Diagnosis of exclusion

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:
 * Fibromyalgia
 * Adult-onset Still's disease
 * Behçet's disease
 * Bell's palsy
 * Burning mouth syndrome
 * Chronic recurrent multifocal osteomyelitis
 * Long COVID
 * Psychogenic polydipsia
 * Schizophrenia
 * Somatic symptom disorder
 * Sudden infant death syndrome
 * Tolosa–Hunt syndrome
 * Systemic-onset juvenile idiopathic arthritis