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Prothrombin time test is used for the monitoring of warfarin but not heparin. Why is this used for warfarin monitoring and why is it not used for heparin monitoring?

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Thats because the mechanism of action is different between warfarin and heparin. Warfarin inhibits the formation of prothrombin (also known as factor II) by inhibiting the metabilization of vitamin K (for extensive details have a look at the Wikipedia article about Warfarin). Heparin inhibits the blood coagulation by binding to antithrombin III which functions as a inactivator of active clotting factors as factors V, IX, X, XI and XII. It also binds calcium which is important for the blood coagulation. For more details see the Wikipedia under Heparin. Therefore Heparin interferes with the formation of Thrombin and the test does not work. A nice image of the coagulation cascade can be found here, which also shows the points where Heparin interferes. The test can be done with Batroxobin, see here.

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So, heparin stops the formation of thrombin and warfarin stops the formation of prothromibin. But i still don't get it that why is PT test not used for heparin monitoring? –  Rafique Jan 4 at 20:57
    
Because you can measure how much thrombin activity is left when the synthesis is inhibited (for warfarin), but when it can't be activated (for Heparin), you will not measure anything. You don't knock-down coagulation completely with Wafarin. The last link in the answer above explains the test. –  Chris Jan 4 at 21:07
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