In medicine we love us a good diagnostic test; we're always looking for the next one. It is strange, then, that we should be so aloof to the basics of diagnostic testing. This month's audio is a primer on testing—and it changes everything. We're going back to basics: if you learn the four axioms of diagnostic testing you'll know more about how to choose and how to interpret diagnostic tests than just about everyone. Weird thing is that it turns out you knew it all already... you just needed a reminder.