Anticoagulants

Haemostasis process

Coagulation cascade

• Vitamin K dependent clotting factors: II, VII, IX, X (+ proteins C and S)

Clot dissolution

• Natural anticoagulants: proteins C and S (inactivate factors V and VIII); antithrombin (inactivates many clotting factors)

Tests

• PT and INR = EXTRINSIC: tissue factor is added to blood in the laboratory to activate the extrinsic pathway. Clotting time (PT) is measured in seconds (normal usually = 12-13). However, the PT may vary depending on the reagent used, so the laboratory can use the PT to calculate the INR to standardise results (normal = 0.8-1.2).
  • WEPT: Warfarin Extrinsic Prothrombin Time

• APTT = INTRINSIC: a contact activator is added to blood in the laboratory to activate the intrinsic pathway. Clotting time (APTT) is measured in seconds (normal usually = 30-50). The APTT can be divided by the mean normal value to calculate the APTT ratio if required.

Types of anticoagulants