Dr. McDonald's recent essay on medical heuristics is interesting and timely. His thesis that a critical review to clarify, improve, and standardize medical heuristics could reduce practice variation and optimize the care process deserves serious consideration. After all, the reduction of practice variation is an activity that consumes increasing amounts of our time. But where do these heuristics come from? How can they be collected for clarification, improvement, and standardization?

Perhaps a place to begin is with a collection of house officer aphorisms that, on reflection, seem to be one of the ways that heuristics are passed on from one generation of physicians to the next. We may not remember the reference to Occam's razor, but the Zebra Rule ("When you hear hoofbeats, think of horses, not zebras"), which describes one of its corollaries, is familiar to all. In the issue of Annals that contained Dr. McDonald's essay, Dr. Stein [2] quotes another house officer heuristic that addresses the futility of end-of-life cardiopulmonary resuscitation: "If you can't keep them alive when they are alive, you can't keep them alive when they are dead."

Finally, in searching my mind for other examples of medical heuristics, I remembered an anatomy professor at St. Louis University, Dr. Calvin Richins, who wrote on the blackboard in the first hour of our first day of class, "Primum non nocere."

Dr. McDonald references this fundamental heuristic in his reference section [3], and I think most would agree that in these days of efforts to provide care and to reduce practice variations, it expresses an overriding principle to guide our relationships both with patients and with the changing health care system.

William J. Oetgen, MD, MBA

Georgetown University

Washington, DC 20007