Laboratory diagnosis of Syphilis

Syphilis is a sexually transmitted disease caused by the bacterium Treponema pallidum. Syphilis has diverse clinical manifestations and shares many clinical features with other treponemal and nontreponemal diseases (because of this,syphilis has been called “The Great Pretender”) . The etiological agent, Treponema pallidum, cannot be cultured, and there is no single optimal alternative test.

Commonly used laboratory diagnosis testing available for syphilis are:

Darkfield micrograph of Treponema pallidum
(source: CDC)
  1. Direct diagnosis:

Direct laboratory methods used for the laboratory diagnosis of syphilis includes the detection of Treponema pallidum by microscopic examination of fluid or smears from lesions, histological examination of tissues or nucleic acid amplification methods such as polymerase chain reaction (PCR).
Tests used for the direct detection of Treponema pallidum are;

  1. Dark-field microscopy
  2. Direct fluorescent antibody test for Treponema pallidum
  3. Nucleic acid amplification (PCR based) methods

 Indirect diagnosis/serological methods:

It is based on serological tests for the detection of antibodies. Serological testing is the mainstay in the laboratory diagnosis and follow-up of syphilis. Serological tests fall into two categories: nontreponemal tests for screening, and treponemal tests for confirmation.

a)      Nontreponemal tests: They measure both immunoglobulin (IgG and IgM) antiphospholipid antibodies formed by the host in response to lipoidal material released by damaged host cells early in infection and lipid from the cell surfaces of the treponeme itself.  Commonly used nontreponemal tests are

  1. Rapid plasma reagin (RPR) test
  2. Toluidine red unheated serum test (TRUST)
  3. Venereal Disease Research Laboratory (VDRL) test

b)      All treponemal tests use Treponema pallidum or its components as the antigen.

Treponemal tests are used as confirmatory tests to verify reactivity in non-treponemal tests. Once positive, treponemal tests remain positive throughout life with or without treatment, so these tests can not be used to know response to treatment.

Commonly used treponemal tests are:

Process of laboratory diagnosis of syphilis:  If lesion exudate or tissue is available, direct examination is performed, followed by a nontreponemal serology test. A reactive nontreponemal test is then confirmed by a treponemal test.

A confirmed serological test result is indicative of the presence of treponemal antibodies but does not indicate the stage of disease and, depending on the test, may not differentiate between past and current infection.