Blood coagulation, Haemostasis, and its Investigation Flashcards
Haemostasis - define
Protective process evolved in order to maintain a stable physiology
“An explosive reaction designed to curtail blood loss, restore vascular integrity and ultimately preserve life”
The Horseshoe Crab - define
Limulus Polyphemus
‘A primitive coagulation pathway can be initiated by endotoxin’
The Haemolymph contains what
The Haemolymph contains amebocytes:
Proteins of the coagulation system
Proteins & peptides of the immune system
Haemostasis - life preserving processes = describe
Life preserving processes designed to maintain blood flow
Respond to tissue injury
Curtail blood loss
Restore vascular integrity & promote healing
Limit infection
Haemostasis - 4 components
Four Key Components Endothelium Coagulation Platelets Fibrinolysis
What makes a Blood Clot?
Fibrin mesh
Platelets
Red blood cells
Coagulation - decribe process briefly
Vessel damage
& Blood loss
Vascular spasm
Platelet Plug forms
Coagulation
Primary haemostasis - description
Primary haemostasis:
Vasoconstriction (immediate)
Platelet adhesion (within seconds)
Platelet aggregation and contraction (within minutes)
Secondary haemostasis - description
Secondary haemostasis:
Activation of coagulation factors (within seconds)
Formation of fibrin (within minutes)
Fibrinolysis - description
Fibrinolysis:
Activation of fibrinolysis (within minutes)
Lysis of the plug (within hours)
Clot strength in terms of stages
Initiation = slow rise ~ 0
Amplification = rapid increase
Stable clot = strongest
Lysis = steady decrease to 0
Characteristic of heamostasis at rest
Haemostasis at rest: triggers and cofactors separated
vWF function under shear forces
Under shear forces, self-association of plasma VWF occurs surface of platelets + endothelial cells = creating fibrillar structures that further facilitate platelet adhesion
Platelet aggregation function
Prevents excessive blood loss at site of injury
When does seconday haemostasis take place
This plug of activated platelets, localised to the site of injury, provides the phospholipid surface upon which Secondary Haemostasis takes place
When does primary haemostasis take place
Primary hemostasis = when platelets attach to damaged endothelium, adhesion allows the platelets to undergo a shape change = aggregation
Once adhered to each other a temporary platelet plug is created
Brief process of haemostatic plug formation
Vessel constriction
Formation of an unstable platelet plug
platelet adhesion
platelet aggregation
(above is primary haemostasis)
Stabilisation of the plug with fibrin blood coagulation (secondary)
Dissolution of clot and vessel repair fibrinolysis
Fibrin mesh function
Fibrin mesh binds and stabilises platelet plug and other cells
Fibrinogen (I) function
Forms clot (fibrin)
Prothrombin (II) function
Its active form (IIa) activates I, V, VII, XIII, protein C, platelets
Tissue factor (III) function
Co-factor of VIIa
V (proaccelerin, labile factor)
function
Co-factor of X with which it forms the prothrombinase complex
VI function
Unassigned - old name of Factor Va
VII (stable factor) function
Activates IX, X
VIII (antihaemophilic factor)
function
Co-factor of IX with which it forms the tenase complex
IX (Christmas factor)
function
Activates X: forms tenase complex with factor VIII
X (Stuart-Prower factor)
function
Activates II: forms prothrombinase complex with factor V
XI (plasma thromboplastin antecedent)
function
Activates XII, IX and prekallikrein
XII (Hageman factor)
function
Activates prekallikrein and fibrinolysis
XIII (fibrin-stabilizing factor)
function
Crosslinks fibrin
von Willebrand factor
function
Binds to VIII, mediates platelet adhesion
FVII deficiency effect
FVII deficiency causes bleeding
FXII deficiency effect
FXII deficiency not associated with bleeding
What does each action in waterfall hypothesis require
Each reaction requires:
Ca2+
Phospholipid
± Specific co-factors
Describe clotting cascade
TF is outside the lumen
Formation of TF-FVIIa complex
Recruitment of FX and formation of thrombin
Fibrinolysis (clot dissolution) function
Main function:
clot limiting mechanism
repair and healing mechanism
Fibrinolysis (clot dissolution) - characteristic of the process
Series of tightly regulated enzymatic steps
- feedback potentiation & inhibition
Fibrinolysis (clot dissolution) - key players
Main Key players:
Plasminogen
Tissue plasminogen
activator (t-PA) & urokinase (u-PA)
Plasminogen activator inhibitor -1 and -2
α2-plasmin inhibitor
Formation of plasmin - describe
Plasminogen → plasmin
through tissue plasminogen
activator, tPA
Formation of plasmin - effect
D dimers are generated when cross-linked fibrin is degraded.
FDP (Fibrin degradation products) are generated if non-cross linked fibrin or fibrinogen is broken down.
