A<unk>  or by<unk>  (per<unk> em) 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<unk>  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<unk>  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 of exclusion
Dual diagnosis
Tolerogenic therapy
Experimental pathology
Treatment as prevention
Natural history of disease
Immune-related response criteria
Monitoring in clinical trials