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Laboratory Diagnosis

The different strains of treponemes produce similar histological changes and have a high degree of cross-immunity. There is complete cross-protection between the strains of T. pallidum which cause venereal and nonvenereal syphilis, and also between the strains of T. pertenue. There is some cross-immunity between venereal syphilis and yaws. The best and most rapid method to establish the diagnosis is by examining the exudates from early lesions, because they usually contain many treponemes. Dark-field microscopy is required, and the rapid rotation of the spirochetes will be seen.

There are two groups of diagnostic tests. The tests involving animal tissue antigens are highly specific for the treponematoses. These are all precipitation, flocculation, or complement fixation methods. There is the VDRL (Venereal Disease Research Laboratory) flocculation test, the Kolmer complement fixation test, and the RPR (Rapid Plasma Reagin) agglutination test.

Biologically false-positive reactions can occasionally occur in normal individuals and also after smallpox vaccination, leprosy, tuberculosis, malaria, and kala-azar.

The second group of tests depends on the ability of treponema antibodies to immobilize T. pallidum (the TPI or Treponema pallidum immobilization test). It is necessary to estimate the number of spirochetes immobilized over a 48-hour period by the test serum. This test will fail to distinguish between yaws and syphilis, but does exclude the false-positives in the other group of tests. It is possible to perform a similar test by complement fixation (the TPCF or Treponema pallidum complement fixation test) and there are also a fluorescent treponemal antibody test (FTA) and a hemagglutination assay (TPHA).

Whichever tests are employed, it is often necessary to utilize two or three alternatives to establish the correct diagnosis in areas where the treponematoses overlap. Skill and experience are required to provide reliable results. Some believe that no test can separate the various organisms!

Serological tests become active 3-4 weeks after the appearance of the primary lesion. They remain strongly positive in the secondary stage and, usually, in the third stage. If the patient is cured, the serological tests return to normal. Reinfection is possible, but it may be modified by residual immunity.

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