Zebra

Foreman: You learn it in first year medical school; when you hear hoofbeats, think horses not zebras.

House: Are you in first year medical school?

House, introducing his new hire Dr. Foreman, to the realities of the practice of diagnostic medicine in Pilot.

In medical slang, a zebra is a surprising, unexpected diagnosis. In many cases an uncommon or rare disease which has identical symptoms to a common disease qualifies as a zebra. As such, most medical students have to learn, like Foreman did, to assume during a differential diagnosis that the patient's symptoms are caused by a common ailment rather than a rare one. However, as House usually only handles patients where common diseases have been ruled out, he often has to re-educate his fellows to not only consider rare diagnoses as part of the differential, but to prefer them. He also frequently reminds his fellows that they often have to ignore missing symptoms as most symptoms are not found in every patient who has a disease. For example, an immunosuppressed person with an infection will not show a fever.

A large number of House's ultimate diagnoses are zebras. More than one critic has noticed that Princeton-Plainsboro Teaching Hospital has seen more rare diseases in House's four seasons than most hospitals do in their entire operational history.

Some diagnoses that are zebras in one case are not necessarily zebras in every case. For example, leprosy was a zebra in Cursed because the patient had never been to Asia. However, it was not so much of a stretch in Dying Changes Everything, because the patient had an extensive history of worldwide travel.

Some diseases are ultimate zebras - their symptoms are so similar to more common ailments, even though these diseases themselves are very rare, that it is unlikely even a good doctor will consider the diagnosis. For example, Wegener's disease has symtoms that mimic many common ailments. It's only distinguishing feature, granulomas, are also common in many diseases. However, granulomas together with the other symptoms leads to a definitive diagnosis. Lupus is another zebra that often comes up in a differential, but has symptoms that are not unique to the disease and is not easy to test for definitively. A patient showing symptoms of Lupus may have the disease, but is far more likely to have a more common ailment.