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: www.accessmedicine.com. Copyright © The McGraw-Hill Companies, Inc. All rights reserved.
- 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
- Thrombin activates platelets via glycoprotein IIb/II a receptors
- F-Va binds to platelets
- More F-VIIa is released
- 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
- 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
- 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 sample of citrated blood in a rotating cup. ...
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