A diagnosis of exclusion (per exclusionem) is a medical condition whose presence cannot be established with complete confidence from examination or testing. Diagnosis is therefore by elimination of other reasonable possibilities. Perhaps 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, PDD-NOS (an autism spectrum disorder), Sarcoidosis, Tolosa-Hunt syndrome, and Irritable bowel syndrome.
No one test exists to definitively diagnose fibromyalgia, therefore to obtain a fibromyalgia diagnosis, patients must frequently endure tests for many other conditions to eliminate those conditions as possible diagnoses. For that reason, fibromyalgia syndrome is frequently inaccurately referred to as a diagnosis of exclusion. But to obtain a fibromyalgia diagnosis, the patient must show: widespread pain in all four body quadrants for at least three months and tenderness in at least 11 of 18 points when pressure is applied.
The Daily Omnivore 
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