Thromboelastogram (TEG)

Reviewed and revised by Hamish Lala and Chris Nickson

OVERVIEW

Thromboelastography (TEG) is a viscoelastic hemostatic assay that measures the global viscoelastic properties of whole blood clot formation under low shear stress

  • TEG shows the interaction of platelets with the coagulation cascade (aggregation, clot strengthening, fibrin cross-linking and fibrinolysis)
  • does not necessarily correlate with blood tests such as INR, APTT and platelet count (which are often poorer predictors of bleeding and thrombosis)
  • This page describes TEG® predominantly, ROTEM® is the alternative viscoelastic hemostatic assay that is widely available commercially

METHOD

  • TEG® 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

TEG6s (Haemonetics)

  • This newer machine no longer uses the ‘pin-in-cup’ technique (as did its TEG5000 predecessor)
  • It uses ‘resonance’ where blood is exposed to a fixed vibration frequency range and the detector measures the vertical motion of blood meniscus under LED illumination and transforms that movement into tracing of clot dynamics
  • With pre-prepared cartridges, there is no longer any pipetting required!

USE

Indications

  • prediction of need for transfusion (maximum amplitude (MA) is a useful predictor in trauma)
  • guide transfusion strategy

Studies show cost-effectiveness and reduction in blood products in:

  • liver transplantation
  • cardiac surgery

Maybe 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:

  • low molecular weight heparin (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)
    • initiation phase
    • dependent on clotting factors
  • K = kinetics (s)
    • time taken to achieve a certain level of clot strength (amplitude of 20mm)
    • amplification phase
    • dependent on fibrinogen
  • alpha = angle (slope of line between R and K)
    • measures the speed at which fibrin build up and cross-linking takes place, hence assesses the rate of clot formation
    • “thrombin burst” / propagation phase
    • dependent on fibrinogen
  • 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
    • dependent on platelets (80%) and fibrin (20%) interacting via GPIIb/IIIa
  • A30 or LY30 = amplitude at 30 minutes
    • percentage decrease in amplitude at 30 minutes post-MA
    • fibrinolysis phase
  • CLT = clot lysis time (s)

Approximate normal values (kaolin activated TEG, values differ if native blood used, and between types of assay)

  • R: 4-8 min
  • K: 1-4 min
  • α-Angle: 47-74°
  • MA: 55-73mm
  • LY 30%: 0-8%

Corresponding terminology for ROTEM

ROTEMTEG
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

IMPORTANT PATTERNS

TEG AS A GUIDE TO TREATMENT

  • Increased R time => FFP
  • Decreased alpha angle => cryoprecipitate
  • Decreased MA => platelets (consider DDAVP)
  • Fibrinolysis =>  tranexamic acid (or aprotinin or aminocaproic acid)

Or use this handy guide (-:

Source: @DocXology

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 and user familiarity
  • 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

EVIDENCE

  • Cochrane review (2015)of the use of TEG and ROTEM in traumatic bleeding advised that they should be used for research only, due to a lack of evidence for the accuracy of the assays.

References and Links

Journal articles

FOAM and web resources


CCC 700 6

Critical Care

Compendium

Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also the Innovation Lead for the Australian Centre for Health Innovation at Alfred Health and Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.

After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.

He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE.  He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.

His one great achievement is being the father of two amazing children.

On Twitter, he is @precordialthump.

| INTENSIVE | RAGE | Resuscitology | SMACC

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