Point of care coagulation testing Flashcards
(37 cards)
What is point-of-care testing?
POCT is a rapid specific testing of bodily fluids at the bedside
What do the prothrombin time and activated partial thromboplastin time provide information about?
The 1st phase of clotting up to fibrin formation
What are the advantages of post-of-care coagulation testing?
- fast turnaround
- whole blood used to allow interaction between plasma clotting factors, plts and red cells
- real time visual display of clot evolution
- reduction in non-evidence-based transfusion
What are the disadvantages of point of care coagulation testing?
- measure coagulation under artificial conditions rather than flow within an endothelialized blood vessel
- training and competency of non-haematological staff members required
- quality standards more difficult to institute outside the lab
- may be more expensive than conventional tests
What does thromboelastography do?
It provides continuous measurement and display of the ciscoelastic properties of a whole blood sample from the initial phase of fibrin formation to clot retraction and fibrinolysis
How does thromboelastography work?
360 micro litres of whole blood is added to activators in 2 disposable cups at 37 degrees. A pin on a torsion wire is immersed in the blood and the cup rotates in each direction for 10s. As the blood clots the rotational movement of the cup is transmitted to the pin. A transducer converts torsion on the pin to the TEG tracing.
How does the ROTEM analyser work?
300 microlitres of whole blood with activators is incubated in a disposable cuvette and placed in a 37 degree heater. A pin on a fixed axis stabilized by a ball bearing is immersed into the blood. The pin is rotated in either direction while the cuvette stays stationary. The pin encounters resistance as the clot strength increases. Detected by a light emitting diode, a mirror and an electronic camera and is translated into a tracing which various parameters are derived from.
What signifies the concentration of soluble clotting factors in the plasma on the TEG and ROTEM analysers?
TEG = R (reaction time) (time until initiation of fibrin formation, taken as 2mm amplitude on the tracing)
ROTEM = CT (clotting time), INTEM and EXTEM
What does K time and CFT time mean on the TEG and ROTEM analysers?
The time period for the amplitude of the tracing to increase from 2 to 20mm.
It’s a measurement of clot kinetics.
ROTEM = CFT (clot formation time)
What does the α angle mean in TEG and ROTEM analysers?
Angle between a tangent to the tracing at 2mm amplitude and the horizontal midline.
Indicates the rapidity of fibrin build up and cross-linking.
What is the MA (maximum amplitude) in TEG and MCF (maximum clot firmness) in ROTEM analysers?
The greatest vertical width achieved by the tracing reflecting max clot strength.
Indicates the number and function of platelets and fibrinogen concentration.
What is the CL30/LY30 in TEG/ROTEM analysers?
Per cent reduction in amplitude 30 min after MA
Indicates clot stability and fibrinolysis
What do the TEG and ROTEM analysers incorporate in it’s cuvettes?
TEG = kaolin
ROTEM = tissue factor in the EXTEM cuvette (extrinsic pathway allowing faster assessment of clot formation and fibrinolysis) and contact activator in the INTEM cuvette (intrinsic pathway)
Both devices have heparinase containing cuvettes to allow monitoring of coagulation while a patient is fully heparinized eg on cardiopulmonary bypass.
What are the strengths of TEG/ROTEM analysers?
- rapid assessment of overall coagulation status
- derived parameters can be used to guide administration of blood products
- analyse all 3 phases of coagulation, initiation, amplification, propagation - reflecting the interactions of the cellular and plasma components of coagulation and the fibrinolytic system activity
- TEG and ROTEM based transfusion algorithms reduce rates of transfusion of blood products and reduces rates of surgical re-exploration
- can detect hypercoagulable states in post-op patients (best predictive parameter being an increased MA or MCF
- useful to detect inadequate heparin reversal or heparin rebound
What are the limitations of TEG/ROTEM analysers?
- can’t detect conditions affecting platelet adhesion (eg Von Willebrands) as can’t reflect endothelium contribution to coagulation
- preop baseline TEG/ROTEM = poor prediction of postop bleeding
- will not reflect effects of hypothermia as measurement is taken at 37 degrees
- insensitive to aspirin and clopidogrel
- not standardised in terms of sample collection/processing, activators used and other modifications so can’t compare results between institutions
- ?adequate quality control as ran away from lab
- no agreement between TEG and ROTEM
What is the role of platelets?
- haemostasis - when a vessel is damaged, platelets adhere to the exposed subendothelium, resulting in activation, continued recruitment and formation of primary platelet plug
- provide phospholipid surface rich in receptors for activation of soluble coagulation factors
- produces thrombin burst for stable fibrin clot formation
What does the PFA-100 measure?
- aspirin
Cheap, rapid, sensitive assessment of Von willebrand disease and aspirin effects
But wide normal range, influenced by thrombocytopenia and anaemia, can’t measure thienopyridines and GPIIb/IIIa inhibitors
What does Plateletworks measure?
- aspirin
- P2Y12 antagonists
- GPIIb/IIIa antagonists
Cheap, rapid, provides a platelet count, assessement of all 3 classes of antiplatelet agents.
Requires a cell counter, blood must be tested within 10mins.
What does TEG platelet mapping measure?
- aspirin
- P2Y12 antagonists
- GPIIb/IIIa antagonists
Assesses all 3 classes of antiplatelet.
Expensive and slow.
What does VerifyNow measure?
- aspirin
- P2Y12 antagonists
- GPIIb/IIIa antagonists
Rapid assessment of all 3 classes of antiplatelets.
Expensive, effects of thrombocytopenia unknown, interpretation may be complex
What does MultiPlate measure?
- aspirin
- P2Y12 antagonists
- GPIIb/IIIa antagonists
Cheap, assessment of all 3 classes of antiplatelets; good agreement with light transmission aggregometry.
Manuel pipetting required - increased staff training, relatively short shelf life of reagents.
How do platelet function analysers measure platelet adhesion and aggregation?
Under high shear conditions.
A vacuum induces flow of citrated whole blood across a collagen-coated membrane with a 150 micro m aperture to which the platelet activators epinephrine or ADP are bonded. This results in platelet activation and aggregation.
The time taken for the platelet plug to occlude flow is defined as the closure time (CT) (normally 5-8mins) and reflects platelet function.
How does Von Willebrand’s disease show up on platelet function analysers?
Both epinephrine and ADP CTs are prolonged in qualitative and quantitative abnormalities of Von Willebrand factor (affects 1% of patients).
How does Plateletworks measure platelet function?
Uses impedance cell counter to perform a platelet count in a baseline whole blood EDTA sample. This is compared to platelet count in the citrated sample plus agonist (arachidonic acid or ADP). The agonist causes platelet activation, leading to formation of aggregates which exceed the threshold for platelet size and are not counted.
