Question
Standard tests for syphilis
Answer
Standard tests for syphilis (caused by Treponema pallidum):
1. Non-treponemal (screening) tests: detect non-specific antibody (reagin) against a cardiolipin-lecithin-cholesterol antigen — VDRL (Venereal Disease Research Laboratory) test and RPR (Rapid Plasma Reagin) test. Inexpensive, useful for screening and monitoring treatment response (titres decline with successful treatment), but can give false positives (autoimmune disease, pregnancy, other infections).
2. Treponemal (confirmatory) tests: detect specific antibody against T. pallidum antigens — TPHA (Treponema pallidum Haemagglutination Assay), FTA-ABS (Fluorescent Treponemal Antibody-Absorption test), and TPPA (Treponema pallidum Particle Agglutination assay). More specific, but remain positive lifelong even after treatment, so not useful for monitoring treatment response.
3. Direct demonstration: Dark-field microscopy of chancre exudate (early primary syphilis) — direct visualization of motile spirochetes, since T. pallidum cannot be cultured on artificial media.
Standard testing algorithm: a reactive non-treponemal screening test (VDRL/RPR) is confirmed with a treponemal test (TPHA/FTA-ABS/TPPA); some laboratories use a reverse algorithm starting with an automated treponemal test.