transmission, and the etiological agent (i.e., the human immunodeficiency virus) (Buehler et al., 1993). The most recent revision to the case definition was in 1993, a full 12 years after the first cases were recorded and 9 years after the cause was discovered. This attests to the protean nature of case definitions even, as in this example, with the advantage of objective physical findings and identified etiology. Eventually, the name of the condition was changed to “human immunodeficiency virus disease” to denote the etiological agent and to capture the full course of the disease, from primary infection, to asymptomatic stage, to late stage (WHO, 1992). Although AIDS is the term reserved for the late stage, nosologists would likely favor a name such as “late-stage HIV disease,” but common usage reigns.

Systems of Disease Classification

Formal classifications of diseases, disorders, and syndromes are found in the latest modifications of the International Classification of Diseases (ICD-10) and the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV).5 These two compendia are issued about every 10 years by the World Health Organization (WHO) and the American Psychiatric Association (APA), respectively. As the most authoritative sources for public health, government agencies, and health insurers, they have a myriad of applications for medicine, health statistics, reimbursement, disability claims, and medical record keeping. Numerous health and disability statutes at the federal and state levels require a code from one of these classification manuals. Thus, from a legal and statistical perspective, a new listing in these manuals marks the official “arrival” of a new clinical entity (Wegman et al., 1997), even though it may come as little surprise to many clinicians and researchers. From their point of view, a new listing is for coding and classification purposes, not necessarily for fundamental insights into etiology, diagnosis, and treatment. A listing can be construed as the reflection of consensus of health professionals, rather than the instigator.

The formal decision to place a new clinical entity in one of these volumes is made by health professionals organized under the auspices the sponsoring agency. WHO and APA have established procedures for making revisions to ICD-9 and DSM-IV, respectively (APA, 1994; ICD-9, 1999). Yet there are no explicit criteria underlying these procedures. Neither organization publishes explicit criteria for the types of research and clinical evidence needed to revise an existing listing or to add a new listing.6 APA furnishes the most guidance about its procedures and the types of evidence that would warrant changes. It


DSM-IV is a listing of mental disorders and ICD-10 is a listing of all somatic and mental disorders. The former enumerates specific criteria for making a diagnosis, whereas the latter does not. Its list of mental disorders is almost identical to those categorized by, yet expanded upon, in DSM-IV.


WHO is presently engaged in the long-term process of developing an updating process between revisions that will include “an effective updating mechanism” (WHO, 1992).

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