A diagnosis of exclusion or by 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 history, examination or testing. Such elimination of other reasonable possibilities is a major component in performing a differential 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.

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 prerequisite for making a functional diagnosis.

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:

• Chronic fatigue syndrome^[1]
• Fibromyalgia^[2]
• Adult-onset Still's disease^[3]
• Behçet's disease^[4]
• Bell's palsy^[5]
• Burning mouth syndrome^[6]
• Chronic recurrent multifocal osteomyelitis^[7]
• Long COVID^[8]
• Psychogenic polydipsia^[9]
• Schizophrenia^[10]
• Somatic symptom disorder^[11]
• Sudden infant death syndrome^[12]
• Tolosa–Hunt syndrome^[13]