W7CL1 - Blood Loss Anaemia & Approach to Disorders of Haemostasis Flashcards
Acute Blood Loss
Initially lose blood volume
- i.e. proportional loss of plasma & cells
Loss of up to ~20% of blood volume is tolerated
Hypovolaemic shock occurs with loss of ~40% of blood volume
Loss of ~ 50% of blood volume results in death
Blood volume can be estimated as approximately 70 mL/kg for adults (80 mL/kg in children and 100 mL/kg in neonates)
Response to Acute Blood Loss
Plasma volume expansion to maintain blood volume
May take up to 48h
RBC fraction relatively & progressively diluted as plasma volume expands
↑ EPO due to hypoxia => release of reticulocytes stored in the marrow within hours
Mechanisms of Haemorrhagic Diathesis
Overwhelm normal mechanisms of haemostasis
- e.g. trauma, surgery, giving birth (post-partum haemorrhage)
Defective mechanisms of haemostasis
- e.g. Haemophilia, Von Willebrand Disease, platelet disorders
Post-Partum Haemorrhage
Primary post-partum haemorrhage is defined as a blood loss of >500ml within 24 hours of the birth
It is further classified into minor (500-1000ml) or major (> 1000 ml) with a further sub classification into moderate (1000-2000 ml) or severe (> 2000 ml or > 30% of blood volume)
Post Partum Haemorrhage - History to Consider
If there is a known (previously diagnosed) history of a patient’s bleeding disorder it can be managed/prevented/reduced by targeted therapy especially when it is known a haemostatic challenge will occur
If there is no known coagulopathy with the patient or their relatives consider:
- type of bleeding e.g. rapid, diffuse, oozing
- onset, duration and severity
- drug taking (prescribed and non-prescribed)
Deficiency of Several of Vitamin K Dependent Factors
Clinical
- both warfarin therapy and liver disease => multiple factor deficiency
- may exhibit purpura and petechiae => compound factor deficiency
Assays
- PT, aPTT, factor assays, TT, Fibrinogen
- e.g. Liver function tests
Circulating Anticoagulant Present
Clinical - e.g. Heparin, factor antibodies (e.g. FVIII ‘inhibitors’), lupus anticoagulant Assays - aPTT with heparin neutralising agent - aPTT with 1:1 mix with plasma - aPTT with excess phospholipid - Bethesda assay
Consumptive Coagulopathy
Clinical
- e.g. Disseminated Intravascular Coagulation (DIC)
- inciting causes; sepsis, trauma, liver disease etc.
Assays
- DIC screen
- platelet count, PT, aPTT, FDP, D-dimer
Laboratory Investigations of Bleeding Disorders
Quality assurance
Platelet disorders (& VWF)
Coagulation factor disorders
Fibrinolysis
Quality Assurance Issues
Many haemostasis tests may be affected by medications
- e.g. aspirin affects platelet function
- must be known for meaningful interpretation of results
Haemolysis icterus, lipaemia may affect some assays & should not be analysed
Reference intervals should be established wherever possible
Measurement of Platelets
Automated analysers Must reliably distinguish platelets from RBC Measure: - platelets - large platelets - platelet distribution width - immature platelets - plateletcrit
Platelet Function Analysers - PFA-100
Mimics platelet adhesion and aggregation at a site of vessel injury
- high shear rates
- collagen exposure
Membrane coating
- collagen
- agonist (e.g. Epinephrine, ADP)
Time to membrane occlusion recorded (closure time)
Sensitive to platelet adhesion, aggregation deficiencies
Von Willebrand Factor
Used to determine the amount and functionality of VWF
Important in the diagnosis of Von Willebrand disease and ‘ruling out’ VWD in the investigation of platelet disorders
Assays include:
1. Von Willebrand factor antigen
- assesses the total amount of VWF
- commonly measured by ELISA
2. Ristocetin cofactor assay
- assesses functionality of VWF
