Which of the following is done for a patient on coumarin (warfarin) therapy:

When a coumarin, such as warfarin, is given to a patient, the amounts of active prothrombin and Factors VII, IX, and X, all  formed by the liver, begin to fall.  Prothrombin time gives an indication of the concentra­tion of prothrombin in the blood. The normal prothrombin time is about 12 seconds. Reference:  Ganong Physiology 25th ed page no 492,493