Abstract: A test article for determining coagulation capability in a blood sample comprises a porous membrane having a coagulation initiator and substrate impregnated therein. The pore dimensions and composition of the membrane are selected so that only blood plasma can pass into the interior of the membrane, where coagulation is initiated. The substrate is activated by a component of the coagulation pathway, typically thrombin, and produces a detectable signal upon activation. By utilizing membrane matrix materials which are substantially free from interference with the coagulation pathway, accurate results can be achieved.