"When you hear hoofbeats behind you, don't expect to see a zebra," said Theodore Woodward, Professor at the University of Maryland School of Medicine to his medical residents and students.  Since zebras are much less common than horses in North America, he was telling his medical residents and students they should not look for an unusual medical diagnosis when a commonplace explanation was more likely.  Zebras represented a medical possibility that was deemed unlikely.  The expression became widely embraced in medical circles and is still a deeply held tenant by some health providers.   It broadly refers to a rare disease, which includes the estimated 171,000 neuroendocrine cancer patients in the United States.

 

The problem, of course, is that concluding something is unlikely to occur doesn't mean that it will never occur.  This is a base rate fallacy that occurs when people rely too heavily on prior information, instead of focusing on the current situation.  For providers who are seeing their very first neuroendocrine cancer patient,  this can lead to incorrect judgments and decisions unless they look more closely at symptoms. When an illness is correctly diagnosed for an individual, calculations of probability have no meaning. The pertinent question is whether the disease is present or not.

 

Whether any disease is rare or common does not change the odds in a single patient.   When you hear our hoofbeats, think zebras!

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