A user interface for a medical instrument such as a dialysis machine is described which uses both a touch screen and at least one hard key off of the touch screen to effectuate a change in a parameter associated with the operation of the machine or the treatment session. The user interface is connected to a central computer control system having a host microprocessor and a backup safety microprocessor. The hard key is directly wired to the safety microprocessor. After the user selects a new parametric value on the touch screen, the user presses a hard key. The host and safety microprocessors implement a verification routine to insure that the entered parameter is appropriate for the patient's treatment and the display on the touch screen. If the verification procedure ends in a positive result, the user is prompted to presses a second hard key to confirm the change, causing an additional verification check to be preformed. If the additional verification check is successful, the parametric value is entered into a memory (such as a hard disk) storing instructions for operation of the medical instrument.