Syphilis Screening Tests

Rapid Plasma Reagin (RPR) is a blood-screening test which detects antibodies that are present if you have syphilis. Normally a negative (non-reactive) RPR test result means you don't have syphilis. However, the RPR test is most sensitive (almost 100 per cent accurate) during the middle stages of the disease. In the early or late stages of syphilis, RPR blood-screening tests have often produced false negative results. This is usually because your body does not always produce antibodies in response to syphilis. There have also been documented cases of syphilis where there was so much antibody present in a patient's blood that the RPR test was non-reactive. When the patient's blood was diluted, however, the RPR testing results were positve.

Because RPR testing is highly reactive, the test also occasionally produces false positive results. Various conditions can sometimes be the cause of a false positive test, such as:

  • HIV
  • lyme disease
  • certain types of pneumonia
  • malaria
  • systemic lupus erythematosus
  • infectious mononucleosis
  • antiphospholipid antibody syndrome
  • hepatitis A
  • leprosy
  • (occasionally) pregnancy

If your RPR test result is positive, a syphiiis diagnosis can be confirmed using a more-refined test. Fluorescent Treponemal Antibody – Absorbed (FTA-ABS) is one type of blood-screening test that shows whether or not antibodies that are specific only to the syphilis organism are present. FTA-ABS tests (like RPR) can occasionally produce false negative results in the early or late stages of syphilis. The FTA-ABS test is more expensive and time-consuming then tests (such as RPR) which detect more general-type antibodies.

A positive FTA-ABS test result (for the most part) indicates that the patient has (at some point) been infected with syphilis. However, patients who have had syphilis will always test positive using FTA-ABS — even when the disease has been successfully treated. For this reason, FTA-ABS tests cannot be used to measure the effectiveness of treatment for syphilis.

Once a positive syphilis test is confirmed, your blood is then diluted in half and tested again using RPR. This is done to determine how advanced the syphilis infection is. If the RPR test result is still positive (reactive), the blood is then diluted to one quarter and retested. The rate that the blood is diluted is consequently increased (1:8, 1:16, 1:32 and so on), until the RPR tests are no longer reactive. The more the blood is diluted and still tests positive (called "titres."), the more advanced the stage of the disease.

RPR testing is also used to measure how effective a patient's treatment for syphilis has been. Following treatment with antibiotics, the levels of syphilis antibodies should fall. These levels are monitored using titres (the dilute-and-test process). If the treatment is working, the amount the patient's blood needs to be diluted in order for the test to be non-reactive should gradually decrease. Eventually, the patient's RPR test results should be negative without diluting the blood at all. If the titre (the ratio the blood is diluted and still tests positive) does not drop (or increases), then there the syphilis infection is persistent. There have been rare cases documented where RPR titres indicated that the treatment of syphilis was successful, but the patient was later found to have advanced late-stage syphilis disease.

Another form of antibody test used to detect syphilis is the Venereal Disease Research Laboratory (VDRL) test. In most places, the use of this earlier type of syphilis test (first developed in 1906) has now been replaced with the more-sensitive RPR test.

created: Sep. 19, 2010
last modified: Sep. 19, 2010
S.O.S. S.O.S.
Box 3075, Main Post Office
Vancouver, BC V6B 3X6 CA