diagnosis of exclusion 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: 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