Diagnosis of exclusion

A diagnosis of exclusion (per exclusionem) is a diagnosis of a medical condition reached by a process of elimination, which may be necessary if presence cannot be established with complete confidence from examination or testing. Such elimination of other reasonable possibilities is a major component in performing a differential diagnosis.

Perhaps the largest category of diagnosis by exclusion is seen among psychiatric disorders where the presence of physical or organic disease must be excluded as a pre-requisite for making a functional diagnosis. Diagnosis by exclusion tends to occur where scientific knowledge is scarce, specifically where the means to verify a diagnosis by an objective method is absent. As a specific diagnosis cannot be confirmed, a fall back position is to exclude that group of known causes that may cause a similar clinical presentation.

Examples
An example of such a diagnosis is "fever of unknown origin": to explain the cause of elevated temperature the most common causes of unexplained fever (infection, neoplasm, or collagen vascular disease) must be ruled out.

Other examples include:
 * Behcet's Disease
 * Bell's Palsy
 * schizophrenia
 * Tolosa-Hunt syndrome
 * Irritable bowel syndrome
 * New daily persistent headache
 * Sudden infant death syndrome
 * Burning mouth syndrome.