A patient with secondary syphilis has a negative VDRL test despite high clinical suspicion. The laboratory reports that the test was performed without dilution. What is the most likely explanation?

• A Window period
• B Prozone phenomenon ✓
• C Biological false positive
• D Laboratory error

Explanation

The prozone phenomenon occurs when very high antibody titers, common in secondary syphilis, prevent visible antigen-antibody complex formation in undiluted serum, causing a false-negative result. The test must be repeated with serial dilutions. The window period refers to the time before antibodies develop, which would not cause a prozone effect. Biological false positives occur with conditions like SLE but cause false-positive, not false-negative, results.

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

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