Explanation

TPHA titer (syphilis): what is measured?

Syphilis is an STI caused by the bacterium Treponema pallidum. The TPHA (Treponema pallidum hemagglutination assay) looks for antibodies aimed specifically at this bacterium. For a titer measurement, the laboratory dilutes the blood step by step and checks up to which dilution the antibodies are still detectable; the result is a ratio such as 1:80 or 1:1280. According to the LCI guideline (RIVM), these antibodies usually remain detectable for life, even after proper treatment. Whether an infection is active is shown by a different test, the RPR.

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
A syphilis you have had and been treated for in the past

Result

What does the result mean?

Positive result

A positive result means that antibodies against the syphilis bacterium have been found. This may fit an active syphilis, a previously treated syphilis or, less often, a false positive result. According to the LCI guideline (RIVM), the height of the titer says little about whether the infection is active or needs treatment; that requires a confirmation test and an RPR test. 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 can damage the brain, eyes, heart and blood vessels after years, 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) detects the bacterium itself. After using antibiotics shortly after the infection, 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 testing for HIV and other STIs is also done. With an active syphilis, treatment with antibiotics follows and sexual partners are informed, tested and treated if needed. Syphilis is not notifiable for individual cases.

Reliability

How reliable is the test?

The TPHA is a treponemal test: the antibodies are specific to the syphilis bacterium and, according to the LCI guideline (RIVM), usually remain detectable for life, even after proper treatment. A positive result is not a diagnosis on its own; the guideline names a confirmation test (IgG immunoblot) and an RPR test as the next step. According to the guideline, the TPHA titer usually gives no extra insight and is not suitable for monitoring the effect of treatment; the RPR titer, by contrast, does fall after successful treatment.

What can affect the result?

In the first weeks after infection, the test is often still negative; if early syphilis is suspected, a repeat after 3, 6 and 12 weeks is needed.
After treated syphilis, the test usually stays positive and then says nothing about a new infection.
The height of the titer says little about the activity of the infection or the need for treatment.
Cross-reaction with related tropical treponemal infections gives a positive result that cannot be distinguished from syphilis.

Who it is for

Who is the test useful for?

Useful for

The result is most meaningful 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. For people who have had syphilis before, this test says nothing about a new infection; an RPR titer then says more.

How common is it?

In 2024, syphilis was diagnosed in 1,798 people at the Sexual Health Centers; 92 percent of them were men who had sex with men. Syphilis is also more common in people with HIV. Almost no babies with syphilis are born in the Netherlands, because all pregnant women are screened.

The RIVM (Sexual Health Centers, 2024)

Drawbacks and risks

A positive result that fits a syphilis treated long ago, or that turns out to be false on confirmation, can cause unnecessary worry.
A high titer can wrongly be read as a sign of an active infection.

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