SEROLOGIC TESTING FOR SYPHILIS
Both nonspecific nontreponemal reaginic antibodies and specific antitreponemal antibodies are measured in the serologic testing for syphilis, and they are generally measured in tandem to confirm the diagnosis. rxnorth.com
Nontreponemal Reaginic Tests
The nontreponemal antibody test is a rapid and inexpensive test used for screening large numbers of sera for syphilis and to monitor disease activity and response to treatment. Patients infected with T. pallidum generate immunoglobulin M and immunoglobulin G antibodies that cross-react with cardiolipin antigens that are used for testing. The standard nontreponemal test is the Venereal Disease Research Laboratory (VDRL) slide test, which utilizes a cardiolipin-cholesterol-lecithin antigen. A modified test used by most laboratories and blood banks is the rapid plasma reagin (RPR) card test.
Levels of nontreponemal antibody vary during the course of the disease, with the highest titer being in the secondary and early latent periods. In cases of primary syphilis, antibodies will not be present until 1 to 4 weeks after the chancre has formed. In cases of later disease, antibody titers generally decline to less than 1:4, with about one quarter of untreated cases eventually developing negative titers. It is important to remember that a prozone phenomenon may occur in 1% to 2% of patients with secondary syphilis, in which the spirochetal antigen load is the highest, and during pregnancy. Therefore, appropriate dilutions should be done to rule out the diagnosis when there is a high clinical suspicion.
Because of the non-specificity of the test, false-positive reactions are common, especially in conditions of immune upregulation, such as in cases of chronic infectious diseases (such as HIV), autoimmune disease, and intravenous drug use.
Specific Treponemal Tests
The specific treponemal antibody tests are the fluorescent treponemal antibody, absorbed (FTA-ABS) test, and the T. pallidum hemagglutination assays (TPHA and MHA-TP). They are used primarily to verify a positive nontreponemal antibody test finding. The FTA-ABS is an indirect immunofluorescent antibody assay that tests the patient's serum against T. pallidum cultivated in rabbit testes. Once a patient has a positive treponemal antibody test finding, it usually remains positive for life, despite treatment, although reversion to a negative status may occur in some patients, especially if treated early. Specific treponemal antibody tests are highly sensitive and specific, although false-positive results may occur in about 1% of the population in association with certain conditions, such as systemic lupus erythematosus.