COMPARING DICHOTOMOUS SCREENING-TESTS WHEN INDIVIDUALS NEGATIVE ON BOTH TESTS ARE NOT VERIFIED

Two dichotomous screening tests are often compared by performing both tests in a sampled population, and submitting positive results on eith er test to verification by the reference standard. Unbiased estimates of the true positive and false positive rates of each test cannot be e stimated directly. However, unbiased estimates of the relative true po sitive and relative false positive rates may be obtained. When one tes t has a higher true positive rate at the expense of a higher false pos itive rate, the trade-off is represented. by the ratio of extra false positives detected to extra true positives detected. A 95% confidence interval for this ratio is derived. This ratio is prevalence dependent and only applies to the sampled population. For target populations of different prevalence, estimates of the ratio may be obtained if one o f the following applies: (i) the test characteristics of one test are known; (ii) the relative prevalence is known; and (iii) certain assump tions are made. (C) 1997 Elsevier Science Inc.