Coagulopathy Testing

Outline the methods for assessing coagulation, platelet function and fibrinolysis

Coagulation Factors

All these tests measure how long it takes to make fibrin. They evaluate different parts of the coagulation cascade, which help localise where a coagulopathy may be occurring.

In these tests:

  • Citrate is added to blood
    Binds calcium and prevents clotting.
  • Sample is centrifuged
  • Plasma decanted
  • Calcium (to replace the calcium lost by binding to citrate) and a reagent is added
  • Time taken to clot measured

Prothrombin Time/INR

The prothrombin time measures the extrinsic pathway. Tissue factor has to be added to the sample in order start clotting - this is why it is known as the extrinsic pathway as a substance extrinsic to the sample must be added. As the PT varies significantly between different labs, the INR is used to allow values to be compared.

Any disorder of the extrinsic or common pathways will prolong the PT, i.e. deficiency or inhibition of:

  • Factor VII
  • Factor X
  • Factor II (prothrombin)
  • Factor V
  • Factor I (fibrinogen)

Although warfarin affects factors in all three pathways, its clinical effects are measured using INR. This is because:

  • Factor VII has the shortest half-life of the clotting factors affected by warfarin
    Therefore so its levels will fall the quickest.
  • Therefore a fall in Factor VII levels is the earliest indication of changes in coagulation status due to warfarin
  • As factor VII is only in the extrinsic pathway, the PT/INR are the only tests which can evaluate its function

(Activated) Partial Thromboplastin Time

The partial thromboplastin time measures the intrinsic pathway, which begins produce fibrin when activated by the addition of phospholipid to the sample (phospholipid is contained in platelets, and so is not technically "extrinsic"). The activated partial thromboplastin time is the same test, except an activating agent is added to speed up the reaction.

Any disorder of the intrinsic or common pathways will prolong the APTT, i.e. deficiency or inhibition of:

  • Factor XI
  • Factor IX
  • Factor VIII
  • Factor X
  • Factor V
  • Factor II (prothrombin)
  • Factor I (fibrinogen)

Heparin affects both sides of the pathway (IIa, IXa, Xa, XIa) however typically affects intrinsic factors more than extrinsic.

In addition, anti-phospholipid antibodies will also prolong the APTT by binding the added phospholipid.

Activated Clotting Time

Activated Clotting Time is used to for the dosing and reversal of heparin in cardiopulmonary bypass and other extracorporeal circuits.

Fresh whole blood is added to a tube with an activator (e.g. glass beads) to stimulate the intrinsic pathway. The time until clot formation is measured in seconds. Different activators will have different normal ranges, and target ranges for the circuit in use.

Platelet Function

Evaluate how well platelets aggregate in response to factors like ADP, collagen, arachidonic acid, and adrenaline (i.e., endogenous stimulators of platelet aggregation).

In this test, the aggregating agent is added to a tube of platelets, and the change in turbidity measured. Different patterns of response (or non-response) can be diagnostic of different platelet function disorders.

Point of Care Testing

Point of care coagulation testing:

  • Involves testing of whole blood
    Traditional testing uses plasma only.
    • Therefore includes the cell-based model of coagulation
      May better represent actual clotting function compared with traditional coagulation factor testing.
  • Provides information on all phases of clotting

Viscoelastic Methods

Include:

  • Thromboelastography (TEG)
    Continuous measurement and display of viscoelastic properties of a blood sample from initial fibrin formation to clot retraction, and ultimately fibrinolysis. Involves:
    • A known volume (typically 0.36ml) of whole blood added to activators in two disposable cuvettes (cups) heated to 37°C
      • Contact activators (such as kaolin) are added to the blood to accelerate clotting
      • A heparinase cuvette is also commonly used so clotting function can be measured during full anticoagulation (e.g. CPB)
    • Pin attached to torsion wire immersed into blood Torsion on the pin is converted (by a transducer) into a TEG tracing.
    • Cuvette rotates through 4°45′ in alternate directions
      Each rotation takes 10s.
    • Pin initially remains stationary as it rotates through the un-clotted blood
      This is represented by a straight line on the tracing.
    • As blood clots, cup rotation exerts torque on the pin
    • The stronger the blood clot, the greater the torque exerted on the pin
  • Rotational Thromboelastometry (ROTEM)
    Modified version of TEG:
    • A pin fixed to a steel axis is rotated in blood via movement of a spring
      The cuvette remains stationary.
    • Two samples are used:
      • Tissue factor is added to measure the extrinsic pathway (known as the ENTEM cuvette)
      • Contact activator is added to measure the intrinsic pathway (INTEM cuvette)
    • Impedance to rotation is detected by an optical system:
      • LED
      • Mirror on the steel axis
      • Electronic camera
    • Uses different reference ranges and nomenclature to TEG

Advantages and Disadvantages of TEG/ROTEM

Advantages Disadvantages
Rapid compared with traditional testing Still measures coagulation in artificial conditions
Uses whole blood, providing a more complete picture of plasma-RBC-platelet interaction Does not measure contribution of endothelium and therefore conditions affecting platelet adhesion (e.g. von Willebrand's disease)
Real-time display of clot evolution Harder to institute QA outside of laboratory
Reduces non-evidence-based transfusion Measurement methodology is not yet standardised between institutions
Predictive of post-operative hypercoagulable states Baseline measurement does not predict post-operative bleeding
Very sensitive to heparin effect Does not measure effect of hypothermia
Requires training and competency of non-lab staff
More expensive than traditional testing

Interpreting TEG/ROTEM

Note that reference ranges are not included here, and will vary depending on the:

  • Technique (TEG/ROTEM) used
  • Activator used
  • Adjuvants added
    e.g. Citrated vs. recalcified samples.

Parameter (TEG) Parameter (ROTEM) Definition Relevance
R (reaction) time CT (clotting time) Time until 2mm amplitude Time until initial fibrin formation, dependent on plasma concentration of clotting factors
K time CFT (clot formation time) Time for amplitude to increase from 2-20mm Measurement of clot kinetics (clot amplification), dependent on fibrinogen
α angle α angle Angle between the tangent to the tracing at 2mm and the midline Rapidity of fibrin formation and cross-linking. Alternate measure of clot kinetics, dependent on fibrinogen
MA (maximum amplitude) MCF (maximum clot thickness) Greatest amplitude Indicates point of maximal clot strength, dependent predominantly on platelets (80%) and fibrinogen (20%), binding via GPIIb/IIIa. Treatment with platelets or DDAVP.
CL 30 (clot lysis 30) LY 30 Percent decrease in amplitude 30 minutes after MA Clot stability, dependent on fibrinolysis. Reduced CL 30 can be treated with an antifibrinolytic, such as TXA

References

  1. Krafts K. Clot or Bleed: A Painless Guide for People Who Hate Coag. Pathology Student.
  2. Activated Clotting Time - Practical Haemostasis.
  3. Srivastava A, Kelleher A. Point-of-care coagulation testing. Contin Educ Anaesth Crit Care Pain. 2013;13(1):12-16.
Last updated 2019-07-18

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