Why do Direct Thrombin Inhibitors spare PT and PTT?

[ChessMaster3000]

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In the explanation of a question, it notes that DTIs would prolong thrombin time (TT), but not PTT and PT. Given that the PTT and PT measure the length of time it takes to clot which INCLUDES the thrombin step of converting fibrinogen to fibrin, WHY is this not affected by a DTI?

 

[Myxedema]

Can you quote the entire question? Because direct thrombin inhibitors are monitored by aPTT as well (like heparin), so they most certainly prolong aPTT.

 

[ChessMaster3000]

Can you quote the entire question? Because direct thrombin inhibitors are monitored by aPTT as well (like heparin), so they most certainly prolong aPTT.

I don't want to because its a UWorld question and don't want to give it away. But youre right, DTI is monitored by aPTT--so maybe UW messed up?

 

[ulikedaggers]

In the explanation of a question, it notes that DTIs would prolong thrombin time (TT), but not PTT and PT. Given that the PTT and PT measure the length of time it takes to clot which INCLUDES the thrombin step of converting fibrinogen to fibrin, WHY is this not affected by a DTI?

I thought PT and PTT were dependent on factor II (among others) being present, whereas TT actually measures fibrinogen to fibrin. So since DTIs don't affect factor II (not IIa) then there's no affect on PT or PTT.

 

[Myxedema]

I thought PT and PTT were dependent on factor II (among others) being present, whereas TT actually measures fibrinogen to fibrin. So since DTIs don't affect factor II (not IIa) then there's no affect on PT or PTT.

As the name suggests, direct thrombin inhibitors block the action of thrombin (Factor IIa). Factor II is prothrombin. Also, DTIs prolong aPTT. If someone can confirm the question in the Usmleworld, maybe we can say more about this issue.