Which test is treponemal-specific for syphilis?

• A. VDRL
• B. RPR
• C. TP-PA
• D. VDRL with fluorescent antibodies

Answer: (C)

The test that is specific for antibodies against the organism Treponema pallidum is the TP-PA. This treponemal test uses purified Treponema pallidum antigens to detect antibodies directed against the actual pathogen, so a positive result indicates exposure to or infection with syphilis. Treponemal antibodies tend to remain detectable for life, even after treatment, which is why this type of test is ideal for confirming infection after an initial screening.

In contrast, the other commonly used tests are non-treponemal and detect antibodies to cardiolipin rather than to the bacteria itself. Those reagin-type tests (used for screening and monitoring therapy) can yield false positives in other conditions and their titers change with disease activity, which is why they’re not considered treponemal-specific. The option that blends a non-treponemal framework with fluorescent labeling doesn’t change this fundamental specificity.