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.^[1]

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. It can also commonly occur where objective diagnostic tests do exist, but extensive diagnostic testing or sufficient exploration of differential diagnosis by a multidisciplinary team is not undertaken due to financial constraints or assessment bias (health inequity).^{[2][3][4][5][6]}

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.^[7][8]

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:

• Fibromyalgia^[9]
• Adult-onset Still's disease^[10]
• Behçet's disease^[11]
• Bell's palsy^[12]
• Burning mouth syndrome^[13]
• Chronic recurrent multifocal osteomyelitis^[14]
• Long COVID^[15]
• Inappropriate sinus tachycardia^[16]
• Psychogenic polydipsia^[17]
• Schizophrenia^[18]
• Somatic symptom disorder^[19]
• Sudden infant death syndrome^[20]
• Tolosa–Hunt syndrome^[21]
• Systemic-onset juvenile idiopathic arthritis^[22]

• Idiopathic
• Wastebasket diagnosis