Evaluation and comparison of the accuracy of two diagnostic medical devices have been typically conducted using fixed sample designs with subjective sampling ratios between the case and the control. On the other hand, group sequential design as an alternative of the fixed sample design addresses both cost and ethical concerns in diagnostic trials. In addition, optimal sampling ratio may be an efficient choice to minimize the total required sample size and reduce the overall costs of diagnostic studies. In this talk, we provide a brief overview of recent development on group sequential design methods and optimal sampling ratios in medical diagnostics. We discuss the application of these design methods to non-inferiority studies.