The concept of diagnostic hierarchy was explained for psychiatry a century ago by the German psychiatrist Karl Jaspers. The idea is that not all diagnoses are created equal; some are more important or more primary than others. For instance, alcohol-induced psychosis, not schizophrenia, should be diagnosed in the setting of delirium tremens.
I would suggest that the concept also is implicit more generally in the standard “medical model” which we are taught in medical school and which the average internist applies without much conscious cogitation – the principle of differential diagnosis. As is well known by the average physician, the idea is that the physician might think the patient’s syndrome matches a certain diagnosis best, but before settling on that “working” diagnosis, the physician should “rule out” a few others. Typically one begins with more severe diseases that might be more rare, but would be important to treat. One then moves to other less severe or more common conditions, or other diagnoses which might be polysymptomatic than the working diagnosis. A common error is to think only in terms of etiology, ruling out other diagnoses which for which we might know the “cause.” This isn’t necessary. The differential diagnosis approach to hierarchical diagnosis is not about knowing the cause; it can be about other conditions (of unknown causation) which could still “cause” the clinical syndrome presented to us. For instance, a patient with cognitive impairment needs to be ruled out for a delirium of unknown cause before attention deficit disorder is diagnosed. The concept of “working” diagnosis and “ruling out” other diagnoses is a basic application of the diagnostic hierarchy concept, used in all of medicine except DSM-based psychiatry.
Related, the DSM system, from the 3rd edition onward, has explicitly rejected the hierarchy concept on the false grounds that it could be applied only if one knew the etiologies of other conditions that could cause the presenting syndrome. Since etiologies are mostly unknown in psychiatry, the hierarchy concept was rejected. Instead the “comorbidity” concept replaced it, with DSM ordering clinicians on pain of death to diagnose as many diagnoses as possible. No one could be excluded in favor of another (with a few exceptions, such as the important schizophrenia versus mood disorder distinction). This is anti-medical, going against the differential diagnosis tradition, and has led to psychiatry’s unmedical unscientific practice of DSM-based hyperdiagnosis, leading to false “comorbidity” and polypharmacy. The Dutch psychiatrist Hermann van Praag’s identified this risk over two decades ago, writing about the consequence of “nosologomania”.
The ”comorbidity” problem is false, a result of the fact that DSM has made psychiatry uniquely wrong and different from the rest of the profession of medicine in rejecting the differential diagnosis concept as entailed in the notion of a diagnostic hierarchy.