The term diagnosis of exclusion (per exclusionem) refers to a medical condition whose presence cannot be established with complete confidence from examination or testing. Diagnosis is therefore by elimination of other reasonable possibilities.

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.

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

• Idiopathic