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; inexpensive, useful for screening and monitoring treatment response (titres decline with successful treatment).
- VDRL (Venereal Disease Research Laboratory) test.
- RPR (Rapid Plasma Reagin) test. Limitation: can give false positives (e.g., in autoimmune disease, pregnancy, other infections), requiring confirmation.
2. Treponemal (confirmatory) tests: detect specific antibody against T. pallidum antigens; more specific, but generally remain positive for life even after successful treatment (so not useful for monitoring treatment response).
- TPHA (Treponema pallidum Haemagglutination Assay).
- FTA-ABS (Fluorescent Treponemal Antibody-Absorption test) — historically considered the gold-standard confirmatory test.
- TPPA (Treponema pallidum Particle Agglutination assay).
Direct demonstration methods (in early primary syphilis, from chancre exudate): Dark-field microscopy — direct visualization of motile spirochetes.
Standard testing algorithm: a reactive non-treponemal screening test (VDRL/RPR) is confirmed with a treponemal test (TPHA/FTA-ABS/TPPA) to establish a definitive diagnosis; some laboratories now use a “reverse algorithm” starting with a treponemal test (e.g., automated EIA/CLIA) followed by a non-treponemal test for titre monitoring.