Do you know the two-finger rule for finding ripe watermelon? This is an easy way to determine if you should buy the giant fruit you’re eyeing at the grocery store. The two-finger rule is very simple.

Jab a finger, draw up a spot of blood with a test strip, let it dry, then mail it off to your doctor. Could testing for Alzheimer’s disease become that simple? Quite possibly. Modern immunoassays are ...