The Serology Test for Syphilis

Blood serum, the clear-liquid component of the blood, is tested to determine whether a person has been exposed to syphilis. Serological tests for syphilis are used for diagnosis, for evaluating the effectiveness of treatment and for screening members of the general population for their possible exposure to syphilis. These tests primarily detect antibodies that are formed in the blood during the course of syphilitic infection. Several types of blood tests for syphilis are used in the United States. Some are used in follow-up monitoring of patients as well as initial diagnosis.

Sampling Procedure

• Routine procedures are used for blood sampling.

  Nothing is unusual or specific about the blood-sampling procedure for serological testing for syphilis. Usually about 5 to 10 mL of blood are extracted into a glass tube and then sent to a lab equipped for the special tests. This sampling can be performed in almost any major population center in the United States or Canada. No special preparation by the patient is required.

Screening

• A relatively simple class of tests, called Nontreponemal Antigen Tests, is generally used for screening. A sample of the blood is mixed with special substances. If the mixture forms clumps, the test is positive. A doctor's interpretation and further testing are often required, because false-negative and false-positive results are possible. False-positive results can be caused by other infectious diseases, including mononucleosis, leprosy, rheumatoid arthritis, lupus and HIV. False-negatives can occur when patients are tested too soon after exposure to syphilis.

Follow-Up

• A more complex variety of tests called Treponemal Antibody Tests is used to rule out false-positive results tests resulting from the screening tests. They measure the presence of only those antibodies that are specific to syphilis. The patient's blood serum is mixed with a preparation that prevents interference from antibodies caused by infections other than syphilis. Then, the test serum is put on a slide containing a special substance. If the serum is from a syphilitic person, syphilitic antibodies appear on the slide, which are then detected by ultraviolet light.

Benefits of Testing

• A 2009 task force of the U.S. Agency for Healthcare Research and Quality found that screening tests can accurately detect syphilis infections that can be subsequently treated with antibiotics. Screening may also result in potential harms, such as false-positive results, unnecessary anxiety to the patient and issues associated with antibiotic use, but on the whole, the benefits of screening outweigh the disadvantages. Evidence also showed that the universal screening of pregnant women decreases the proportion of infants with syphilis infection.

Results of Testing

• In an article in the journal Sexually Transmitted Diseases, Allyn Nakashima and others report that historically, syphilis was distributed widely throughout the U.S. but declined rapidly after the introduction of penicillin therapy and broad-based public health programs, attaining its lowest levels in the 1950s. However, in the late 20th and early 21st centuries, the disease has returned and become focused in the southern region and in urban areas outside that region.