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Reliability and Validity of Clinical Diagnosis

Reliability and Validity of Clinical Diagnosis As indicated, clinical diagnosis usually results in a specific categorization or label. After the patient has been appraised, the clinician offers a diagnosis (e.g., schizophrenia, paranoid type). If the diagnosis reflects a specific disease process that is known and understood, there is a reasonable probability that most diagnosticians might agree on the correct diagnosis. However, if the diagnostic category lacks preciseness and covers a moderate variety of behaviors, the reliability of the diagnosis may be impaired. This is of some importance for both practice and research. For example, if a specific treatment is indicated for a given disorder, a misdiagnosis may lead to improper treatment. In research investigations, unreliable diagnoses for the subjects studied may lead to unreliable or invalid results. Thus, the reliability of the diagnosis is of some consequence; most of the research in the past has not secured high agreement among psychiatrists who provided diagnoses on the same group of subjects. Although several of these studies have flaws, they do illustrate the problem of the lack of reliability in psychiatric diagnosis. The percentage of agreement between two psychiatrists for specific subtype diagnosis, excluding organic brain syndromes, varied from 6 to 57% in one study (Schmidt & Fonda, 1956). In a study of three psychiatrists who worked in the same hospital with comparable groups of patients, one of the psychiatrists diagnosed two-thirds of his patients as schizophrenic, compared with 22 and 29% by the other two psychiatrists. Anyone who has worked for a time in a psychiatric hospital has had the opportunity to observe the diagnostic preferences and biases of other staff members. Other examples of the possible unreliability of psychiatric diagnosis can be found in differences in proportions or distribution of diagnoses over time and in different countries or geographic locations. Kramer, for example, collected data that showed large variations over time in first admission rates to state mental hospitals in the United States for different diagnoses. The rates of schizophrenic admissions were 17.2, 22.0, and 25.3 per 100,000 for the years 1940, 1950, and 1960, respectively. The 1960 rate of admission for patients diagnosed as schizophrenic exceeded the 1940 rate by 47%. The rates of affective psychosis, on the other hand, showed a steady decline from 11.2 to 9.5 to 7.4 for the same periods, or a decline of 33.9%. Does this represent a real increase in rates of admission for schizophrenia and a real decrease in such rates for affective psychosis, or does this reflect inconsistency in applying diagnostic criteria? Similar questions can be raised concerning other diagnoses for which large differences in rates of admission are observed. ‘‘The large increases in the rates for the psychoneuroses, personality disorders, and alcoholic addictions raise questions as to whether they represent true increases in rates of admission for these diagnostic groups or differences in diagnostic fads and criteria or other factors that lead mental hospital psychiatrists to place nonpsychotic diagnoses on increasing numbers of patients’’. It is reasonably clear from the kinds of data mentioned before, as well as from studies comparing psychiatric diagnoses obtained in different countries, that unreliability in diagnosis has constituted a problem of some importance. It is certainly a serious problem for research on psychopathology. If there is limited reliability for the diagnostic groups studied, the results secured for any investigation may lack stability, and replication and generalization of results will be difficult. Clinically, lack of reliability may lead to incorrect diagnosis and treatment. As a result of faulty diagnosis, individuals may be socially stigmatized or inappropriately institutionalized. Thus, reliability of diagnosis is not merely an academic issue, even though our concern here with methodological issues may pertain more directly to problems of research in psychopathology. Although the reliability of clinical diagnosis is important, there is also the matter of the validity of the diagnoses secured. Reliability, although necessary, does not guarantee validity. As emphasized in a critical review, ‘‘Even if the reliability of schizophrenia could be assured, therefore, the validity of the concept would require further demonstration’’. The importance of the validity of clinical diagnoses in psychiatry, however, has not received very much research attention, perhaps because of the difficulties in securing adequate criteria for evaluative purposes. How does one decide if a diagnosis of anxiety disorder or schizophrenia is valid? In essence, in evaluating disorders without physical components or clearly known etiologies, one must rely on clinical judgments or psychological tests, which in turn have been based largely on clinical judgments. In the absence of some standard or accepted criteria against which to evaluate clinical diagnoses, the process of validation is difficult and thus somewhat neglected. Reliability, by contrast, is much easier to appraise and has received increased attention in the preparation of DSM-IV. Nevertheless, the problem of validity remains. It may not be viewed as of great importance when there are no treatments of proven worth or preventive strategies for different diagnostic categories. However, where different and effective treatments or methods of prevention are available, the validity of differential diagnosis becomes important. It is, however, always significant for studies of particular types of disorders and may be particularly so for certain types of research. One kind of research in which the validity of clinical diagnoses is of great importance is that conducted with subjects at high risk for schizophrenia. Although the problem of low reliability of psychiatric diagnoses can be overcome with competent and specially trained diagnosticians who are provided with accurate and detailed information and with structured interview guidelines, the matter of validity is more complex and difficult. Nevertheless, ‘‘For high-risk researchers, the issue of the predictive validity of the diagnoses of schizophrenia is of central importance’’.

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