Question 1

How does the cell-based model differ from the cascade model?

Accepted Answer

The cell-based model has three phases (initiation, amplification, propagation) and better explains platelet role and localisation of clotting. The cascade model remains useful for interpreting lab tests.

Question 2

What does an isolated prolonged aPTT suggest?

Accepted Answer

Suggests a deficiency or inhibitor in the intrinsic pathway: factor VIII (haemophilia A), IX (haemophilia B), XI, or XII deficiency, or lupus anticoagulant.

Question 3

How does warfarin affect the cascade?

Accepted Answer

Warfarin inhibits vitamin K epoxide reductase, reducing activation of factors II, VII, IX, and X. PT prolongs within 24-48 hours, but full antithrombotic effect requires 3-5 days.

Question 4

What factor deficiencies cause prolonged PT with normal aPTT?

Accepted Answer

An isolated prolonged PT suggests factor VII deficiency or vitamin K deficiency. Factor VII deficiency is a rare autosomal recessive disorder.