Clinical meaning
Treponema pallidum evades immune surveillance through antigenic variation of its outer membrane proteins and minimal surface antigen expression. The humoral immune response produces both non-treponemal antibodies (anti-cardiolipin, detected by RPR/VDRL) and treponemal-specific antibodies (detected by FTA-ABS, TP-PA). Non-treponemal titers correlate with disease activity and decline with treatment, while treponemal antibodies persist lifelong. The traditional screening algorithm uses non-treponemal tests first (RPR/VDRL), confirmed by treponemal tests. The reverse algorithm, increasingly adopted, screens with treponemal immunoassay (EIA/CIA) first. Discordant results (positive treponemal, negative non-treponemal) require TP-PA confirmation and may represent very early infection, late latent syphilis, or previously treated disease. The clinician must interpret complex serologic patterns, manage stage-appropriate treatment, evaluate for neurosyphilis, and manage syphilis in special populations including HIV co-infection and pregnancy.