My mother-in-law’s arms look like she’s been in a fight. The bruises don’t hurt, but they’re embarrassing. They’re likely due to the drug Plavix, a trade-off for preventing clots. But we don’t know if the drug is actually helping, because she started it before the FDA urged physicians to use a pharmacogenetic (PGx) test to distinguish patients likely to respond to the drug from “poor metabolizers,” who won’t. And no one’s thought to test her since.