[Sensitivity, specificity and predictive values of methods of evaluation in psychiatry]

Clinical assessments, psychological examinations, biological parameters and other indices are commonly used in psychiatry to measure disease status. These evaluation tools are not perfect and, in general, they will fail to perfectly discriminate "cases" from normal controls. When a gold standard is available, their performances can be evaluated. If the results of a measurement are tabulated with true disease status in a contingency table, the proportions of cases and normals correctly identified (sensitivity and specificity) are useful coefficients of validity of the instrument studied. It is shown that unbiased estimates of the prevalence rate cannot however be derived directly from the proportion of sample respondents identified by a test. Practical examples with a psychiatric rating scale are provided which illustrate the importance and magnitude of this bias. Formulas are subsequently given which permit the calculation of the "true" prevalence rate from the proportion of screen positives when the sensitivity and the specificity of the test are known. The proportion of true cases among those individuals identified by a test is another important property of psychiatric measures, particularly for screening purposes. It is also called the positive predictive value and, like sensitivity and specificity, predictive values are probabilities for which high values are desirable. However, this critical characteristic of a test depends on the prevalence rate, the sensitivity and the specificity of the test used. Using an example, major changes in the probability of a screen positive individual to be truly diseased are shown to depend upon varying levels of the base rate of the disease.(ABSTRACT TRUNCATED AT 250 WORDS)