Close to 4 million babies are born in the United States every year, and within their first 48 hours nearly all are pricked in the heel so their blood can be tested for dozens of life-threatening genetic and metabolic problems. The heel-stick test is considered such a crucial public health measure that states typically require it and parents aren’t asked for their permission before it’s done. But the lab tests for newborn screenings generally don’t use all of the half-dozen or so drops of blood collected on filter paper cards. So states hold on to the leftover “dried blood spots,” as they’re ca…