Question 9

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

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).