Validity of psychiatric diagnoses

By: Peter Tyrer and Kenneth R. Silk

There has long been argument about the terms ‘reliability’, ‘utility’ and ‘validity’ of psychiatric diagnoses. Reliability, as a psychological construct, is the easiest of the three to resolve, as it is merely a measure of agreement between assessors of the same information. Thus for a diagnosis of a patient to be reliable, it is necessary for several people to see the same patient (preferable) or a set of proxy data (video recordings or transcripts) and show a level of agreement (measured by a standard measure of correlation) that is preferably above a level of 0.75 (Cicchetti & Sparrow, 1981). Confusion only arises when the lay interpretation of reliability is used (e.g. in a court of law counsel often asks if ‘the evidence is reliable’ when they really mean ‘is the evidence valid’, or can we really be confident that this evidence is a true record?). Validity is a much more difficult construct to achieve. Consider, for example, this quotation from two noted authorities on classification:

We suggest, therefore, that a diagnostic category should be described as valid only if one of two conditions has been met. If the defining characteristic of the category is a syndrome, this syndrome must be demonstrated to be an entity, separated from neighboring syndromes and normality by a zone of rarity. Alternatively, if the category’s defining characteristics are more fundamental – that is, if the category is defined by a physiological, anatomical, histological, chromosomal, or molecular abnormality – clear, qualitative differences must exist between these defining characteristics and those of other conditions with a similar syndrome. (Kendell & Jablensky, p. 7)

When current diagnostic practice is examined there can be only one, rather depressing, conclusion:

At present there is little evidence that most contemporary psychiatric diagnoses are valid, because they are still defined by syndromes that have not been demonstrated to have natural boundaries. This does not mean, though, that most psychiatric diagnoses are not useful concepts. In fact, many of them are invaluable. But, because utility often varies with the context, statements about utility must always be related to context, including who is using the diagnosis, in what circumstances, and for what purposes. (Kendell & Jablensky, p. 8)

Many, though not all, of the diagnostic concepts represented by the categories of disorder listed in contemporary nomenclatures such as DSM–IV and ICD–10 are extremely useful to practising clinicians, and most clinicians would be hard put to cope without them. Diagnostic categories provide invaluable information about the likelihood of future recovery, relapse, deterioration and social handicap; they guide decisions about treatment; and they provide a wealth of information about similar patients encountered in clinical populations or community surveys throughout the world. Diagnostic categories allow us to identify cohorts of like unwell people for whom we can collate their frequency in the population, their demographic characteristics, family backgrounds and premorbid personalities, their symptom profiles and the evolution of those symptoms over time found in the results of clinical trials of different therapies. Research can then take place on the aetiology of the syndrome. This is all very useful and often provides invaluable information, whether or not the category in question is valid. Its usefulness depends mainly on two things:

(1) The quantity and quality of the information in the literature (which depends on how long the category has been recognized and provided with adequate diagnostic criteria and how much competent research the category has generated) and

(2) Whether the implications of that information, particularly about aetiology, prognosis and treatment, are substantially different from the implications of analogous information about other related syndromes.

But in recognizing the merits of usefulness, we must not go too far and imply validity to the diagnostic edifice we have constructed; it is a pragmatic solution, not a real one, and new data may quickly sweep it away. We then might consider the following in attempting to rate or score the strength or clinical utility of a given diagnosis.

References

Peter Tyrer and Kenneth R. Silk, Cambridge Textbook of Effective Treatments in Psychiatry, Cambridge University Press 2008.

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