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Sexual health testing guide · 2026
Syphilis is the STI where the report is hardest to read. Two antibody tests, a number that goes up and down, and a result that stays positive for life.
Syphilis is diagnosed with two types of blood test used together: a treponemal test (such as TPHA, TPPA or a treponemal immunoassay), which usually stays positive for life once infected, and a non-treponemal test (RPR or VDRL), which produces a titre that rises with active infection and falls with successful treatment. Neither is sufficient alone. Where an ulcer is present, PCR from the lesion can confirm infection before antibodies develop. Syphilis is entirely curable with penicillin-based treatment, and the RPR titre is how response is monitored.
| Treponemal tests | TPHA, TPPA or treponemal immunoassay — usually positive for life |
|---|---|
| Non-treponemal test | RPR or VDRL — reported as a titre; used to monitor treatment |
| Lesion test | PCR or dark-field from an ulcer, useful before antibodies develop |
| Serology window | Usually 3–6 weeks after exposure; may take up to 90 days |
| Price | Listed on the MedEx STD and STI testing page and the lab test catalogue |
| If positive | Same-day doctor consultation for treatment and partner advice |
The two test types answer different questions.
Treponemal tests detect antibodies specific to the organism. They become positive early and, importantly, usually remain positive for life even after successful treatment. So a positive treponemal test tells you that you have been infected at some point — not whether you have active infection now.
Non-treponemal tests (RPR, VDRL) measure antibodies to material released by damaged cells. They are reported as a titre — 1:2, 1:8, 1:32 and so on. The titre rises with active infection and falls after effective treatment, often becoming non-reactive. That makes it the marker used to monitor response.
Laboratories may run either test first. Under a reverse-sequence algorithm, a treponemal immunoassay is run first and reactive samples are then tested by RPR, with a second, different treponemal test used to resolve discordant results. Ask which algorithm your laboratory uses when reviewing the report with a clinician.
| Treponemal positive, RPR reactive with a titre | Consistent with active or recently treated infection. Staging and treatment history determine what happens next. |
|---|---|
| Treponemal positive, RPR non-reactive | Often previously treated infection, or very early or very late disease. Treatment history is essential to interpretation. |
| Treponemal negative, RPR reactive | Suggests a biological false positive, which can occur in pregnancy, autoimmune disease and some infections. Needs confirmation. |
| Both negative | No evidence of infection, but repeat if exposure was recent, since antibodies may take up to 90 days. |
Because the treponemal test stays positive for life, people who were treated years ago are frequently alarmed by a positive screen. Keeping a record of the date and result of treatment, and of the RPR titre at the time, saves a great deal of trouble later.
A painless ulcer is worth testing even after it heals.
Primary: a single painless ulcer (chancre) at the site of infection, appearing roughly 3 weeks after exposure and healing on its own within a few weeks. Because it is painless and may be internal, it is frequently missed. This is the stage where lesion PCR is most useful, since serology may still be negative.
Secondary: weeks to months later, a rash that characteristically involves the palms and soles, mucous patches, patchy hair loss, swollen lymph nodes and flu-like symptoms. These also resolve without treatment, which is why the infection is so easily missed.
Latent: no symptoms, positive serology. Divided into early and late latent, which affects the treatment course.
Tertiary: years later, potentially affecting the heart, brain and other organs. Neurological, ocular or ear involvement can occur at any stage, and new visual or hearing symptoms in someone with syphilis need urgent assessment.
Syphilis in pregnancy can be transmitted to the baby with serious consequences, so screening is a routine part of antenatal care and treatment is time-critical.
Syphilis is curable. Benzathine penicillin remains the treatment of choice, with the number of doses determined by stage. Alternatives exist for penicillin allergy, though penicillin desensitisation is preferred in pregnancy. This is not a condition to self-treat or to treat with a partial course.
Follow-up is by RPR titre. A fourfold fall (for example 1:32 to 1:8) indicates an adequate response, usually assessed at intervals over 6 to 12 months. A titre that fails to fall, or that rises fourfold, suggests treatment failure or reinfection and requires reassessment.
Retest at the same laboratory where possible, since titres are not perfectly comparable across laboratories.
Partner notification is essential, with the look-back period depending on stage. Testing for HIV, chlamydia, gonorrhoea and hepatitis should be done at the same time, since co-infection is common — see the full STI panel guide. Where ongoing risk exists, discuss PrEP and prevention options.
A treponemal test confirms you have been infected at some point and usually stays positive for life. A non-treponemal test such as RPR gives a titre that rises with active infection and falls with successful treatment, so it is used to monitor response. Neither is sufficient alone.
Serology usually becomes positive 3 to 6 weeks after exposure but can take up to 90 days. Where an ulcer is present, PCR from the lesion can confirm infection before antibodies develop.
The treponemal test usually stays positive for life. The RPR titre should fall fourfold or become non-reactive after successful treatment, and that is what is used to confirm cure.
It reflects the level of non-treponemal antibody and is used comparatively rather than as an absolute. A fourfold fall after treatment indicates an adequate response; a fourfold rise suggests reinfection or treatment failure.
Yes. A reactive RPR with a negative treponemal test suggests a biological false positive, which can occur in pregnancy, autoimmune disease and some other infections. Confirmation is needed.
Yes. Benzathine penicillin is the treatment of choice, with the number of doses determined by the stage of infection. Follow-up RPR testing confirms the response.
Confusing result from an old infection? Bring your treatment history.
Medical disclaimer: This article is general health information and does not replace medical advice, diagnosis or treatment. Laboratory reference ranges differ between laboratories and results must be interpreted alongside your symptoms, medicines and medical history. Speak with a qualified clinician before starting, stopping or changing any treatment. Service details, inclusions and prices can change — confirm them with MedEx before booking.
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