DIAGNOSIS

Health care practitioners suspect primary syphilis if people have a typical chancre. They suspect secondary syphilis if people have a typical rash on the palms and soles. Laboratory tests are needed to confirm the diagnosis. Two types of blood tests are used:

A screening test, such as the Venereal Disease Research Laboratory (VDRL) or the rapid plasma reagin (RPR) test, is done first. Screening tests are inexpensive and easy to do. But they may need to be repeated because for 3 to 6 weeks after the initial infection, results can be negative even though syphilis is present. Such results are called false-negative. Screening test results are sometimes positive when syphilis is not present (false-positive) because another disorder is present.

A confirmatory test must usually be done to confirm a positive screening test. This blood test measures antibodies specific to the bacteria that cause syphilis, Treponema pallidum. Results of confirmatory tests may also be false-negative during the first few weeks after initial infections and thus may need to repeated.

NB: Screening test results may become negative after successful treatment, but the confirmatory test results stay positive indefinitely.

In the primary or secondary stages, syphilis may also be diagnosed using darkfield microscopy. A sample of fluid is taken from a skin or mouth sore and examined using a specially equipped light microscope. The bacteria appear bright against a dark background, making them easier to identify.

In the latent stage, antibody tests of blood and spinal fluid are used to diagnose syphilis.

In the tertiary stage, the diagnosis is based on symptoms and antibody test results. Depending on which symptoms are present, other tests are done. For example, a chest x-ray may be taken or another imaging test may be done to check for an aneurysm in the aorta. If neurosyphilis is suspected, a spinal tap (lumbar puncture) is needed to obtain spinal fluid, which is tested for antibodies to the bacteria