ESTIMATING THE PREVALENCE OF DRUG-USE FROM SELF-REPORTS IN A COHORT FOR WHICH BIOLOGIC DATA ARE AVAILABLE FOR A SUBSAMPLE

Diagnostic procedures, used singly or in combination, are crucial in t he determination of the presence and prevalence of medical and other c onditions. In the absence of a ''gold standard,'' two or more measures or diagnostic tests are often available that may be used to estimate true prevalence, The authors have developed a statistical method with which to calculate more precise estimates of a condition in the presen ce of two diagnostic measures, one measurement being performed on the entire study sample and a second, more precise one being made in a ran dom sample of the study sample, This method uses the well-known equati ons which express the probabilities of the four possible outcomes of t he two measures in terms of the sensitivities and specificities of the measures and the prevalence of the condition and some properties of m aximum likelihood estimates to obtain an expression for the estimated true prevalence and its precision, The method is illustrated by applyi ng it to data collected by urinalysis and self-report in 1992-1993 in a national multisite study-the Cocaine Treatment Outcome Study, Throug h application of this methodology, a more precise estimate of the true prevalence of substance use can be obtained from two measures, one bi ologic and the other self-reported, Detailed equations and expressions are provided so that the method can be applied in other situations wh ere diagnostic data from two different sources or procedures are avail able.