title: Detection rates and false positive rates for Down's syndrome screening: how precisely can they be estimated and what factors influence their value?
abstract: Down's syndrome screening is currently carried out using a combination of biochemical markers measured in maternal serum samples; these include MSAFP, Total hCG, uE3 and Free beta-hCG. Recently a number of papers have compared the effectiveness of different combinations of these markers. Some recommend MSAFP, Total hCG and uE3 (triple test) while others advocate MSAFP and Free beta-hCG (double test). The cases put forward to support these tests rely on estimated detection and false positive rates for the proposed test. A recent paper by Wright used simulation techniques to estimate the effects of sampling error on such error rates. In prospective studies there are two methods commonly used for estimating these rates. We obtain formulae for the standard deviations of these estimates and show that one of them always gives a smaller standard error than the other. We also show that in such studies the accuracy of estimating detection rates and false positive rates depends not only upon the method of calculation but also on the age distribution of pregnant women and the parameters used to calculate patient specific risk. We show that these effects can result in estimation errors of such magnitude that many observed differences in detection rates could be of questionable significance, a conclusion also reached by Wright.
authors.affiliation: Department of Medical Computing and Statistics, University of Wales College of Medicine, Heath Park, Cardiff, U.K.
authors.forename: F D
authors.initials: FD
authors.lastname: Dunstan
authors.forename: J C
authors.initials: JC
authors.lastname: Gray
authors.forename: A B
authors.initials: AB
authors.lastname: Nix
authors.forename: T
authors.initials: T
authors.lastname: Reynolds
mesh_terms: D000328:Adult; D004314:Down Syndrome / Q000453:epidemiology* / Q000517:prevention & control; D005260:Female; D006801:Humans; D007231:Infant, Newborn; D008403:Mass Screening / Q000706:statistics & numerical data*; D008423:Maternal Age; D015233:Models, Statistical; D011237:Predictive Value of Tests; D011247:Pregnancy; D011296:Prenatal Diagnosis / Q000706:statistics & numerical data*; D018570:Risk Assessment