In tertiary neurosyphilis, the Treponema pallidum Particle Agglutination (TPPA) test is positive but VDRL is negative. This serological pattern is known as:

• A Serofast reaction — persistently positive TPPA despite successful treatment, with VDRL becoming non-reactive after adequate treatment ✓
• B Prozone phenomenon — excess antibody inhibiting agglutination of VDRL cardiolipin antigen
• C Biological false positive — VDRL reacts to cardiolipin antigen not specific to treponema
• D Re-infection state requiring repeat penicillin treatment even with VDRL negative

Explanation

The TPPA (treponemal test) detects anti-treponemal antibodies that remain positive for life in most treated patients ('serofast treponemal test'). In contrast, the VDRL (non-treponemal test) measures reagin (IgG/IgM against cardiolipin), which falls and eventually becomes non-reactive after adequate treatment of early syphilis. In late/tertiary syphilis after prior successful treatment, the TPPA can remain positive indefinitely while VDRL is non-reactive — this is NOT treatment failure, just the expected serological pattern. The prozone phenomenon occurs in early secondary syphilis when very high antibody titres block agglutination; VDRL becomes positive after serum dilution. For neurosyphilis diagnosis, CSF-VDRL is specific but insensitive; CSF-FTA-ABS is sensitive. Treatment: aqueous crystalline penicillin G IV for 10–14 days.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.