Question

Consider the following serological tests:
1. Treponema Pallidum Immobilization Test (TPI)
2. T. Pallidum Haemagglutination Assay (TPHA)
3. Fluorescent Treponemal Antibody Absorbed Test (FTA-ABS)
4. Venereal Disease Research Laboratory Test (VDRL)
Which of these are specific for diagnosing syphilis?

• A. 1 only
• B. 2 and 4
• C. 3 and 4
• D. 1, 2 and 3

Hint:

VDRL is a screening test, not a treponemal-specific one.

Answer 1

The Correct Option is D

Syphilis serology is split into two groups of tests: non treponemal (screening) tests and treponemal (confirmatory, specific) tests. This question asks which of the four listed tests are specific for the organism itself.

1. TPI, Treponema Pallidum Immobilization Test: This is a treponemal test, it uses live Treponema pallidum and checks whether the patient's antibodies immobilize the organism. It is specific for syphilis.
2. TPHA, T. Pallidum Haemagglutination Assay: Also a treponemal test, detecting antibodies specific to Treponema pallidum antigens using a hemagglutination method. It is specific.
3. FTA-ABS, Fluorescent Treponemal Antibody Absorbed Test: Another treponemal test, and in fact one of the most sensitive and specific tests, often used to confirm a reactive screening test.
4. VDRL, Venereal Disease Research Laboratory Test: This is a non treponemal test. It detects antibodies against cardiolipin, a lipid released from damaged host cells and from the treponeme, not the organism itself. It is useful for screening and following treatment response, but it is not specific and can give false positives in conditions like pregnancy, autoimmune disease and some infections.

So TPI, TPHA and FTA-ABS are the specific treponemal tests, while VDRL is a non specific screening test.

Let's summarize:

• Treponemal tests (TPI, TPHA, FTA-ABS) directly detect antibodies against Treponema pallidum antigens and are specific.
• VDRL is a non treponemal, cardiolipin based screening test with possible false positives.

So the tests specific for diagnosing syphilis are TPI, TPHA and FTA-ABS.