A patient with syphilis serology shows a 'prozone phenomenon' on VDRL testing. This occurs due to:
- A Insufficient antigen in the VDRL reagent preparation
- B Interference from concurrent HIV co-infection
- C Excess antibody in undiluted serum blocking antigen-antibody lattice formation ✓
- D Contamination of serum with the patient's own RBCs
Explanation
The prozone phenomenon occurs when antibody is present in great excess relative to antigen, resulting in all antigen binding sites being occupied by individual antibody molecules, preventing cross-linking and the visible flocculation required for a positive VDRL. It is most common in secondary syphilis (peak antibody titres) and pregnancy. Diagnosis requires serial dilution of the serum, at which point the titre falls into the zone of equivalence producing a visible reaction. The prozone effect is a general serological principle, not specific to HIV co-infection.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.