| 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
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| 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
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| 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
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| International normalized ratio (INR) | - Compensates for differences in PT reagents:
- INR = patient PT/control PT
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| 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
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| 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
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| 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
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| Thromboelastography | - 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. A ...
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