Zebra is the American medical slang for arriving at an exotic medical diagnosis when a more commonplace explanation is more likely. It is shorthand for the aphorism coined in the late 1940s by Dr. Theodore Woodward, professor at the University of Maryland School of Medicine, who instructed his medical interns: "When you hear hoofbeats, think of horses not zebras". Since horses are common in Maryland while zebras are relatively rare, logically one could confidently guess that an animal making hoofbeats is probably a horse. By 1960, the aphorism was widely known in medical circles.
As explained by Sotos, medical novices are predisposed to make rare diagnoses because of (a) the availability heuristic ("events more easily remembered are judged more probable") and (b) the phenomenon first enunciated in Rhetorica ad Herennium (circa 85 BC), "the striking and the novel stay longer in the mind." Thus, the aphorism is an important caution against these biases when teaching medical students to weigh medical evidence.
Diagnosticians have noted, however, that "zebra"-type diagnoses must nonetheless be held in mind until the evidence conclusively rules them out:
In making the diagnosis of the cause of illness in an individual case, calculations of probability have no meaning. The pertinent question is whether the disease is present or not. Whether it is rare or common does not change the odds in a single patient. ... If the diagnosis can be made on the basis of specific criteria, then these criteria are either fulfilled or not fulfilled. — A. McGehee Harvey, James Bordley II, Jeremiah Barondess
The term for an obscure and rare diagnosis in medicine is fascinoma.
Necrotic skin lesions in the United States are often diagnosed as loxoscelism (recluse spider bites), even in areas where Loxosceles species are rare or not present. This is a matter of concern because such misdiagnoses can delay correct diagnosis and treatment.
Ehlers–Danlos syndrome and hypermobility syndrome are both medical zebras and yet are commonly under diagnosed and misdiagnosed. The EDS and hypermobility syndrome awareness movements have adopted the zebra as the mascot for their campaigning.
In popular culture
The television series House had the working title "Chasing Zebras, Circling the Drain", a reference to the show's recurring theme of hunting for obscure diagnoses while a patient is in a critical condition. This name was rejected in favor of "House", which references the title character, Dr. Gregory House. House himself rejects the aphorism, claiming that any cases with simple solutions would have been successfully diagnosed by someone else before reaching him.
In the television series, ER, Dr. Weaver tells Dr. Chen to "start hunting for zebras" in the episode "Loose Ends."
Harvard Medical School and Harvard School of Dental Medicine's 2017 music video titled, "IT'S NOT A ZEBRA!" depicts a group of first-year medical students erroneously trying to diagnose a patient with a rare and unlikely disease, when in fact she has the flu.
Other medical aphorisms
- Sutton's law – perform first the diagnostic test expected to be most useful
- Occam's razor – select from among competing hypotheses the one that makes the fewest new assumptions
- Leonard's Law of Physical Findings – it's obvious or it's not there
- Hickam's dictum – "Patients can have as many diseases as they damn well please"
- Sotos (2006) page 1.
- Sotos (2006) page 1.
- Imperato (1979) pages 13, 18.
- Sotos (2006) page 7.
- Harvey (1979) page 15.
- Vetter, Richard S. (2008). "Spiders of the genus Loxosceles (Araneae, Sicariidae): a review of biological, medical and psychological aspects regarding envenomations" (PDF). Journal of Arachnology. 36: 150–163. doi:10.1636/rst08-06.1.
- Why the Zebra|url=http://www.ehlers-danlos.org/about-eds-uk/why-the-zebra
- "House... and Holmes". Radio Times. BBC Magazines Ltd. January 2006. p. 57. Archived from the original on January 18, 2010.
- Sotos (2006) page 15.
- Harvey, A. M.; et al. (1979). Differential Diagnosis (3rd ed.). Philadelphia: W.B. Saunders.
- Imperato, Pascal James (1979). Medical Detective. New York: Richard Marek. ISBN 0-399-90058-6.
- Sotos, John G. (2006) . Zebra Cards: An Aid to Obscure Diagnoses. Mt. Vernon, VA: Mt. Vernon Book Systems. ISBN 978-0-9818193-0-3.