M9 - Haemostasis

What is haemostasis?

The process by which bleeding is spontaneously arrested within the body. It involves the formation of a fibrin clot (thrombus) and its eventual breakdown and removal.

What are the two main functions of haemostasis?

1. Prevention of blood loss from an intact vessels

2. Arrest of bleeding from damaged vessels via vascular reaction, platelet plug formation, and plasma protein coagulation.

Name the four systems involved in haemostasis

1. Vascular system – vasoconstriction.

2. Platelet system - platelet plug

3. coagulation system – fibrin clot

4. Fibrinolytic system - clot dissolution

What does the vascular system do in haemostasis?

Injured blood vessels initiate vasoconstriction, reducing blood flow to the site of injury. Endothelin release and reflex vasoconstriction both contribute.

How does the platelet system contribute to haemostasis?

Injury exposes subendothelial collagen, initiating platelet activation (adhesion, shape change, granule release of ADP and TXA₂, and aggregation) to form a primary haemostatic plug.

What is the role of the fibrinolytic system?

Initiated when fibrin is formed. Injured endothelial cells release plasminogen activators —> plasmin, which breaks down fibrin into fibrin degradation products. Endothelium eventually replaces the fibrin.

What is primary haemostasis and what does it depend on?

Medicated by platelets and vessel wall interaction. Depends on; Vascular integrity, biochemical integrity of endothelium and subendothelium, and platelet reactivity (numbers and normal function).

What is secondary haemostasis and what does it depend on?

Medicated by the coagulation cascade —> formation and stabilisation of fibrin clot. Depends on adequate procoagulants (coagulation factors) and natural anticoagulants (Antithrombin III, protein C, Protein S).

What are the three groups of coagulation factors?

Group 1: Vitamin K-dependent (II, VII, IX, X)

Group 2: Thrombin-Sensitive (I, V, VIII, XIII)

Group 3: Contact factors (XI, XII, prekallikrein, HMWK).

Which coagulation factors are vitamin K-dependent, and what are their active forms?

Factor II (prothrombin), VII (proconvertin), IX (Christmas factor), and X (Stuart Prower factor). All are active as serine proteases.

What are the thrombin-sensitive factors and their active forms?

Factor I (fibrinogen) → fibrin subunit.

Factor V (Labile factor) → cofactor.

Factor VIII (Anti-haemophilic factor) → cofactor

Factor XIII (Fibrin stabilising factor) → transglutaminase

What are the contact factors and why are they significant?

XI (Plasma thromboplastin antecedent), XII (Hageman/contact factor), Prekallikrein (Fletcher), HMWK. They are intrinsic pathway proteins activated by negatively charged surfaces — initiate contact activation.

Describe the intrinsic pathway of the coagulation cascade.

Triggered by exposure to negatively charged surfaces (phospholipids, collagen, subendothelium). Contact activation → XIIa → XIa → IXa + VIIIa → Xa + Va → thrombin → fibrin.

Describe the extrinsic pathway of the coagulation cascade.

Triggered by tissue damage exposing tissue factor (TF). TF complexes with VIIa → activates Xa + Va → thrombin → fibrin clot.

What is the common pathway of coagulation?

Converging point of intrinsic and extrinsic pathways: Xa + Va (prothrombinase complex) → converts prothrombin to thrombin → fibrinogen to fibrin (soft clot) → XIIIa crosslinks to form hard clot.

What role does Factor XIII play and why is it important?

Factor XIII (activated to XIIIa by thrombin) crosslinks fibrin monomers via the D-fragment to form a stable, hard fibrin clot (from soft clot to hard clot).

What are the key properties of the coagulation cascade?

Enzymatic cascade of serine proteases (mostly liver-produced, several requiring Vit K). Also includes 3 protein cofactors. Requires Ca²⁺, is localised to injury site, and is reversible via plasmin.

What is the role of thrombin (IIa) in the coagulation cascade?

Central enzyme of the cascade. Converts fibrinogen → fibrin. Also activates Factors V, VIII, and XIII. Provides positive feedback to amplify coagulation.

Name three natural anticoagulants and their role

1. Antithrombin III - inhibits thrombin and factor Xa

2. Protein C - inactivates Va and VIIIa (activated by thrombin and thrombomodulin)

3. Protein S – cofactor for protein C. Also, TFPI inhibits the TF: VIIa complex.

What screening tests are used for haemostasis?

PT, APTT, TCT (thrombin clotting time), fibrinogen, D-dimer, and skin bleeding time

What does the prothrombin time measure, and what is its reference range?

Measures the extrinsic pathway. Add thromboplastin + CaCl₂ (Thromborel S) to patient plasma at 37°C. Reference range: 11.0–15.0 sec. Converted to INR to standardise results.

How is the INR calculated and why is it used?

PR = PT(patient) ÷ PT(control). INR = PR^ISI. ISI (International Sensitivity Index) corrects for thromboplastin source variation. INR standardises results worldwide and is used to monitor warfarin therapy.

What does the APTT measure and what is its reference range?

Measures the intrinsic pathway. Uses phospholipid + surface activator (e.g. Pathromtin), then CaCl₂ to initiate. Reference range: 25–40 sec (reagent-dependent). Used to monitor heparin therapy.

What does the Thrombin Clotting Time (TCT) measure and what is its reference range?

Directly measures the conversion of fibrinogen → fibrin. Thrombin is added to patient plasma at 37°C. Reference range: 14–18 sec (at Griffith). Prolonged by heparin, low/abnormal fibrinogen, or DIC.

What is the Clauss technique for fibrinogen and what is the reference range?

Patient plasma diluted 1:10, combined with kaolin, then thrombin added. Clotting time read from a standard curve. Reference range: 1.5–4.0 g/L. Also measurable by biochemical, immunological, or clot weight methods.

What are D-dimers and what are their indications

D-dimers are fragments formed when Factor XIIIa-crosslinked fibrin is broken down by plasmin. They indicate fibrin clot lysis. Used to diagnose DIC, Pulmonary Embolus (PE), Deep Vein Thrombosis (DVT), and others.

What is a mixing study and how does it distinguish factor deficiency from inhibitor?

Patient plasma mixed with normal plasma. If a factor deficiency: normal plasma replaces the factor → PT/APTT corrects. If an inhibitor present: normal plasma is also inhibited → no correction. Can use normal plasma, adsorbed plasma, or normal serum.

What does the Skin Bleeding Time assess and what is its reference range?

A small skin cut is made; time to stop bleeding is measured. Assesses platelet function, platelet numbers (thrombocytopenia), and capillary function. Reference range: 2–10 minutes.

What are the expected screening test results for a patient on Warfarin?

PT: increased. APTT: ± increased. TCT: normal. Fibrinogen: normal. Warfarin affects the extrinsic pathway (Vit K-dependent factors II, VII, IX, X). Monitored by INR.

What are the expected screening results on a patient with heparin.

PT: mildly increased. APTT: increased. TCT: increased. Fibrinogen: normal. Heparin affects the intrinsic pathway and is monitored by APTT. Prolonged TCT reflects its direct inhibition of thrombin.

What are the expected screening results in DIC vs Liver Disease?

DIC: PT ↑, APTT ↑, TCT grossly ↑, Fibrinogen reduced (consumed). Liver Disease: PT ↑, APTT ↑, TCT normal/increased, Fibrinogen normal/reduced (impaired synthesis of clotting factors).