The top-down approach to designing a multistage test is relatively understudied in the literature and underused in research and practice. This study introduced a route-based top-down design approach that directly sets design parameters at the test level and utilizes the advanced automated test assembly algorithm seeking global optimality. The design process in this approach consists of five sub-processes: (1) route mapping, (2) setting objectives, (3) setting constraints, (4) routing error control, and (5) test assembly. Results from a simulation study confirmed that the assembly, measurement and routing results of the top-down design eclipsed those of the bottom-up design. Additionally, the top-down design approach provided unique insights into design decisions that could be used to refine the test. Regardless of these advantages, it is recommended applying both top-down and bottom-up approaches in a complementary manner in practice.