Sample size calculation should be performed for design accuracy in diagnostic test studies.

Background And Objectives: Guidelines for conducting studies and reading medical literature on diagnostic tests have been published: Requirements for the selection of cases and controls, and for ensuring a correct reference standard are now clarified. Our objective was to provide tables for sample size determination in this context.

Study Design And Setting: In the usual situation, where the prevalence Prev of the disease of interest is <0.50, one first determines the minimal number Ncases of cases required to ensure a given precision of the sensitivity estimate. Computations are based on the binomial distribution, for user-specified type I and type II error levels. The minimal number N(controls) of controls is then derived so as to allow for representativeness of the study population, according to Ncontrols=Ncases [(1-Prev)/Prev].

Results: Tables give the values of Ncases corresponding to expected sensitivities from 0.60 to 0.99, acceptable lower 95% confidence limits from 0.50 to 0.98, and 5% probability of the estimated lower confidence limit being lower than the acceptable level.

Conclusion: When designing diagnostic test studies, sample size calculations should be performed in order to guarantee the design accuracy.