Treponema pallidum

Treponema pallidum Epidemiology Pathogenesis Signs, Symptoms, & Sequallae
Diagnosis Laboratory Testing Treatment & Prevention References

Laboratory Testing

   During primary or secondary syphilis when a chancre is present it's possible to detect the disease using a dark field microscope. 4 The spirochetes are too small and slender to be seen using bright field microscopy. The chancre is peeled off and a scraping of the base of a lesion can be put on a slide to be examined. In this method individuals are looking for the presence of treponemal spirochetes. The organisms have to be alive and moving to be valid, so prepared slides should be viewed as soon as possible. If the lesion that is scraped is from the oral cavity there is a good chance that nonpathogenic spirochetes could be present and visualized, so follow up testing should be performed. If no spirochetes are present, syphilis can not be ruled out but follow up testing should be performed if there is reason to believe they are infected. 4  


    If no chancre is present then syphilis can be tested using serological tests.2 The two serology test methods are broken down

 into treponemal anitbody tests and nontreponemal antibody tests. The preferred specimen to be used is serum collected by

 venipuncture in a red top tube that has no anticoagulants. Both of these types of serological tests are fairly sensitive in the

 primary stage and then become extremely sensitive when patients are in the secondary stage. 1

Nontreponemal Serologic Tests

        The nontreponemal tests are screening tests used to detect nonspecific antibodies to cardiolipin and other lipids that are formed and released when tissue is being destroyed.2 A specific example of a nonspecific antibody is known as regain. 4 Common assays used here are the Venereal Disease Research Laboratories Test (VDRL) and the Rapid plasma regain test (RPR). Both test methods are examples of a flocculation assay, in which Ab-Ag complexes are formed and the positive result can be viewed as clumping. The VDRL test can be performed using a patient’s serum or their cerebral spinal fluid. 1 The results are best read microscopically. The RPR assay, which uses a patient’s serum, is commonly preferred because flocculation can be seen with the naked eye. A treponemal test with a diluted sample should be performed if any degree of reactivity is viewed.

Treponemal Serologic Tests

        Treponemal antibody tests are used as confirmatory tests. 4 They are testing for specific antibodies against the Treponema pallidum subsp. pallidum organism. The most common assay performed is the Fluorescent Treponemal Antibody Absorption test (FTA-ABS). In this test the patientPerforming an assay sample is mixed with a sorbent so that the nonpathogenic treponemal strains are extracted out. Once the antibodies are specific for T. pallidum subsp. pallidum, the patient sample is added to the antigen fixed on the plate wells. The Ag embedded on the plate is the Nichols strand of T. pallidum subsp. pallidum. If the specific antibodies are present they will bind to the antigen. A second antibody, the conjugate, is added that has a fluorescein isothiocyanate label on it. This conjugate will bind to the antibodies that have bound to the pathogenic treponemal antigen. When viewed in fluorescence microscope a patient who has syphilis will have a green fluorescence that is apparent on the slide. 4 

Test Name Type of Test Normal Reference Range Infected Patient Results Screening/Confirmatory Follow-Up Testing
Rapid Plasma Reagin Non-Specific Negative Positive Screening Dilutions & FTA-ABS
Venereal Disease Research Laboratory Non-Specific Negative Positive Screening Dilutions & FTA-ABS
Fluorescent Treponemal Antibody Absorption Specific Negative Positive Confirmatory _____