Clinical meaning
The coagulation cascade consists of intrinsic and extrinsic pathways converging on a common pathway to form fibrin clot. The prothrombin time (PT) and its standardized form, the International Normalized Ratio (INR), measure the extrinsic pathway (tissue factor/Factor VII) and common pathway (Factors X, V, II/prothrombin, fibrinogen). PT/INR is prolonged by warfarin (inhibits vitamin K-dependent factor synthesis: II, VII, IX, X, and proteins C and S), liver disease, vitamin K deficiency, and DIC. The activated partial thromboplastin time (aPTT) measures the intrinsic pathway (Factors XII, XI, IX, VIII) and common pathway. aPTT is prolonged by unfractionated heparin (potentiates antithrombin III), factor deficiencies (hemophilia A = Factor VIII deficiency, hemophilia B = Factor IX deficiency), lupus anticoagulant (paradoxically increases clotting risk despite prolonged aPTT), and DIC. When BOTH PT and aPTT are prolonged, the common pathway (Factor X, V, II, fibrinogen), liver disease, DIC, or vitamin K deficiency affecting multiple factors should be suspected. Mixing studies differentiate factor deficiency (corrects with normal plasma) from inhibitors (does not correct). The NP interprets these studies to guide anticoagulation dosing, evaluate bleeding risk, identify coagulopathies, and determine reversal strategies.