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.
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. 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.
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:
- Behcet's disease
- Bell's palsy
- Tolosa–Hunt syndrome
- Sudden infant death syndrome[medical citation needed]
- Burning mouth syndrome
- Psychogenic polydipsia
- Chronic recurrent multifocal osteomyelitis 
- Somatic symptom disorder
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- Freudenreich, O (December 2012). "Differential Diagnosis of Psychotic Symptoms: Medical "Mimics"". Psychiatric Times.
- Kwan ES, Wolpert SM, Hedges TR, Laucella M (February 1988). "Tolosa-Hunt syndrome revisited: not necessarily a diagnosis of exclusion". AJR. American Journal of Roentgenology. 150 (2): 413–8. doi:10.2214/ajr.150.2.413. PMID 3257334.
- Maltsman-Tseikhin A, Moricca P, Niv D (June 2007). "Burning mouth syndrome: will better understanding yield better management?". Pain Practice. 7 (2): 151–62. doi:10.1111/j.1533-2500.2007.00124.x. PMID 17559486. S2CID 4820793.
- Prince, Jim McMorran, Damian Crowther, Stew McMorran, Steve Youngmin, Ian Wacogne, Jon Pleat, Clive. "primary polydipsia - General Practice Notebook". www.gpnotebook.co.uk. Retrieved 2016-11-22.
- Ferguson B, Gryfe D, Hsu W (December 2013). "Chronic recurrent multifocal osteomyelitis in a 13 year old female athlete: a case report". The Journal of the Canadian Chiropractic Association. 57 (4): 334–40. PMC 3845477. PMID 24302781.
- Henningsen, Peter (March 2018). "Management of somatic symptom disorder". Dialogues in Clinical Neuroscience. 20 (1): 23–31. ISSN 1294-8322. PMC 6016049. PMID 29946208.