29 - Haemostasis Flashcards
Primary haemostasis
1)
2)
3)
1) Vasoconstriction
2) Platelet adhesion
3) Platelet aggregation
Occurs seconds to minutes after injury
Secondary haemostasis
1)
2)
1) Activation of coagulation factors
2) Formation of fibrin
Occurs minutes after injury
Fibrinolysis
1) Activation of fibrinolysis
2) Lysis of blood clot
Occurs minutes to hours after injury
Virchow’s triad
1)
2)
3)
Description of inappropriate clotting (thrombosis) Factors that can lead to thrombosis are: 1) Blood composition 2) Vessel wall 3) Blood flow
How is functional integrity of vascular wall tested?
It can’t be tested at the moment. No functional test
Coagulation system aspects
1)
2)
3)
1) Complex system of proteins in plasma
2) Interact with platelets and blood vessel wall
3) Clotting and bleeding are opposite ends of spectrum
Starter of the coagulation cascade
Tissue factor (TF) released by damaged epithelial cells
Key enzyme in coagulation cascade that must be controllled
Thrombin
Initiation phase of coagulation 1) 2) 3) 4)
1) Blood vessel wall injured, tissue factor is exposed.
2) TF binds FVIIa, or binds FVII and converts it to FVIIa
3) TF/FVIIa complex activates FIX and FX.
4) FXa binds to FVa on cell surface
Amplification phase of coagulation 1) 2) 3) 4)
1) FXa/FVa complex bound to endothelial wall converts small amounts of prothrombin to thrombin
2) Small amount of generated thrombin activates FV, FVIII, FXI and local platelets
3) FXIa converts FIX to FIXa (activated)
4) Activated platelets bind to FVa, FVIIIa, FIXa
Propagation phase of coagulation
1)
2)
3)
1) FXIIIa/FIXa complex on platelets activates FX on the surface of active platelets
2) FXa and FVa convert large amounts of prothrombin to thrombin in ‘thrombin burst’
3) Thrombin burst leads to stable formation of fibrin clot
Factors that convert prothrombin to thrombin
FXa and FVa in association with each other
Thrombin funciton
Converts fibrinogen to fibrin.
Activates platelets.
Can negatively-regulate upstream factors in coagulation cascade
Thrombin inactivation
Thrombomodulin-APC binds thrombin, regulates ability to cleave fibrinogen.
Irreversibly inhibited by antithrombin (process increased x1000 by heparin)
Non-coagulation roles of thrombin
1)
2)
3)
1) Activates protease-activated receptors (PARs)
2) Involved in angiogenesis, inflammation, atherosclerosis, neutrophil, monocyte migration
3) Could be involved in tumour spread
Caveat to haemostatic testing
Not reflective of physiology
Three broad types of haemostatic testing
1) Predict bleeding
2) Predict clotting
3) Monitor drugs used to manipulate haemostatic system
Haemostatic tests on platelets
Look at number, function, appearance
Are there global tests for risk of bleeding?
Yes
Are there global tests for risk of clotting?
No
Are there global tests to monitor anticoagulant drugs?
Yes
Are there specific assays for risk of bleeding?
Yes
Are there specific assays for risk of clotting?
Yes
Are there specific assays to monitor anticoagulant dugs?
Yes
Things involved in haemostasis that can be genetically tested
Risk of clotting, bleeding disorders. Maybe different genotypes react differently to anticoagulant drugs.
Key principles for coagulation tests 1) 2) 3) 4) 5)
1) Sample integrity is crucial (EG: can’t have too little blood in sample)
2) Standard curves made
3) Control samples
4) Duplicate testing
5) Multiple consistent tests before diagnosing a patient
Global tests for bleeding
1)
2)
3)
1) APTT
2) PT
3) Thrombin generation
APTT
Activated Partial Thromboplastin Time Take blood at 37 degrees. Add Ca2+ to reverse citrate in tube. Measure time for fibrin to form. Tests cascade from FXII to fibrin
PT
Prothrombin time.
Add Ca2+ to reverse citrate in tube
Thromboplastin activated.
Tests cascade from FVII to fibrin
Bleeding test used to test effect of heparin
Activated partial thromboplastin time (APTT)
Bleeding test used to monitor warfarin
Prothrombin time (PT) test
Examples of specific tests for bleeding
1)
2)
3)
1) Factor assays
2) Collagen assays
3) Fibrinogen assay
Pros and cons of functional clot-based assays
Better reflect physiology, but are technically difficult to perform.
Pros and cons of chromogenic assays
More reproducible, but questionable clinical relevance quesitonable
Problems with global tests
Each lab does it slightly differently with different reagents.
Solution to differing reagents used between labs on global assays
Make international normalised ratio (INI), which is PT result/average of PT results from lab. This fraction is then raised to the power of ISI (international sensitivity index), based on which reagents were used.
Example of a global functional assay
APTT.
Factor deficiency, lupus anticoagulant, heparin monitoring
Example of a specific factor assay
FXIII assay
Example of a chromogenic assay
Anti-FXa assay.
For heparin monitoring.
