Thromboelastography (TEG) is a method of testing the efficiency of blood coagulation. It is a test mainly used in surgery and anesthesiology.
Thromboelastography is a viscoelastic hemostatic assay that measures the global visco-elastic properties of whole blood clot formation under low shear stress. it shows the interaction of platelets with the coagulation cascade (aggregation, clot strengthening, fibrin cross linking and fibrinolysis).
- EG® measures the physical properties of the clot in whole blood via a pin suspended in a cup (heated to 37C) from a torsion wire connected with a mechanical–electrical transducer
- The elasticity and strength of the developing clot changes the rotation of the pin, which is converted into electrical signals that a computer uses to create graphical and numerical output
- point of care test (quick, takes around 30min)
- can be repeated easily and compared and contrasted
- requires calibration 2-3 times daily
- should be performed by trained personnel
- susceptible to technical variations
- kaolin and more recently kaolin + tissue factor (TF) (RapidTEG®) are used as activators, NATEM (TEG® using native whole blood is slower)
- other tests are available including functional fibrinogen, a measure of fibrin-based clot function, and Multiplate which evaluates platelet function
USE
Indications
- prediction of need for transfusion (MA is a useful predictor in trauma)
- guide transfusion strategy
Studies show cost-effectiveness and reduction in blood products in:
- liver transplantation
- cardiac surgery
May be useful in:
- trauma (reduction in blood product use and mortality in cohort studies)
- obstetrics (some data to show that it may decrease transfusion rates; this is controversial)
- early detection of dilutional coagulopathy
Hard to interpret in certain situations:
- LMWH
- aspirin
- post cardiac bypass
- fibrinolysis
- hypercoagulability
NORMAL TEG
Specific parameters represent the 3 phases of the cell-based model of haemostasis: initiation, amplification, and propagation
- R value = reaction time (s); time of latency from start of test to initial fibrin formation (amplitude of 2mm); i.e. initiation
- K = kinetics (s); time taken to achieve a certain level of clot strength (amplitude of 20mm); i.e. amplification
- alpha = angle (slope between R and K); measures the speed at which fibrin build up and cross linking takes place, hence assesses the rate of clot formation; i.e. thrombin burst
- TMA = time to maximum amplitude(s)
- MA = maximum amplitude (mm); represents the ultimate strength of the fibrin clot; i.e. overall stability of the clot
- A30 or LY30 = amplitude at 30 minutes; percentage decrease in amplitude at 30 minutes post-MA and gives measure of degree of fibrinolysis
- CLT = clot lysis time (s)
IMPORTANT PATTERNS
TEG AS A GUIDE TO TREATMENT
- Increased R time => FFP
- Decreased angle => cryopreciptate
- Decreased MA => platelets (consider DDAVP)
- Fibrinolysis => tranexamic acid (or aprotinin or aminocaproic acid)
TEG® VERSUS ROTEM®
Comparison
- Two commercial types of viscoelastic tests are available: thromboelastography =TEG® (developed in 1948, now produced in the USA) and rotational thromboelastogram = ROTEM® (from Germany)
- differences in diagnostic nomenclature for identical parameters between the two
- TEG® operates by moving a cup in a limited arc (±4°45′ every 5s) filled with sample that engages a pin/wire transduction system as clot formation occur
- ROTEM® has an immobile cup wherein the pin/wire transduction system slowly oscillates (±4°45′every 6s)
- results are not directly comparable as different coagulation activators are used
- ROTEM® is more resistant to mechanical shock, which may be an advantage in the clinical setting
Equivalent variables for ROTEM®
- Clotting time (CT) = R value (reaction time)
- α angle and clot formation time (CFT) = K value and α angle
- Maximum clot firmness (MCF) = Maximum amplitude (MA)
- Clot lysis (CL) = LY30
COMPARISON WITH PLASMA CLOTTING TESTS
Pros of viscoelastic hemostatic assays
- assessment of global haemostatic potential provides more information than time to fibrin formation
- can readily differentiate a coagulopathy due to low fibrinogen from one due to thrombocytopenia
- point-of-care (POC) device with rapid turnaround times so that many results available within 5–10 min of starting the test
Cons of viscoelastic hemostatic assays
- variable availability
- marked inter-operator variability and poor precision (UK NEQAS data suggests coefficients of variance ranging from 7.1% to 39.9% for TEG® and 7.0% to 83.6% for ROTEM®)
- may require specialist staff to perform