Diagnosis

When a chancre is present, it may be possible to see the bacteria in a sample taken from the sore under a microscope. T. pallidum has a distinctive spiral appearance.

Two kinds of test are normally used to diagnose syphilis. Both tests detect antibodies, but neither is fully reliable in diagnosing the disease. The choice of test, and how they are used, varies internationally.

The first kind of test detects antibodies to lipids, called 'reagin' antibodies. The Venereal Disease Research Laboratory (VDRL) test is used on blood or sometimes on cerebro-spinal fluid (CSF) taken from the spinal column by lumbar puncture. The rapid plasma reagin (RPR) test is used only on blood. These tests may be positive when people have a range of other infections, including malaria, or during to pregnancy. Reagin antibody levels due to syphilis go down when syphilis is treated, so these tests can be used to monitor treatment.

A second approach looks for antibodies to proteins made by the syphilis bacteria. These 'treponemal' tests include T. pallidum haemagglutination assay (TPHA), T. pallidum particle agglutination test (TPPA) and fluorescent treponemal antibody absorption test (FTA-abs), along with a number of enzyme immuno-assay (EIA) tests which are similar to HIV antibody tests. These are very specific, but usually remain positive even after a patient has been cured of syphilis.

In the United Kingdom, VDRL tests are generally used for initial screening followed by TPHA tests to confirm a diagnosis of syphilis. In some countries, TPHA or TPPA tests have been used for initial screening.

It can take up to 90 days for the body to develop antibodies to the bacterium that causes syphilis, so a blood test immediately after exposure to syphilis may not detect infection. Some studies have suggested that these tests are not as effective in people with HIV: some HIV-infected people who do have syphilis may test negative.

Guideines for the management of sexually transmitted infections in HIV-positive people developed by the British Association for Sexual Health and HIV in 2006 have recommened that syphilis tests be incorporated into routine HIV blood tests every three months. In addition, due to the increasing prevalence of syphilis among women, the Health Protection Agency has called for antenatal screening for syphilis to become universal, in order to prevent the re-emergence of congenital syphilis.