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.

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

• Idiopathic
• Wastebasket diagnosis