- principle: washed platelets do not ‘agglutinate’ with ristocetin unless vWf present
3. Multimeric analysis
- assesses the components of VWF
- used in the classification of subtypes of vWD
Laboratory Studies for Platelet Disorders
Platelet concentration
Platelet function analyser (PFA-100)
Platelet aggregometry
Von Willebrand factor
Laboratory Studies of Coagulation Factor Disorders
Prothrombin time (PT) International normalized ratio (INR) Activated partial thromboplastin time (APTT) Thrombin time Factor assays
Formation and Detection of Fibrin Clot
Generation of ‘thrombin burst’
Thrombin cleaves fibrinopeptides A and B (highly negatively charged) from E domain
Changes charge of E domain from negative to positive
=> Positively charged E domains & Negatively charged D domains
Allows spontaneous fibrin polymer formation
Subsequently stabilized by FXIII
Prothrombin Time (PT)
Also referred to as one stage prothrombin time (OSPT)
Measures the clotting time of re-calcified plasma in the presence of optimal concentration of tissue extract (‘thromboplastin’) => “tissue factor induced plasma coagulation time”
Assesses ‘extrinsic’ (& common) pathway
- => measures activity of plasma clotting factors: VII, (X, V, II, I)
PT & INR
International Normalised Ratio (INR)
Require patient & control times
INR = Raw ratio adjusted to reflect sensitivity and/or reactivity of the thromboplastin reagent
INR = (test/normal)^ISI
ISI = International Sensitivity Index
- a derived figure assigned to the thromboplastin being used
- should preferably be close to 1.0
Interpretation of PT/INR
Common pathological causes for prolonged PT/increased INR
- Oral Anticoagulant Therapy (specifically Warfarin)
- liver disease
- vitamin K deficiency
- DIC
- inherited deficiency/defect of factor VII
Activated Partial Thromboplastin Time (aPTT)
Measures clotting time after contact activation with addition of phospholipid & CaCl2
FXIa and FXIIa must be activated using water wettable surface agents
Assesses intrinsic (& common) pathway
Measures activity of plasma coagulation factors
- XII, XI, IX, VIII ( X , V, II, I)
aPTT Interpretation
Artefactual prolongation of the aPTT may be due to:
- the presence of heparin in the sample
- difficult or slow collection
- addition of an incorrect volume of blood to the tube
- delay in mixing blood with the citrate anticoagulant
- suboptimal specimen storage or a prolonged interval between collection and testing
Common pathological causes of a prolonged aPTT:
- DIC
- liver disease
- heparin treatment
- circulating anticoagulant (‘inhibitor’)
- inherited factor deficiency
Thrombin Time
Thrombin is added to plasma and the clotting time measured
Affected by:
- the concentration of fibrinogen
- the presence of inhibitory substances e.g. heparin, fibrin/fibrinogen degradation products
TT Interpretation
Causes of prolonged thrombin time include:
- congenital deficiency/defect, hypofibrinogenaemia
- DIC
- increased FDPs
- heparin treatment
- hypoalbuninaemia
- paraproteinaemia
Fibrinolysis
Digestion of a fibrin clot
Plasminogen is activated to plasmin
Plasmin => digestion of fibrinogen & fibrin
- formation of fibrinogen/fibrin degradation products (FDPs)
Plasmin digestion of fibrinogen:
- early FDPs: fragments X and Y
- late FDPs: fragments D and E
Plasmin digestion of cross-linked fibrin:
- D-Dimers: D-D fragment
- other combinations
Inhibited by Plasminogen Activator Inhibitors (PAIs)
- e.g. α2 - antiplasmin
Assessment of Fibrinolysis
Fibrin/Fibrinogen Degradation Products (FDPs) - special tube with antifibrinolytic agent & thrombin - latex agglutination method - contain antibodies to fragments D & E - normal values: < 10 mg/mL D-dimers - latex agglutination method - detects cross-linked fibrin D-dimers - normal values: < 200 mg/L
