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Figure 23-1. Classic Model of Coagulation

Classic coagulation cascade. Reproduced from Tintinalli JE, Stapczynski JS, Ma OJ, Cline DM, Cydulka RK, Mckler GD. Tintinalli’s Emergency Medicine: A Comprehensive Study Guide. 7th ed. Figure 229-1. Available at: Copyright © The McGraw-Hill Companies, Inc. All rights reserved.

  • Initiation:
    • Endothelial injury
    • Subendothelial tissue factor (TF) exposed
    • TF + circulating plasma FVII builds TF–FVII complex
    • TF–FVII complex activates F-X, F-IX and a small amount of thrombin forms
  • Amplification:
    • Thrombin activates platelets via glycoprotein IIb/II a receptors
    • F-Va binds to platelets
    • More F-VIIa is released
  • Propagation:
    • Positive feedback:
      • F-Va leads to more release of F-IXa:
        • F-IXa leads to formation of thrombin activatable fibrinolysis inhibitor (TAFI) that prevents fibrin breakdown
      • F-IXa–F-VIII complex activates more F-X:
        • Thrombin burst
        • Fibrin production
        • Fibrin polymerization
  • Clot formation

Assessment of Coagulation and Coagulation Tests
Platelet count
  • Automated counting cannot detect the presence of small or extremely large platelets
  • Manual smear can exclude the presence of pseudothrombocytopenia due to in vitro platelet agglutination
Bleeding time
  • Evaluate platelet–vascular endothelium interaction
  • Prolonged bleeding time may occur in thrombocytopenia (<50,000), qualitative platelet abnormalities (e.g., uremia), von Willebrand disease (vWD), and severe fibrinogen deficiency
  • Does not predict surgical bleeding and is of limited usefulness in clinical bleeding
Prothrombin time (PT)
  • Measures the efficiency of the fibrin production through the extrinsic pathway and the final common pathway:
  • Tissue factor, factor VII (extrinsic pathway), and factors X, V, prothrombin (factor II), and fibrinogen
  • Factors VII, X, and prothrombin are dependent on vitamin K and affected by coumadin; therefore, PT is used to monitor anticoagulation with coumadin
International normalized ratio (INR)
  • Compensates for differences in PT reagents:
  • INR = patient PT/control PT
Activated partial thromboplastin time (aPTT)
  • Measures the intrinsic (factors XII, XI, IX, VIII) and common pathway (factors II, V, X, and fibrinogen)
  • Used to monitor heparin effect and to evaluate deficiencies of all coagulation factors except factors VII and XIII
Thrombin time (TT)
  • Measures the time it takes for a clot to form in the plasma of a blood sample to which an excess of thrombin has been added
  • If a patient is receiving heparin, a substance derived from snake venom called reptilase (not inhibited by heparin) is used instead of thrombin
  • Normal TT: 10–15 s or within 5 s of the control. Normal reptilase time: between 15 and 20 s
  • TT can be prolonged by heparin, fibrin degradation products, factor XIII deficiency, and fibrinogen deficiency/abnormality
Activated clotting time (ACT)
  • An activating agent such as Celite or kaolin is added to a blood sample and the time to clot formation is measured
  • Used to confirm and monitor heparin effect as a point-of-care test during cardiac or vascular surgery
  • Thromboelastography measures clinical clot formation and lysis not specific to coagulation pathways
  • TEG reflects clinically significant hemostasis and can guide transfusion and factor therapy
  • Coagulation is activated by adding calcium to a ...

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