SN7 SHORT NOTE 5M

Describe the normal mechanism of haemostasis. Add a note on the interpretation of the coagulation screening tests.

Pathology · 2nd Professional — Para-Clinical

Haemostasis is the physiological process that arrests bleeding from an injured vessel while keeping the blood fluid elsewhere.

1. Vasoconstriction — transient reflex arteriolar spasm, augmented by endothelin → 2.
Primary haemostasis — platelets adhere to exposed collagen via von Willebrand factor and GpIb → Platelets are activated, change shape and release ADP and TXA2 → They aggregate through GpIIb/IIIa bridged by fibrinogen → the primary platelet plug → 3.
Secondary haemostasis — tissue factor activates the coagulation cascade → thrombin → fibrin → 4.
Stabilisation — polymerised fibrin and platelet aggregates form a permanent plug → 5.
Counter-regulation — tissue plasminogen activator and thrombomodulin limit the plug to the site of injury

Pathway	Factors	Test	Triggered by
Intrinsic	XII, XI, IX, VIII	APTT	Contact with a negatively charged surface
Extrinsic	VII	PT / INR	Tissue factor — the physiologically important trigger
Common	X, V, II (prothrombin), I (fibrinogen), XIII	Both PT and aPTT	—

Vitamin K dependent factors — II, VII, IX and X, plus proteins C and S; mnemonic "1972"
Factor VII has the shortest half-life, which is why the PT/INR rises first in warfarin therapy, liver disease and vitamin K deficiency

CLINICAL PEARL

Platelet disorders and coagulation disorders bleed differently.
Platelet problems give immediate, superficial, mucocutaneous bleeding — petechiae, epistaxis, menorrhagia.
Coagulation problems give delayed, deep bleeding — haemarthrosis, muscle haematoma. The history alone usually indicates which system is at fault.

PT	APTT	Platelets	Suggests
Prolonged	Normal	Normal	Factor VII deficiency; early warfarin, early liver disease, vitamin K deficiency
Normal	Prolonged	Normal	Haemophilia A (VIII) or B (IX); von Willebrand disease; heparin; antiphospholipid antibody
Prolonged	Prolonged	Normal	Common pathway defect; established liver disease; vitamin K deficiency
Prolonged	Prolonged	Low	DIC; massive transfusion
Normal	Normal	Low	Thrombocytopenia — ITP, hypersplenism, marrow failure
Normal	Normal	Normal	Vascular disorder, or a platelet function defect — test bleeding time

Aspirin inhibits cyclo-oxygenase irreversibly, and since platelets have no nucleus its effect lasts their whole 7–10 day lifespan — hence stopping it a week before surgery
Clopidogrel blocks the ADP receptor; abciximab blocks GpIIb/IIIa — each targets a specific step above
Warfarin acts on vitamin K dependent factors and is monitored by INR; heparin acts through antithrombin III and is monitored by aPTT
Haemophilia A is X-linked, presenting with haemarthrosis and deep haematomas; von Willebrand disease is autosomal and mucocutaneous, and is the commonest inherited bleeding disorder
Vitamin K deficiency of the newborn is prevented by a single intramuscular dose at birth — one of the most cost-effective interventions in medicine

All questions in Haemodynamic Disorders