The nurse is administering warfarin (Coumadin) to a client with deep vein thrombophlebitis. Which laboratory value indicates warfarin is at therapeutic levels ？（）
A．Partial thromboplastin time (PTT) 
to 2 times the control.
B．Prothrombin time (PT) to 2 times the control.
C．International normalized ratio (INR) of 3 to 4.
D．Hematocrit of 32%.
参考答案：B
解析：
Warfarin is at therapeutic levels when the client's PT is 
to 2 times the control. Higher values indicate increased risk of bleeding and hemorrhage; whereas lower values indicate increased risk of blood clot formation. Heparin--not warfarin--prolongs PTT. The INR may also be used to determine if warfarin is at a therapeutic level. An INR of 2 to 3 is considered therapeutic. Hematocrit doesn't provide information on the effectiveness of warfarin; however, a falling hematocrit in a client taking warfarin may be a sign of hemorrhage.