Hemostasis and Coagulation
Hemostasis and coagulation testing
Primary hemostasis involves platelet adhesion via GPIb–vWF, activation, and aggregation via GPIIb/IIIa–fibrinogen. Secondary hemostasis is the coagulation cascade: intrinsic (XII, XI, IX, VIII), extrinsic (VII plus tissue factor), and common (X, V, II, I), all phospholipid- and calcium-dependent. Regulation includes antithrombin, protein C/S, TFPI, and fibrinolysis. Screening uses PT (extrinsic/common) and aPTT (intrinsic/common); mixing studies distinguish factor deficiency (corrects) from inhibitor (does not). Thrombin time and fibrinogen assess fibrin formation; D-dimer reflects fibrinolysis. von Willebrand disease causes mucocutaneous bleeding with prolonged bleeding time/platelet function and low vWF antigen/activity; hemophilia A/B causes deep bleeding with prolonged aPTT and normal PT. DIC shows prolonged PT/aPTT, low fibrinogen, high D-dimer, and schistocytes. Lupus anticoagulant paradoxically prolongs aPTT but predisposes to thrombosis. TTP has low ADAMTS13, schistocytes, and thrombocytopenia.
Traps: a mixing study that corrects immediately may still fail after incubation in some inhibitors. Heparin contamination prolongs aPTT and thrombin time. Vitamin K deficiency affects II, VII, IX, X. Liver disease affects all factors. Platelet clumping in EDTA causes pseudothrombocytopenia.
Worked example: aPTT 60 s, PT 12 s, mixing corrects; factor VIII 5% confirms hemophilia A.