Thromboelastography and Thromboelastometry

Thromboelastography and thromboelastometry are viscoelastic tests that follow clot formation, strength and breakdown in whole citrated blood in real time. A pin and cup, with one rotating relative to the other, measure the increasing resistance as fibrin and platelets form a clot, and the trace gives a graph and numerical parameters. Unlike the clotting screen, they show the combined contribution of factors, fibrinogen, platelets and fibrinolysis in whole blood, with results available within 10 to 15 minutes at the bedside.

They are used to direct transfusion in bleeding patients, mainly in cardiac surgery, liver transplantation, major trauma and obstetric haemorrhage. Evidence of reduced transfusion is strongest in cardiac surgery and liver transplantation. Evidence of improved survival in trauma is less firm.

Indications. Major haemorrhage, coagulopathy during or after cardiac surgery, liver transplantation, trauma, postpartum haemorrhage, suspected hyperfibrinolysis, unexplained bleeding with a normal clotting screen, and assessment of hypercoagulability in selected critically ill patients.

Specimen. Citrated whole blood (3.2% sodium citrate), recalcified at the time of the test, rested for about 30 minutes and tested within 2 hours. Samples are tested at 37 °C. Samples should be mixed gently, and a haematocrit that is very high or low alters the trace.

Values are device-, reagent- and activator specific, and local ranges take precedence.

Thromboelastography (kaolin activated).

• R (reaction time, minutes): time to initial fibrin formation. Reflects clotting factors and heparin. About 4 to 8.
• K (clot kinetics, minutes): time to reach a clot strength of 20 mm. Reflects fibrinogen and platelets. About 1 to 3.
• Alpha angle (degrees): speed of clot build up. About 55 to 78.
• Maximum amplitude (mm): peak clot strength, mainly platelets with fibrin contribution. About 51 to 69.
• LY30 (%): percentage fall in amplitude 30 minutes after the maximum amplitude, a measure of fibrinolysis. About 0 to 8.
• Functional fibrinogen assay: maximum amplitude with platelets inhibited.

Thromboelastometry.

• Clotting time (seconds), clot formation time (seconds), alpha angle, amplitude at 5 or 10 minutes, maximum clot firmness (mm) and maximum lysis (%).
• Tissue factor activated assay: factor VII, X, V, II, fibrinogen, platelets and fibrinolysis. Typical clotting time 38 to 79 seconds and maximum clot firmness 50 to 72 mm.
• Contact activated assay: factors XII, XI, IX, VIII and the common steps. Typical clotting time 100 to 240 seconds.
• Fibrinogen specific assay (platelets inhibited): fibrin contribution to clot strength. Typical maximum clot firmness 9 to 25 mm.
• Aprotinin containing assay: corrects the trace if hyperfibrinolysis is present.
• Heparinase containing assay: corrects the trace if heparin is present.

Clinical settings.

• Cardiac surgery: separates heparin effect from factor and platelet deficiency, and directs product use.
• Liver disease: INR overestimates bleeding risk, and viscoelastic traces often show rebalanced haemostasis.
• Postpartum haemorrhage: a fibrinogen specific amplitude at 5 minutes under about 12 mm predicts progression to severe bleeding.
• Trauma: early hyperfibrinolysis and low clot amplitude predict mortality and the need for massive transfusion.

Critical findings needing immediate action.

• Hyperfibrinolysis with active bleeding.
• Very low fibrinogen contribution in major haemorrhage.
• A flat trace, which shows no clot formation.

Pitfalls.

• Insensitive to von Willebrand disease, mild factor deficiency, aspirin and P2Y12 inhibitors (unless platelet specific assays are used), low dose heparin and most direct oral anticoagulants.
• Hypothermia slows enzymatic reactions in the body but the test is run at 37 °C, so it can understate the clinical coagulopathy.
• Delay beyond 2 hours, underfilled tubes, air bubbles, vibration, or poor mixing alter the trace.
• Reference ranges differ between devices, activators and laboratories, and results from different devices cannot be interchanged.
• Treatment thresholds are not universal, and they should be set by local algorithms with the clotting screen, platelet count and Clauss fibrinogen.
• Operator technique and quality control affect reproducibility.

Special groups.

• Pregnancy: the trace is hypercoagulable, with high fibrinogen contribution, and ranges differ from non-pregnant adults.
• Neonates and children: age specific ranges are needed.
• Liver and renal disease: traces are altered by the underlying disorder.
• Anticoagulated patients: use drug specific assays in preference.

Follow up tests. Full blood count, clotting screen, Clauss fibrinogen, D-dimer, platelet function tests, anti-Xa assay, calcium and temperature, lactate, repeat viscoelastic test after intervention.