Plasma thromboplastin antecedent (PTA)= Factor XI
“International normalized ratio (INR) is a calculation made to standardize prothrombin time. INR is based on the ratio of the patient’s prothrombin time and the normal mean prothrombin time.”
INR = a patient’s PT time / PT of a standardized pt (average PT). The INR really measures the same thing as PT (the extrinsic pathway). However, the international normalized ratio was brought in to make PT values comparable across hospitals.
You order an INR when a patient is taking Warfarin (coumadin).
Warfarin acts by inhibiting the synthesis of vitamin K-dependent clotting factors, which include Factors II, VII, IX, and X, and the anticoagulant proteins C and S. Vitamin K is an essential cofactor for the post ribosomal synthesis of the vitamin K-dependent clotting factors. Read more here. Note that factors VII, X, and II are all part of the extrinsic pathway. Only factor IX is out of the extrinsic pathway.
You order PTT or aPTT when a patient is taking Heparin.
Heparin acts at multiple sites in the normal coagulation system. Heparin binds to antithrombin III, catalyzing inactivation of thrombin and other clotting factors. PTT looks at the intrinsic pathway.