Thromboelastography (TEG) and rotational thromboelastometry (ROTEM) are both laboratory methods for testing how well the blood coagulates. Rather than measuring the function or concentration of specific components of the coagulation pathways, like the PT or aPTT do, TEG and ROTEM measure the functional abilities of the overall coagulation pathways. They are intended to take into account everything that may be affecting coagulation, including such things as platelet function, fibrinolysis, illness, and medications. Thus both TEG and ROTEM are improvements on the old bleeding time test and also on the platelet function analysis (PFA).
In both cases reagents are added to a small sample of blood and the speed and size of clot formation is detected. Rotem works by inserting a pin into the sample and rotating the pin, with optical detection of the rotations. As clot formation occurs, it creates shear and drag on the pin, which is detected. TEG works essentially in the same way, but rotates the sample rather than the central pin. In both cases a set of parameters are measured and a visual graph of the coagulation is produced as the coagulation proceeds.
The main parameters that are produced and reported include:
- R value – the reaction time in minutes, the time from beginning the test until clot formation begins
- K value – the speed of clot formation in minutes, once clot formation has begun
- Angle – a tangent drawn to the curve created as K is reached. This parameter is reported in degrees and gives mostly the same information as K. It is often reported rather than K.
- Maximum Amplitude (MA) – the widest point of the curve in mm and an indication of the strength of the clot that has formed.
- G value – the clot strength in dyn/cm2 .
These parameters and the shape of the graph that is produced can be interpreted in real time to indicate the efficiency of the patient’s coagulation system. TEG and ROTEM are most frequently used during surgery to keep track of the patient’s ability to coagulate and thus the level of anticoagulation being maintained. These are not tests that should be performed Point-of-Care even though surgeons who use this testing often like to see the shape of the curve as the clot formation occurs. Thus many places perform the test in the lab but have linked computer screens in the OR so that the surgeons can see the graph shape and parameter results in real time.
-Patti Jones PhD, DABCC, FACB, is the Clinical Director of the Chemistry and Metabolic Disease Laboratories at Children’s Medical Center in Dallas, TX and a Professor of Pathology at University of Texas Southwestern Medical Center in Dallas.