Abstract
The high-dose hook effect, in which a very high antigen concentration saturates both capture and detection antibodies in a
two-site immunometric assay and produces a falsely low or even normal result, is a well-described but infrequently encountered
analytical pitfall that can delay diagnosis when the falsely reassuring result does not match the clinical picture. We report a case
in which a patient with advanced ovarian carcinoma had a CA-125 result within the reference range despite a clinical
presentation strongly suggestive of advanced malignancy, delaying oncology referral by several weeks until an alert laboratory
scientist, noting the mismatch, requested sample dilution, which revealed a true CA-125 concentration exceeding the assay's
normal reportable range by more than one-hundred-fold. We describe the clinical and laboratory clues that should prompt
consideration of a hook effect, and recommend conditions under which routine dilution should be considered for clinically
discordant tumour marker results.
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