Thromboelastography

Thromboelastography (TEG) is a method of testing the efficiency of blood coagulation. It is a test mainly used in surgery and anesthesiology, although few centers are capable of performing it. More common tests of blood coagulation include prothrombin time (PT,INR) and partial thromboplastin time (aPTT) which measure coagulation factor function, but TEG also can assess platelet function, clot strength, and fibrinolysis which these other tests cannot. Thromboelastography (TEG) is a method of testing the efficiency of blood coagulation. It is a test mainly used in surgery and anesthesiology, although few centers are capable of performing it. More common tests of blood coagulation include prothrombin time (PT,INR) and partial thromboplastin time (aPTT) which measure coagulation factor function, but TEG also can assess platelet function, clot strength, and fibrinolysis which these other tests cannot. Thromboelastometry (TEM), previously named rotational thromboelastography (ROTEG) or rotational thromboelastometry (ROTEM), is another version of TEG in which it is the sensor shaft, rather than the cup, that rotates. A small sample of blood is taken from the selected person and rotated gently through 4º 45', six times a minute, to imitate sluggish venous flow and activate coagulation. A thin wire probe is used to measure, which the clot forms around. The speed and strength of clot formation is measured in various ways, typically by computer. The speed at which the sample coagulates depends on the activity of the plasma coagulation system, platelet function, fibrinolysis and other factors which can be affected by genetics, illness, environment and medications. The patterns of changes in strength and elasticity in the clot provide information about how well the blood can perform hemostasis, and how well or poorly different factors are contributing to clot formation. Four values that represent clot formation are determined by this test: the reaction time (R value), the K value, the angle and the maximum amplitude (MA). The R value represents the time until the first evidence of a clot is detected. The K value is the time from the end of R until the clot reaches 20mm and this represents the speed of clot formation. The angle is the tangent of the curve made as the K is reached and offers similar information to K. The MA is a reflection of clot strength. A mathematical formula determined by the manufacturer can be used to determine a Coagulation Index (CI) (or overall assessment of coagulability) which takes into account the relative contribution of each of these 4 values into 1 equation.The G-value is a log-derivation of the MA and is meant to also represent the clot strength using dynes/sec as its units. There are some studies which suggest that an elevated G-value is associated with a hypercoagulable state and therefore increases the risk for venous thromboembolic disease. However, there are no studies dosing of prophylactic heparin products based on the G-value. TEG also measures clot lysis which is reported as both the estimated percent lysis (EPL) and the percentage of clot which has actually lysed after 30 minutes (LY 30,%). Although a normal EPL can be as high as 15% and a normal LY 30 can be has high as 8%, some studies in the trauma population suggest that a LY30 greater than 3% is associated with risk of hemorrhage. Thromboelastometry (TEM), previously named rotational thromboelastography (ROTEG) or rotational thromboelastometry (ROTEM), is another version of TEG in which it is the sensor shaft, rather than the cup, that rotates. Blood (300 µl, anticoagulated with citrate) is placed into the disposable cuvette using an electronic pipette. A disposable pin is attached to a shaft which is connected with a thin spring (the equivalent to Hartert's torsion wire in thrombelastography) and slowly oscillates back and forth. The signal of the pin suspended in the blood sample is transmitted via an optical detector system. The test is started by adding appropriate reagents. The instrument measures and graphically displays the changes in elasticity at all stages of the developing and resolving clot. The typical test temperature is 37 °C, but different temperatures can be selected, e.g. for patients with hypothermia.