A method for operating a dialysis cassette including a flexible membrane that covers a pump chamber includes allowing a source of fluid to fluidly communicate with the pump chamber of the dialysis cassette, filling the pump chamber with the fluid from the source, mechanically extending the flexible membrane into the pump chamber with a piston head to expel the fluid from the pump chamber through a flow path, and directly sensing a pressure of the fluid flowing through the flow path at a location of the dialysis cassette adjacent to the pump chamber and using the sensed pressure to perform a test prior to delivering fluid to a patient.
