A patient with florid secondary syphilis has a non-reactive undiluted VDRL, yet the TPHA is strongly positive. Repeating the VDRL on serial dilutions of the same serum gives a reactive result at 1:16. What phenomenon explains the initial non-reactivity?
- A Biological false positive reaction
- B Seroconversion lag before antibody appearance
- C Prozone phenomenon due to antibody excess ✓
- D Antigen excess in the test system
Correct answer: C. Prozone phenomenon due to antibody excess
Explanation
In the prozone phenomenon, extremely high titres of anticardiolipin antibody interfere with the flocculation reaction, producing a falsely non-reactive result in undiluted serum. Dilution of the serum restores the optimal antigen-antibody ratio and the test becomes reactive. It is classically described in secondary syphilis. Option B applies to primary syphilis before seroconversion, and option D describes postzone, not prozone.
Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.
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