Coagulation: Pathways, Tests, and Anticoagulant Monitoring
Core concepts
Know the coagulation cascade in practical terms: the intrinsic pathway (factors VIII, IX, XI, XII) is measured by the aPTT; the extrinsic pathway (factor VII) by the PT/INR; the common pathway (X, V, II, fibrinogen) by both. Understand that the PT is most sensitive to factor VII (shortest half-life), making it the earliest marker of liver synthetic failure or warfarin effect. Know the INR calculation (patient PT / mean normal PT)^ISI and why it standardizes warfarin monitoring across reagent lots.
Platelets and special tests
Know the role of platelets in primary hemostasis and how platelet function is assessed (aggregometry, PFA-100). Understand von Willebrand factor's role and the mixing study concept: correction of a prolonged aPTT with normal plasma suggests factor deficiency; failure to correct suggests an inhibitor (e.g., lupus anticoagulant, factor VIII inhibitor). Know heparin monitoring: aPTT for unfractionated heparin, anti-Xa for low-molecular-weight heparin.
Traps
Questions test that a normal PT with prolonged aPTT points to the intrinsic pathway, and that a mixing study result must be interpreted at both immediate and incubated time points. Beware the trap that a prolonged aPTT always means bleeding risk - lupus anticoagulant prolongs aPTT but is prothrombotic in vivo.
Worked example
PT normal, aPTT prolonged, mixing study corrects → factor deficiency in the intrinsic pathway (VIII, IX, XI, XII). If it does not correct, suspect an inhibitor. Rule of thumb: corrects = deficiency; doesn't correct = inhibitor.