Overview: Thromboelastography and the resulting thromboelastograph (TEG) provide point of care testing of the coagulation cascade, platelet function, platelet-fibrin interaction, and fibrinolysis.
Technique: Whole blood is placed in a cuvette and a pin and torsion wire mechanism applied while the cuvette oscillates and the blood clots. A recording of the signal from the torsion wire is generated and a graphical representation created.
Interpretation: The overall shape of the TEG may be as useful as the values generated that describe specific characteristics of its shape. Values generated are: R (reaction time): measures from time zero to the beginning of clot formation, when the width (amplitude) of the TEG reaches 2mm (some sources say 1mm). This represents the contributions of the intrinsic and extrinsic coagulation cascades, and the final common pathway to fibrin formation. The normal range depends on whether an activator is added to the whole blood sample, and which activator is used (1-3 minutes is normal for use of Tissue Factor activator, 7-14 minutes for Celite, Kaolin may also be used). K (coagulation time): a measure of speed of clot formation and strengthening, it is equal to the time from amplitude 2mm to 20mm. This relies on fibrinogen. Normal values depend on which activator is used. Alpha angle: also a measure of speed of clot formation, it is defined in one of two ways, either as the angle between baseline and a line defined by the points R and K, or as the angle between baseline and a line tangential to the TEG at R (2mm amplitude). Like the K value, this relies on fibrinogen. Normal is 45-55 degrees. MA (maximal amplitude): measures maximal clot strength, and is equal to the maximal width of the TEG. This represents clot strength as determined by platelet number and function, as well as fibrin cross-linking to form a stable clot. Normal is 50-60mm. Times to amplitudes or amplitudes at times after MA (for example A30 or A60) are measures clot lysis.
Treatment: As follows from the above discussion of the meaning of measures related to the TEG, causes of abnormalities and logical treatments may be identified. Prolonged R values suggest coagulation cascade abnormalities and may result from the presence of heparin or factor deficiency. Heparinase may be added to perform TEG in heparinized patients, for example those on cardiopulmonary bypass. Factor deficiency could be corrected by transfusion of FFP. Shortened R values could result from aggressive factor replacement or hypercoagulable state. Prolonged K values suggest inadequate fibrinogen Low MA values suggest quantitative of qualitative platelet dysfunction or inadequate fibrinogen. Measures of clot lysis consistent with dramatically narrowing amplitudes suggest fibrinolysis.
Studies have shown TEG has greater predictive value than specific coagulation tests in cardiac surgery patients. [Tuman K.J., Spiess B.D., McCarthy R.J., Ivankovich A.D.: Comparison of viscoelastic measures of coagulation after cardiopulmonary bypass. Anesth Analg 1989; 69:69.] TEG has also been used in the setting of liver trasplant, obstetric hemorrhage, and vascular surgery.
Specific modifications to the TEG assay can be used to monitor the anticoagulant effect of platelet inhibitors (ADP-receptor blockers clopidogrel (Plavix) and ticlodipine (Ticlid); and GPIIb/IIIa receptor blockers Abciximad (ReoPro), eptifibatide (Integrilin), tirofiban (Aggrastat)).