In medical slang, a zebra is a surprising, unexpected diagnosis derived from the phrase "if you hear hoofbeats, think horses and not zebras." In many cases, an uncommon or rare disease which has identical symptoms to a common disease qualifies as a zebra. As such, during differential diagnoses most medical students (such as Foreman) have learnt to assume 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 seven 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, granulomatosis with polyangiitis has symptoms that mimic many common ailments. Its 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.
Here on the wiki, for many episodes, we try to give each final diagnosis a "zebra factor", based on how likely it would be that a patient with that disorder would actually walk into (or be dragged into) Princeton-Plainsboro Teaching Hospital in Princeton, New Jersey. A "1" denotes a fairly common disease, and usually the delayed diagnosis is the result of the patient lying about their medical history. A "10" (such as Chimerism) is a disease no doctor is likely to see a case of in their lifetime. Some of the "tens" we have seen (both as a final diagnosis and during the course of treatment) are:
- CIPA (Fewer than 400 recorded cases)
- Giovannini Mirror Syndrome (1 previous recorded case)
- Erdheim-Chester disease (fewer than 200 recorded cases)
- Plague (Fewer than 100 cases in the U.S., mostly in the Southwest)
- Rabies (Fewer than a dozen cases a year in the U.S.)
- Leprosy (Fewer than 100 cases a year in the U.S.)
- Medical definition of "zebra" at: ZebraCards.com.