Hormone Assay Interference

Almost all routine hormone measurement is immunoassay, which measures antibody binding rather than the molecule itself. Interference is anything that changes that binding without changing the hormone concentration, and it comes in three families: substances that disrupt the detection chemistry, biotin in streptavidin-biotin systems being the standard example; antibodies already in the sample, such as heterophile and human anti-mouse antibodies, which bridge or block the capture and detection reagents; and molecules similar enough to the analyte to be counted as it, such as cross-reacting steroids or macroprolactin.

The FDA issued a safety communication on biotin interference in 2017 and updated it in 2019 after a reported death following a falsely low troponin. High biotin intake lowers sandwich-assay results such as TSH and raises competitive-assay results such as free T4, imitating Graves disease in someone taking a hair and nail supplement. The hook effect works differently: at extremely high analyte concentrations, as in a macroprolactinoma, capture and detection antibodies saturate separately and the signal falls, so a hugely elevated prolactin can report as normal.

The decision this changes is whether to act on an implausible number. A result that does not fit the patient should be challenged: repeat on a different platform, dilute serially and check linearity, precipitate with polyethylene glycol, or use mass spectrometry for steroids.

The commonest failure grants a result more precision than its method has. Direct free-testosterone analog assays and estradiol immunoassays at male concentrations get quoted to a decimal place in online bloodwork discussions, when the method's uncertainty is wider than the difference being argued about. The second is assuming biotin only matters at prescription doses; ordinary supplements supply several milligrams a day.