How many people carry enough biotin to skew a test depends where
In a US emergency department, 7.4% of samples held biotin at the lowest level known to shift a test. In another US cohort it was 1%. In 524 UK samples, none did.
What the study found
Three studies counted the same thing and got 7.4%, 1% and zero
In a US emergency department, 7.4% of plasma samples held biotin at or above 10 nanograms per millilitre, the lowest level at which any affected blood test is documented to shift. In a different US cohort it was 1%. In 524 routine UK samples, not one reached it.
The caveat · High biotin is not a failed test
Being above the threshold is not the same as a test actually failing, and none of the three reports a wrong result or a harmed patient. All three read exposure off leftover samples or medical records rather than asking people directly, which undercounts a vitamin that is often an unadvertised ingredient in a beauty product. The UK samples were anonymised, with no record of who those patients were or what they were taking.
What we don't know
There is no single rate of people carrying enough biotin to skew a blood test. Three studies using one threshold disagreed widely, and the gap tracks who was sampled and where, so a clean result may only mean few there take supplements.