The Ornithopter

Biotin (AKA vitamin B7 and Vitamin H) is often included in supplements marketed for thinning hair and brittle nails. But it can interfere with various medical tests, causing inaccurate low, normal, or high results. It is recommended to stop taking the supplements 3 to 7 days before any such test.

Biotin (Wikipedia)

Biotin and the risk of false lab test results

Biotin: Interference with Laboratory Assays