Warfarin and PT/INR delay

[GomerPyle]

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Hey all,

I'm having a hard time here with Warfarin. I understand it takes 4-5 days to take full effect and this is because Factor 2 has a half life of 42-72 hours and thus PT/INR takes that long to become prolonged.

So if you bridge with heparin, the PTT is going to be prolonged since you are blocking factor 2 and 10....but the PT is still normal until factor 2 depletes from the warfarin...I'm having a hard time here because factor 7 is the first to get depleted and I'm wondering why the PT/INR isnt prolonged after that? Seems like Factor 2 would be gone from the heparin....

 

[GomerPyle]

Anyone?????

 

[slz1900]

Do you mean increase instead of decrease? This article says that PT will rise first since like you said factor 7 is the first one depleted.

 

[amberkre]

Heparin use effects PTT whereas Warfarin use effects PT and PT/INR. So when they talk about a prolonged PTT they are saying heparin is working. When they say PT and PT/INR are normal that means the warfarin hasn't kicked in yet due to the increased half lives of the vitamin K dependent factors. I'm not sure what your confusion is, but yes Vitamin K dependent factors are factors II, VII, IX, X. Warfarin takes a while for the anticoagulant properties to kick in because its waiting for the factors still around to be used up, since it interferes in the activation of new factors. Heparin works by potentiating antithrombin III. AT III inhibits already active factors II, VII, IX, X, XI, XII (major targets are thrombin and Xa) which is why you use heparin to bridge the time needed for Warfarin's anticoagulant effects to kick in.

 

[GomerPyle]

Heparin use effects PTT whereas Warfarin use effects PT and PT/INR. So when they talk about a prolonged PTT they are saying heparin is working. When they say PT and PT/INR are normal that means the warfarin hasn't kicked in yet due to the increased half lives of the vitamin K dependent factors. I'm not sure what your confusion is, but yes Vitamin K dependent factors are factors II, VII, IX, X. Warfarin takes a while for the anticoagulant properties to kick in because its waiting for the factors still around to be used up, since it interferes in the activation of new factors. Heparin works by potentiating antithrombin III. AT III inhibits already active factors II, VII, IX, X, XI, XII (major targets are thrombin and Xa) which is why you use heparin to bridge the time needed for Warfarin's anticoagulant effects to kick in.

Thanks, but I understand all of that. My confusion is that factor 7 doesn't take long to go away, so why doesn't the PT/INR get prolonged after that? Why does it have to wait for factor 2 (which has the longest halflife) before the PT is prolonged, especially since factor 2 would already be depleted by the heparin bridge.