Basics of Reading a Thromboelastography
TEG evaluates clotting in three distinct phases: (1) clot initiation, (2) clot propagation, and (3) fibrinolysis or clot lysis. Clot initiation will measure the action of the clotting cascade and factors. Clot propagation is a measure of the strength of fibrin cross-linking and platelet aggregation. Clot lysis measures the amount of fibrinolysis, or clot breakdown, which occurs after clot formation. It is important to note that traditional clotting tests like PT/PTT only measure clot initiation. By contrast, TEG allows each aspect of the natural clotting pathway to be evaluated quickly and accurately and addressed as needed.
The TEG measurements come in a standard series and order. It is not possible to isolate one value without obtaining the others. Each value obtained has a normal value or range based on the company making the TEG equipment.
R-time, the first measurement obtained, constitutes the period of time when the test begins through the point when clotting factors become activated and clot initiation starts, which is noted by the split in the TEG tracing (Figure 2). In the setting of a hypocoagulable state, the R-time will be prolonged; conversely, in a hypercoaguable state, the R-time will be shortened (Figure 3). The R-time is available within minutes and allows for the early management of patients' coagulopathy. For instance, in patients with prolonged R-time (denoting a hypocoagulable state), the transfusion of fresh-frozen plasma (FFP) should be indicated to reverse the coagulopathy (Table 1).
Thromboelastography basic patterns. TEG® Hemostasis Analyzer Image used by permission of Haemonetics Corporation.
The next TEG value measures the effect of fibrin cross-linking and includes the K-value and α-angle (Figure 2). The K-value is the time from clot initiation to 20 mm of clot formation, whereas α-angle represents the rate of clot formation by measuring the slope between the R-time and K-value. The K-value and α-angle are generally inverse. A large K-value and small α-angle reflect low fibrinogen levels (Figure 3), which is best managed with a cryoprecipitate (Table 1). A small K-value and large α-angle indicate adequate fibrinogen cross-linking (Figure 3).
Platelet aggregation is then measured by the maximum amplitude (MA) of the tracing (Figure 2). Larger MA values indicate a stronger platelet plug, whereas a narrow amplitude MA denotes poor platelet aggregation (Figure 3). This provides a functional measurement of platelet activity and is not limited by platelet count. In the case of a low MA, platelet transfusion may be of some benefit (Table 1).
Fibrinolysis (Clot Lysis)
The final measurement obtained on the TEG graph is fibrinolysis, which is represented by the value LY30. In the setting of TEG, LY30 is the percentage of clot fibrinolysis that occurs within 30 minutes after the MA point on the TEG graph (Figure 2). Some degree of fibrinolysis is normal as part of the system's natural inhibition to the coagulation cascade. However, dysregulation of this system can be pathologic, and TEG helps identify when fibrinolysis is abnormal. Increased LY30 values indicate that fibrinolysis of the clot is abnormally high (Figure 3). To help prevent clot lysis, the patients can be treated with tranexamic acid or Amicar (aminocaproic acid) in an appropriate clinical setting (Table 1).
J Am Acad Orthop Surg. 2019;27(14):503-508. © 2019 American Academy of Orthopaedic Surgeons