Which statement is true regarding serologic tests for diagnosing syphilis?

• A. False-positive tests are more frequent with the fluorescent treponemal antibody absorbance (FTA-ABS) test than with the rapid plasma reagin (RPR) assay.
• B. Inactivated Treponema pallidum serves as the antigen in the Venereal Disease Research Laboratory (VDRL) assay.
• C. The antibody titer in the Treponema pallidum antigen (TP-PA) test will decline if the patient is adequately treated.
• D. The tests are usually positive (high sensitivity) in secondary syphilis.

Answer: (D)

In secondary syphilis, the antibody response is strong, so serologic tests for syphilis are highly sensitive and most patients will have a positive result. This includes both non-treponemal tests (like RPR and VDRL) and treponemal tests (like TP-PA and FTA-ABS), reflecting active antibody production at this stage. Therefore, the statement that these tests are usually positive in secondary syphilis is the true one.

Context helps: non-treponemal tests detect antibodies to cardiolipin and are useful for screening and monitoring treatment because their titers rise with infection and fall after effective therapy. Treponemal tests detect antibodies against the bacterium itself and typically remain positive for life, even after successful treatment, so they’re not used to monitor response.

The other statements aren’t correct: VDRL uses cardiolipin, not Treponema pallidum, so the antigen described is incorrect. Treponemal tests like TP-PA/FTA-ABS usually stay positive for life and do not reliably decline after treatment. And false-positive results are more of a concern with non-treponemal tests than with treponemal tests, so the claim about FTA-ABS having more false positives than RPR isn’t accurate.