphilis in 1,798 people; 92 percent of them were men who have sex with men. Syphilis is also more common in people with HIV. Almost no babies with syphilis are born in the Netherlands (zero to three per year), because all pregnant women are screened.

RIVM (Sexual Health Centers, 2024); LCI guideline on syphilis

Drawbacks and risks

A positive screening result that turns out to be false positive on confirmation, or that fits syphilis treated long ago, can cause unnecessary worry.
Testing shortly after a risk contact can give false reassurance if no retest follows.

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