A pregnant woman with clinical features suggestive of secondary syphilis has a non-reactive undiluted VDRL. When the serum is diluted further, the VDRL becomes reactive with a high titre. What is this phenomenon called?

• A Biological false positive reaction
• B Serofast state
• C Seroreversion
• D Prozone phenomenon ✓

Explanation

In the prozone phenomenon, very high antibody titres interfere with the flocculation reaction of non-treponemal tests such as VDRL and RPR, giving a falsely non-reactive result in undiluted serum. Dilution of the serum restores reactivity. Biological false positivity refers to a reactive non-treponemal test with negative treponemal tests in conditions like SLE and pregnancy. Seroreversion is loss of reactivity after successful treatment, and serofast state is persistence of low titres after adequate therapy.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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