Thromboelastography (TEG): What Does It Mean?
Kelly Blackstock, DVM, DACVECC
Coagulation, an imperative balance
Every day the blood vessels within our patients are microscopically damaged. If the body can’t repair them fast enough or breaks down the clots too quickly, they bleed. If the body clots too aggressively or maintains clots too long, there is an increased risk of thrombosis. The coagulation system has to strike a fine balance between all of these factors in hopes that the end result will be preservation of blood flow to the tissues. Unfortunately, many disease states interfere with this elaborate system, creating potentially life-threatening complications. Diagnostic testing of such a system is challenging, and, even when using multiple tests, we only gain a small window of insight into what is really happening within our patients.
What coagulation tests are available?
Prothrombin time (PT) and activated partial thromboplastin time (aPTT): PT evaluates both the extrinsic and common pathways (factors I, II, III, V, VII, X), whereas aPTT evaluates the intrinsic and common pathways (I, II, V, VIII, IX, X, XI, XII). They are limited to detecting hypocoagulable states created by factor depletion. Historically, it was thought that these tests could detect hypercoagulable states, but this was later found to be untrue.1
Proteins induced by vitamin K antagonism or absence (PIVKA): This is a modified PT, detecting deficiencies in factors II, VII, and X. Because the clotting times are longer than PT, it may be more sensitive for abnormalities of these factors.
Buccal mucosal bleeding time: The BMBT is a subjective assessment of primary hemostasis. This includes platelet number/function, rare vessel wall defects and von Willenbrand factor (vWf) disorders.
Platelet function analyzers (PFA): A relatively new screening test that measures platelet adhesion and aggregation, i.e. primary hemostasis
Fibrinogen degradation products (FDPs)/D-dimers: Elevations of these factors can suggest hypercoagulability, as they accumulate from the breakdown of either fibrinogen or cross-linked fibrin.
Thromboelastography (TEG): This is the only test that assesses the entire coagulation system, from initiation of the clot to its dissolution. In addition, we gain added information about the quality of the clot and the dynamics of its formation.
Why should you care about TEG?
TEG is quickly becoming one of the most frequently used diagnostic tests in research today. Since 2000 there have been more than 60 veterinary publications, most in the last few years.2 An investigative group called PROVETS recently published procedural guidelines in the Journal of Veterinary Emergency and Critical Care in hopes of standardizing the testing. This will likely pave the way for further research. Understanding this test will allow you to better interpret the results of the literature. In addition, this is a bench-top test that is easy to run. We anticipate that it will be utilized in more specialty hospitals and emergency settings in the near future.
How does TEG work?
The basic parts of the machine include a plastic cup, a pin, a torsion wire and a transducer. The sample (fresh or citrated blood) is added to the cup, which includes calcium and an activator. The pin is dropped into the sample and oscillated. As fibrin develops, it connects the pin to the cup, creating torque. This torque is sensed by the transducer, which creates the tracing. There are three phases to the tracing: precoagulation, coagulation and fibrinolysis.
What do all the values on the TEG tracing mean?
- R: procoagulation time. It is the distance from the onset of the test until the diverging lines are 1mm apart. Influenced by: intrinsic pathway (VIII, IX, XI, XII)
- K: clot formation time. It is the distance from the end of R until the divergent lines are 20 mm apart. Influenced by: II, VIII, platelet number/function, thrombin, HCT
- α (angle): rate of clot formation. It is the angle of the line drawn from R to K. Influenced by: (same as K)
- MA (max amplitude): final clot strength. It is the maximum distance of the divergent lines. Influenced by: XIII, fibrin, fibrinogen, platelet number/function, thrombin, HCT
- A30/LY30: amount of lysis. It is the distance between divergent lines 30 minutes from MA.
Utilitity of TEG
TEG appears to be a much more sensitive coagulation test. In people, it is used to assess the individual patient’s risk of bleeding due to low platelet count, to monitor anticoagulant/antiplatelet therapy, detect minor factor abnormalities, and monitor risk of bleeding during interventions. In veterinary medicine, we are only scratching the surface of investigation. It has been used to confirm hypercoagulable states in parvovirus, protein-losing nephropathies and IMHA.1 Keep your eyes open though, because there is surely more to come.