Question 9

Outline how the following tests assess coagulation:
a. Prothombin Time (PT)
b. Activated Partial Thromboplastin Time (APTT)
c. Activated Clotting Time (ACT)
d. Thromboelastography (TEG or ROTEM)

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College Answer

Many candidates incorrectly stated that the PT assessed the intrinsic system and that the APTT assessed the extrinsic system. This led to subsequent errors in relating a coagulation test to the appropriate coagulation factors that it assessed. Some candidates produced elaborate diagrams of the coagulation cascade in isolation without relating it to the question.

Discussion

  • Prothrombin Time (PT)
    • Laboratory test for the extrinsic and common pathways 
    • Centrifuged plasma sample is activated with tissue factor
    • Normal = 10-13 sec; therapeutic = 20-30 sec (corresponds to INR 2.0-3.0)
    • Abnormal in:
      • factor deficiency (Vit K sensitive factors II, VII, X)
      • warfarin therapy 
      • direct thrombin inhibitor therapy
      • direct Xa inhibitor therapy
  • activated Partial Thromboplastin Time (aPTT)
    • Laboratory test for the intrinsic and common pathways 
    • Normal = 30-40 sec; therapeutic = 50-90 sec 
    • Centrifuged plasma sample is activated with kaolin (negatively charged surface) and phospholipid (to replace platelets)
    • Abnormal in:
      • factor deficiency (XII, XI, X, IX, II)
      • heparin therapy 
      • direct thrombin inhibitor therapy
      • direct Xa inhibitor therapy
      • antiphospholipid syndrome
  • Activated Clotting Time (ACT)
    • Point of care test for whole clotting system
    • Whole fresh blood activated by kaolin
    • Normal 110-130, target ~200 for ECMO, ~400 for bypass
    • Abnormal in any coagulopathy (i.e. nonspecific)
  • TEG and ROTEM
    • ​​​​​​​​​​​​​​Point of care test for whole clotting system
    • Whole fresh blood activated in presence of selected reagents (eg. heparinase)
    • Tests for a large range of variables, including:
      • Factor deficiency or presence of factor inhibitor
      • Fibrinogen deficiency
      • Poor platelet function (or thrombocytopenia)
      • Hyperfibrinolysis

References

Curry, Andy NG, and JM Tom Pierce. "Conventional and near-patient tests of coagulation." Continuing Education in Anaesthesia, Critical Care and Pain 7.2 (2007): 45-50.

Raber, Martin N. "Coagulation tests." Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition (1990).