What is it?
Thromboelastography, or TEG, is a whole blood viscoelastic test. Without going too deep into materials science, it is a test that tests the viscosity of blood (or it's ability to resist shear flow and strain when a stressor is applied) and its elasticity (which measures blood's ability to be stretched and then return to its original state once the stretch stressor is removed). Put simply, TEG applies these global properties to whole blood and its ability to form a clot under a low shear stress.
Background: Like a lot of our research for EM and trauma, TEG was first developed in University of Heidelberg in 1948 and first applied in battlefield medicine during the Vietnam war. It was used as an attempt to determine who should get transfusion of blood products on the battlefield. In the next several decades it was used before and during transplant and cardiac surgeries, and now finds its use in EM and trauma as a potential method to identify acute coagulopathies before transfusion and possible surgery.
How does it work? The blood sample is placed into a disposable cup in the in which a detection pin is suspended in the center. The system oscillates the cup around the detection pin, and measures the degree of pin movement as a function over time.
Liquid, non-clotted blood has a low viscosity, and initially the pin passes easily through it as the cup oscillates, producing little to no movement of the detection pin.
As the blood clots, the detection pin meets more resistance as it passes through the blood- leading to a higher amplitude of movement.
As fibrolysis begins, and as the clot breaks up, the pin meets less resistance, and records less changes in its movement.
Measured as amplitude (or movement of the detection pin) vs time, we get something that looks like this:
https://www.wikem.org/w/images/TEG-Legend.png
Legend:
R : Reaction time; time between start of test to fibrin formation. Measurement of clotting factors
K: Kinetics: time to achieve clot strength of 20mm amplitude; dependent on fibrinogen
a: alpha angle; slope of line between R and K; measures speed of fibrin buildup and cross linking, AKA rate of clot formation
TMA: time to maximum amplitude
MA: Maximum amplitude; the max strength of the fibrin clot/stability of the clot; a measure of platelets and fibrin interacting via GP2b and GP3a
LY30: amplitude at 30 minutes, measure of the fibrinolysis phase
How to interpret: Depending on what value may be greater or less than normal, can diagnose coagulopathy and help guide treatment:
http://www.emdocs.net/wp-content/uploads/2016/12/Screen-Shot-2016-12-20-at-10.23.52-PM-300x290.png
https://lh3.googleusercontent.com/proxy/TuLwySE2hYy1o5Cy9aS_JL-5hDiq5kshcStOZV6rOMaD89fznwGAKRrcNCzz_URHsbVYY8vBebAwMxK3AC-xKgUsXgdCJAnmnKuaKuKPZsaHqIywri4e0IHGi9TrLXzx7A_uRospSbjjUkB7Hif03B-p
Advantages + Disadvantages: So why do this over the coag panel we send from the ED? Why not just send the entire coagulation panel as we click off the boxes in our EMR orderset?
TEG uses whole blood in a point of care test to assess platelet function, coagulation, and fibrinolysis. As a point of care test, we can get near immediate information on a bleeding patient so that we may more expeditiously deliver treatment in a potentially unstable patient.
In cardiac surgery patients, TEG has found its place to detect, monitor, and manage hemostasis. It can be used to guide perioperative bleeding.
In the trauma setting, however, there is mixed and a current lack of high quality evidence to support its routine use over a conventional coag panel. A Cochrane systematic review in 2015 demonstrated insufficient data to compare the accuracy of TEG versus PT and INR as a reference standard in the diagnosis of trauma induced coagulopathy.
In one study examining the reproducibility of TEG measurements and standardization of it as a laboratory test, R time had a large degree of variability and poor correlation with INR. In patients who are on Warfarin, for example, R time may be normal in a TEG study, and thus may miss a potentially significant coagulopathic state. In this situation, INR is still the gold standard. TEG also cannot replace P2Y12 platelet function to guide clopidogrel therapy, D-Dimer to exclude VTE risk, and other thrombophillic diagnostic tests.
But there are several studies suggesting its potential superiority as a fast and reliable point of care test. One study in 2012 by Holcomb et al. showed TEG was better at predicting the need for transfusion of FFP, RBCs, and platelets compared to PT, aPTT, INR, plt count, and fibrinogen in 1,974 trauma patients. Another study performed by Da Luz et al. demonstrated TEG as an effective method to diagnose coagulopathy and predict blood components transfusion and mortality in trauma patients, with the important caveat that there was no significant improvement in patient mortality or morbidity.
There seem to be a growing number of studies advocating for potential use for TEG in the setting of acutely bleeding trauma patient, but there does not yet seem to be a robust foundation of literature at this time to support its routine use. The potential of TEG to provide quick and reliable information is valuable and keeps the hope alive for its viability as a point of care test in the (hopefully) near future. In fact, this POTD mostly covers TEG alone and does not spend much time delving into ROTEM and r-TEG, examples of alternative and developing variations of TEG that continue to improve on the original concept.
Till next time,
-SD
Sources:
https://wikem.org/wiki/Thromboelastography_(TEG)
https://litfl.com/thromboelastogram-teg/
https://www.ncbi.nlm.nih.gov/books/NBK537061/
https://pubmed.ncbi.nlm.nih.gov/24841452/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4206701/
https://pubmed.ncbi.nlm.nih.gov/22868371/
http://www.emdocs.net/thromboelastogram-teg-five-minute-primer-emergency-physician/