SCREENING-TESTS - CAN WE GET MORE BY DOING LESS

Estimating the prevalence of the human immunodeficiency virus (HIV) in a group is challenging; this is especially so when the prevalence is small. One reason is that the presence of measurement errors resulting from the limited precision of tests makes estimation, using tradition al methods, impossible in some screening situations. Measurement error is real, ignoring it leads to severe bias, and inference about the pr evalence becomes unsatisfactory. Indeed, in a low prevalence situation the expected number of false positives is very high, often even highe r than the number of true positives. The second reason is that in the low prevalence areas the large sample is needed in order to obtain non -zero estimate. This is usually a very costly, and often unrealistic, solution. This paper considers the advantages and disadvantages of poo led testing as an alternative solution to this problem. We show that b y pooling sera samples we not only achieve a cost saving but also, whi ch is counterintuitive, an increase in the estimation accuracy. We als o discuss the statistical issues associated with the resulting estimat or.