Karl E. Jandrey,     Davis, California

Benjamin M. Brainard,     Athens, Georgia

Thromboelastography

The principle behind thromboelastography is the measurement of the viscoelastic characteristics of clotting blood. Clot formation occurs in a rotating plastic cylindrical cuvette (the cup) with a stationary suspended piston (the pin) that is lowered into the center of the cuvette. The cup rotates through an angle of 4°45′ with a 10-second cycle period. The pin is attached to a thin metal torsion wire. As clot formation progresses, the fibrin that is generated physically links the pin to the cup. As this connection strengthens, the rotation of the cup is transmitted to the pin and this torque is translated into the TEG tracing by the torsion wire. The graphic tracing is displayed in real time via the computer interface and proprietary software.

The thromboelastograph generates a qualitative tracing, as well as quantitative values to describe the clot (Figure 15-1). The R-time is the time in minutes from when that blood is placed in the TEG until initial fibrin formation. Reaction time generally reflects coagulation factor levels but does not always correlate with prothrombin time (PT) and activated partial thromboplastin time (aPTT). The K-time measures in minutes the time it takes from initiation of clotting for a TEG tracing to reach a predetermined level (20 mm) of clot strength. The α-angle measures in degrees the rate of fibrin buildup and cross-linking as a function of amplitude and time. Both K-time and α-angle are affected by the availability of fibrinogen, factor XIII, and to a lesser degree platelets. The maximum amplitude (MA, measured in mm), the widest part of the TEG tracing, is a direct result of fibrin production and platelet function and represents the final clot strength. Another measure of clot firmness is the elastic shear modulus (G, in dynes/second/cm²) and is calculated from the MA using the equation:

In addition to clot production, the TEG also demonstrates fibrinolysis; the percent decrease in the amplitude of the tracing 30 and 60 minutes following the MA is indicated as the lysis parameters (CL 30 and LY 30 and CL 60 and LY 60, respectively, measured in %). Lysis parameters are a measure of clot stability. Table 15-1 lists normal values for TEG parameters that have been reported in the literature.

TEG Operation

There are no standard protocols for veterinary TEG; in general, samples are run as recalcified citrated samples (typically 3.2% sodium citrate at a 1 : 9 v/v ratio), at 37° C, and after a 30-minute rest period at room temperature. Vibration, shock, mixing, and rapid shifts in temperature during the rest period should be avoided. Although some laboratories use only recalcification with calcium chloride before starting the assay, this method depends on contact activation (intrinsic pathway) and may result in a prolonged assay time, especially in horses. This method is also associated with operator variability. The use of very dilute (1 : 50,000) or moderately dilute (1 : 3400) tissue factor (TF; Innovin, Siemens Healthcare) has been described in many species and decreases interoperator variability. Haemonetics provides prefilled vials containing a liquid kaolin solution, which strongly activates coagulation via the intrinsic pathway. Kaolin activation also decreases variability and has been described in small animal species. A recent publication that studied various activators (native, TF, and kaolin) for TEG analysis of cat blood showed that the activators are not interchangeable due to large coefficients of variation between activators.

Because of variability between users and institutions, each institution should develop a specific protocol, decide on a specific reference interval, and identify a limited number of operators to maximize useful information from the assay. Protocols should specify guidelines for sample collection, a standard sample rest period following blood sampling, and a standard temperature for both the rest period and the TEG analysis. For research studies, a single operator is preferable, and duplicate testing should be considered if possible. If limiting the number of operators is not feasible, an activator such as TF or kaolin should be used.