Which of the following is done for a patient on coumarin (warfarin) therapy:: (A) Partial thromboplastin time (PTT), (B) Prothrombin time (PT), (C) Bleeding time (BT), (D) Capillary fragility test (CFT)
Answer is B. 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