Design of early validation trials of biomarkers

The design of early-phase studies of putative screening markers in clinical populations is discussed. Biological, epidemiological, statistical and computational issues all affect the design of early-phase studies of these markers, but there are frequently little or no data in hand to facilitate the design. Early-phase studies must be designed as part of a development program, considering the final use of the marker, directly informing the decision to made at the study's conclusion. Therefore, they should test for sensitivity and specificity that would be minimally acceptable to proceed to the next stage of development. Designing such trials requires explicit assumptions about prevalence and false positive and negative costs in the ultimate target population. Early discussion of these issues strengthens the development process, since enthusiasm for developing technologies is balanced by realism about the requirements of a valid population screen. Receiver operating characteristic (ROC) curves, which are useful descriptive tools, may be misleading when evaluating tests in low-prevalence populations, because they emphasize the relationship between specificity and sensitivity in the range of specificity likely to be too low to be useful in mass screening applications.