idk, the rainbow family (lsd distribution network) requiring inductees to take a thumbprint to test that they aren't cops and see if they can handle the inevitable accidental dosing that can occur when laying blotter makes a lot of sense to me.

I wouldn't assert that it's 'fact' but seems very plausible. (A full 'thumbprint' is probably uncommon, but ingesting a visible amount of crystal from a fingertip seems believable)