Affects of drugs on PT, aPTT ?

[anbuitachi]

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uworld says unfractionated heparin bind to antithrombin III and mainly bind to factor Xa which mainly prolongs aPTT
Also uworld says direct thrombin inhibitors like dabigatrin, mainly prolongs thrombin time, with less effect on aPTT and PT.

How does this work? Xa is in both external and internal so why is unfractionated heparin mainly aPTT as opposed to both PT and aPTT?

And also if you stop thrombin, shouldn't that prolong both Pt and aPTT since a clot can't form if thrombin is gone.. thus you are kind of stopping the coagulation cascade... how can Pt and aPTT not be increased ??

Thanks

 

[xeroun]

uworld says unfractionated heparin bind to antithrombin III and mainly bind to factor Xa which mainly prolongs aPTT
Also uworld says direct thrombin inhibitors like dabigatrin, mainly prolongs thrombin time, with less effect on aPTT and PT.

How does this work? Xa is in both external and internal so why is unfractionated heparin mainly aPTT as opposed to both PT and aPTT?

And also if you stop thrombin, shouldn't that prolong both Pt and aPTT since a clot can't form if thrombin is gone.. thus you are kind of stopping the coagulation cascade... how can Pt and aPTT not be increased ??

Thanks

"Whether the patient is anticoagulated with heparin or warfarin, both the PT and PTT are prolonged, because both inhibit factors in the final common pathway. Experience has shown that the PT performs better in monitoring warfarin, while PTT performs better in monitoring heparin." - Goljan Rapid Review of Pathology page 256

Thrombin time for Dabigatrin is the one that have been found to have most linear relationship so it is used. Can also use ecarin clotting time, and INR. aPTT can be used to approximate effect of drug but it is not as accurate as others.

 

[anbuitachi]

thanks!!!