Uses of tPA and streptokinase
tPA and a bacterial activator, streptokinase, are
used in therapeutic thrombolysis for myocardial
infarction (Clot busters)
Describe haemostasis and thrombosis in relation to one another
Haemostasis and Thrombosis: a Balance
Normal haemostasis: a state of equilibrium
Fibrinolytic factors,
anticoagulant proteins
VS
Coagulation factors,
platelets
Cuase of thrombosis
↑ Coagulation
factors,
platelets
and
↓ Fibrinolytic factors,
anticoagulant proteins
Chronic venous insufficiency effects
Atrophic changes
Hyperpigmentation
Ulceration
Infection
Cause of bleeding
↑ Fibrinolytic
factors,
Anticoagulant
proteins
and
↓ Coagulation factors,
Platelets
Define ecchymosis
Easy bruising
(ecchymosis) = a discoloration of the skin resulting from bleeding underneath, typically caused by bruising
Virtually all bleeding disorders and often in normals
Principles of clotting tests
Incubate plasma with reagents necessary for coagulation:
Phospholipid, co-factors
Trigger or activator
Calcium
Measure time taken to form fibrin clot
Prothrombin time (PT) - define and describe characteristics
Prothrombin time (PT) is a blood test that measures how long it takes blood to clot
Sensitive to extrinsic pathway and to a lesser extent common pathway
TF driven
APTT (Activated Partial Thromboplastin Time) - define and describe characteristics
The partial thromboplastin time (PTT) or activated partial thromboplastin time (aPTT or APTT) is a blood test that characterizes coagulation of the blood
Sensitive to intrinsic pathway and to a lesser extent common pathway
Contact activated
TT (Thrombin Time) - define and describe characteristics
Thrombin time (TT), a blood test that measures the time it takes for a clot to form in the plasma of a blood sample containing anticoagulant, after an excess of thrombin has been added.
Sensitive to defects in conversion of fibrinogen to fibrin
Overview of Coagulation Pathways
Activated partial thromboplastin time
(APTT) = 1. Surface activating agent (e.g. kaolin)
2. Phospholipid
3. Calcium
= Intrinsic pathway
PT = 1. Thromboplastin
Tissue factor
Phospholipid
2. Calcium
= Extrinsic pathway
both feed into
Thrombin time = thrombin
= common pathway
All combining to form
= fibrin clot
Accuracy of haemostasis laboratory tests depends on what
Accuracy of haemostasis laboratory tests depends on the quality of the specimen submitted
Effect of under filling the tube
Under filling the tube yields grossly inaccurate results.
Problems with blue-top tube = pre-analytical error
Partial fill tubes
Vacuum leak and citrate evaporation
Problems with phlebotomy = pre-analytical error
(using a needle to take blood from a vein)
Heparin contamination Wrong label Slow fill Underfill Vigorous shaking Difficult venepuncture
Laboratory errors = pre-analytical error
Delay in testing
Prolonged incubation at 37°C
Freeze/thaw deterioration
Biological effects that are pre-analytical errors
Hct ≥55 or ≤15
Lipaemia, hyperbilirubinaemia, haemolysis
Mixing studies - describe
Mix patient and normal plasma in equal volumes (50:50 mix)
Repeat abnormal coagulation test
Test normalises – factor deficiencies
Test remains abnormal – inhibitor (usually antibody)
What tests do you do if you obtain an normal PT + abnormal APTT, include results
Repeat at 50:50 mix
If 50:50 abnormal:
Test for inhibitor activity:
Specific factors: VIII,IX, XI
Non-specific (anti-phospholipid Ab)
If 50:50 mix normal
Test for factor deficiency:
Isolated deficiency in intrinsic pathway (factors VIII, IX, XI)
Multiple factor deficiencies (rare)
What tests do you do if you obtain an abnormal PT + normal APTT, include results
Repeat at 50:50 mix
If 50:50 abnormal:
Test for inhibitor activity:
Specific: Factor VII (rare)
Non-specific: Anti-phospholipid (rare)
If 50:50 mix normal:
Test for factor deficiency:
Isolated deficiency of factor VII (rare)
Multiple factor deficiencies (common) (Liver disease, vitamin K deficiency, warfarin, DIC)
What tests do you do if you obtain an abnormal PT + abnormal APTT, include results
Repeat at 50:50 mix
If 50:50 abnormal:
Test for inhibitor activity:
Specific : Factors V, X, II, I (rare)
Non-specific: anti-phospholipid (common)
If 50:50 mix normal:
Test for factor deficiency:
Isolated deficiency in common pathway: Factors V, X, Prothrombin, Fibrinogen
Multiple factor deficiencies (common) (Liver disease, vitamin K deficiency, warfarin, DIC)
What tests do you do if you obtain an abnormal PT + abnormal APTT, include results
Repeat a normal test
If abnormal: Dysfibrinogenaemia Abnormal fibrinolysis (e.g. α2 anti-plasmin def) Elevated FDPs
If normal: Von Willebrand’s disease Platelet disorder FXIII deficiency Non-coagulation defect (e.g. vascular disorder)
D-dimer Testing - characteristics
A measure of the D-dimer, a fibrin degradation product
Found elevated in the situation of enhanced fibrinolysis (Thrombosis, DIC)
Not specific for thrombosis also elevated as an acute phase reactant
A Negative result is useful if clinical suspicion of VTE is low
