A diagnosis of exclusion is a diagnosis that in most cases can only be confirmed by ruling out every other possible disease that exhibits the same symptoms. In many cases, these disease cannot even be differentiated by the effect of treatment, as several different diseases will also respond to the same treatment and will still have to be eliminated. A diagnosis of exclusion is, by definition, a zebra.
Some of the diseases that fall within this category are:
- Behcet's disease, which shares symptoms with several other autoimmune conditions
- Sarcoidosis, which is why it is such a common differential on the show
- Irritable bowel syndrome, which is essentially a non-diagnosis as it merely describes a set of symptoms for which every other cause has been eliminated.