The use of Baye's theorem and other indices of agreement in evaluating the use of reagent strips in screening rural schoolchildren for Schistosoma haematobium in Zambia.

The diagnosis of Schistosoma haematobium disease on the basis of history and physical examination alone is often difficult. Tests have thus been developed to allow an early and more accurate diagnosis. However these tests have substantial imperfections and many different results obtained from these tests must be integrated into a diagnostic conclusion about the probability of disease in a given patient. Also the accuracy of these tests in detecting S. haematobium disease is critically dependent not only on its sensitivity and specificity but also on the prevalence or pretest likelihood of disease in the population under study. The diagnostic accuracy of haematuria, proteinuria and the combined criteria tests in detecting S. haematobium eggs in schoolchildren are evaluated by calculating the sensitivity and specificity and also by the use of 'Bayes' theorem of conditional probability. The graphic relation between the predictive value of a given test result and the pretest risk of disease in the test subjects was obtained for each of these tests. This method reveals that the prevalence of S. haematobium disease is an important determinant of the predictive value of any test result in the individual patient.