RPR (rapid plasma reagin) syphilis test: screening, confirmation and monitoring

Syphilis is caused by the bacterium Treponema pallidum. Most tests look for the body's response to it. Lab Tests Online UK explains that there are treponemal tests, which look for antibodies directed specifically at the bacterium, and non-treponemal tests such as the RPR, which look for antibodies produced in response to the infection but not targeted at the bacterium. These non-treponemal antibody levels fall with successful treatment, which makes them particularly useful for monitoring whether treatment is working.

Treponemal tests, by contrast, usually remain positive for life, even after successful treatment, so they cannot alone tell whether an infection is current or past.

You may be tested if you have a sore, ulcer or unusual rash, if a partner has syphilis, if you are being tested for another STI, if you are pregnant (the source says testing is offered to all pregnant women in the UK as part of routine antenatal care), or to monitor treatment. The NHS says that having a test is the only way to confirm syphilis, that tests are free, and that you can be tested at a sexual health clinic, at GP surgeries, in pregnancy, or using a self-test kit sent to a laboratory.

In the UK, laboratories usually follow a two-step process: a screening test first and, if positive, further tests to confirm and identify the stage. A repeat blood sample is usually requested to confirm any new positive result.

No special preparation is needed for a blood test. If you have a sore, a doctor or nurse may swab it for PCR or dark-ground microscopy, and if the nervous system may be involved a lumbar puncture may be arranged. You may also be offered tests for other STIs at the same time.

Antibodies may not appear until several weeks after infection, so a negative result soon after exposure may need repeating. A positive screening result does not always mean syphilis, because false positives occur; the laboratory automatically carries out additional tests and you are usually asked for a repeat sample. Non-treponemal tests can give false positives in some other conditions, including pregnancy, lupus, Lyme disease, some viral infections and in people who use intravenous drugs.

Some people raised in certain tropical regions may have been infected by related infections (yaws, pinta, bejel), which are not sexually transmitted and which syphilis tests cannot distinguish from syphilis. The pattern of positive tests and the RPR level help the specialist work out the stage.

The NHS says syphilis is treated with antibiotics given as injections, tablets or capsules, sometimes started before the test result is known, and for a length of time depending on the stage. Some people get flu-like symptoms within the first 24 hours of treatment. You should avoid sex until two weeks after you and your partner have finished treatment, and current and recent partners need testing and treatment; clinics can contact them without naming you.

You return several times in the first 12 months to be retested. A falling RPR level shows the treatment is working, while a failure to fall or a rise suggests treatment failure or reinfection.

A confirmed result is managed by a sexual health or infection specialist, who also offers screening for HIV and other infections. Complications of untreated syphilis include heart problems, brain and nerve problems; seek prompt advice for neurological symptoms such as vision change, confusion or weakness, and call 999 for sudden severe symptoms.