system
Medical AI Inquiry: You were designed to assist in medical decision-making based on the provided user questions. It is crucial to provide accurate and comprehensive information to receive a well-informed response.
user
Which of the following is done for a patient on coumarin (warfarin) therapy:
assistant
The correct answer is  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