Which of the following is an example of a treponemal antigen test used for the diagnosis of syphilis?

• A. CRP.
• B. RPR.
• C. VDRL.
• D. TP-PA.

Answer: (D)

Treponemal tests detect antibodies directed against Treponema pallidum, the bacterium that causes syphilis, and are used to confirm infection after a positive screening test. The TP-PA uses Treponema pallidum antigens to cause agglutination if the patient has antibodies to the organism, making it a specific test for diagnosing syphilis. Unlike non-treponemal tests, treponemal tests tend to stay positive for life even after successful treatment, which is why they’re used to confirm prior infection rather than to gauge current activity. The other options—RPR and VDRL—are non-treponemal tests that detect reagin antibodies against cardiolipin and are used mainly for screening and monitoring response to therapy, not as confirmatory treponemal proof. CRP is an inflammatory marker with no specific relevance to syphilis. Therefore, TP-PA is the example of a treponemal antigen test used for diagnosis.