Theory and Practice in Measuring Hemostasis
By Martin H. Kroll, MD
The most commonly used laboratory tests for evaluating hemostasis measure the ability of blood to coagulate. The activated partial thromboplastin time (aPTT) and the prothrombin time (PT) measure the time it takes blood to form a fibrin clot via the intrinsic and extrinsic pathways, respectively, and platelet counts provide information on the ability of platelets to aggregate.
Another less commonly used method to evaluate the ability of blood to form a clot is thromboelastography (TEG). First developed in Germany by Hellmut Harter in 1948, TEG was not introduced into the U.S. until the 1960s. Improvements over the decades have made TEG a valuable laboratory tool for monitoring patients’ hemostasis as a whole dynamic process.
This article will describe the theory of TEG and the specific applications of the assay in laboratory medicine for determining the kinetics of clot formation and growth, as well as the strength and stability of formed clots.
Hemostasis is a well-regulated process: the blood remains fluid under normal conditions, but forms localized clots when the integrity of the vascular system is breeched. Trauma, infection, and inflammation all activate the blood’s clotting system, which depends on the interaction of two separate systems: enzymatic proteins (clotting factors) and platelets (non-nucleated cells). The two systems work in concert to plug defects in the broken vessels.
The clots that form in this process need to be of sufficient strength to resist dislodgement by circulating blood or mechanical movement. If a particular clotting factor is dysfunctional or absent, as in hemophilia, an insufficient amount of fibrin forms. Similarly, massive consumption of clotting factors in a trauma situation decreases the amount of fibrin formed. Inadequate numbers of platelets resulting from trauma, surgery, or chemotherapy also decrease platelet aggregation, as do genetic disorders, uremia, or salicylate therapy. Ultimately, reduced fibrin formation or platelet aggregation results in clots of inadequate tensile strength. This hypocoagulable state makes the patient prone to bleeding. Conversely, endothelial injury, stasis, cancer, genetic diseases, or other hypercoagulable states lead to thrombosis formation, exemplified in deep-vein thromboses, pulmonary emboli, and arterial occlusions such as stroke and myocardial infarction.
In the 1960s, researchers described the enzymatic portion of the coagulation system as a cascade (Figure 1). When the initial coagulation factors in the cascade become activated, they initiate enzymatic activity that activates additional factors in an amplifying fashion. PT and aPTT evaluate the effectiveness of the clotting cascade by measuring the time it takes to form a clot. A prolonged clotting time indicates decreased concentrations of the factors or inhibition of the cascade.
The Coagulation Cascade
This enzymatic cascade of coagulation factors is responsible for forming a fibrin clot.
Platelets are also involved in the hemostasis system. Produced by megakaryocytes in the bone marrow, these small cytoplasmic vesicles, about 1µ in diameter, are full of active biological agents. Just as the enzymes of the coagulation cascade need to be activated to form a fibrin clot, four agents—adenosine diphosphate (ADP), epinephrine, thrombin, and collagen—activate platelets. Thrombin, derived from the coagulation cascade, and collagen, exposed by injury to the endothelial layer of the vasculature, act as strong activators, while ADP and epinephrine are weak activators.
Vascular injury also exposes the subendothelial matrix. The platelet surface receptor glycoprotein (Gp) Ib-IX-V complex facilitates adhesion of platelets to the von Willebrand factor (vWF) protein located in this matrix. Once platelets adhere to the subendothelial matrix, they aggregate to form a platelet plug.
An adhesive protein from the integrin family called glycoprotein IIb-IIIa (Gp IIb-IIIa) mediates platelet aggregation. The procoagulant factor, fibrinogen, attaches to this receptor, linking the platelets to each other. The bridging, which is linked by fibrinogen, represents the main source of aggregation. Surgery or trauma exposes the procoagulant factors to the tissue factor, triggering the cascade, as mentioned previously. Besides transforming fibrinogen into fibrin, a polymer that strengthens clots, the coagulation cascade produces large amounts of thrombin, the main activator of platelets.
TEG: A Measure of Coagulation Status
Modern TEG instruments record the kinetic changes of clot formation, retraction, and lysis. Furthermore, TEG is sensitive to the interaction of the enzymatic and cellular components and can provide results in 60 minutes or less. The resulting data provides information on both the activity of patients’ clot-forming elements and clot strength.
In principle, TEG measures clot formation via the tensile strength of the fibrin-polymer-platelet complex. To run the assay, the lab technologist places the patient specimen into a sample cup. A metal pin goes into the center of the cup. Contact with the walls of the cup or addition of a clot activator to the cup, such as celite, initiates clot formation. The instrument then rotates the cup in an oscillating fashion, 4.45 degrees every 10 seconds. As fibrin and platelet aggregates form, they connect the inside of the cup with the metal pin, transferring the energy used to move the cup in the pin. A torsion wire connected to the pin measures the strength of the clot over time, with the magnitude of the output directly proportional to the strength of the clot. Lysis or retraction of the clot decreases the strength of the clot, causing the amplitude (A) of the TEG tracing to decrease (Figure 2).
Normal TEG Tracing
The following measurements are shown: R (time of formation of the fibrin strand polymers); K (speed at which the clot forms); α (the slope drawn from R to K) and MA (strength of the clot) measurements.
By measuring the tensile strength of the clot, TEG results reflect both platelet aggregation and fibrin polymerization. The greatest excursion of the amplitude is called the maximum amplitude (MA), which reflects the overall maximum strength of the clot (Figure 2). Typically, the amplitude is expressed as the shear elastic modulus strength, F, in units of 10-3 dynes/cm2.
However, the key metric in TEG measurements is the velocity at which the clot forms. The first part of clot formation begins with activation of the enzymatic cascade and ends with formation of the fibrin strand polymers (R time). The second phase measures the speed at which the clot forms (K time). MA measures the strength of the clot, which is an overall reflection of the structural interactions and fibrinogen, interlaced with fibrin polymers. Platelet function and aggregation have the greatest effect on MA.
Decreases in coagulation factors, the presence of heparin, or dysfunctional factors, all prolong the R time. Hypercoagulation also causes shortening of this time. The value for K is arbitrarily chosen to be the time it takes the tracing to go from an amplitude of 2 mm to 20 mm (Figure 2). The angle a refers to the slope drawn from the R to the K value. Overall, the R and K times and a angle assess contact activation and fibrin formation using circulating coagulation factors and inhibitors, as well as platelets.
To perform the test, the lab technologist begins the clotting process by adding calcium to citrated blood. Alternatively, addition of celite, thrombin, or tissue factor will activate the clotting cascade more rapidly. Labs use special heparinase-coated cups to evaluate clotting in patients on heparin therapy, as heparinase interferes with heparin’s action without affecting the TEG variables. Testing a patient’s blood in both heparinase-coated and non-coated cups can be useful for determining whether the individual has residual heparin that is inhibiting coagulation. This approach is especially valuable for assessing coagulation status both during and after cardiopulmonary bypass surgery.
Applications of TEG
Although TEG is not a routine lab test, it has proven useful for managing post-operative complications. Surgery causes multiple vascular defects, which activate platelets that subsequently plug the defects. As soon as surgery begins, the cascade system is activated. Any type of cardiovascular surgery raises the risk of thrombosis, and this is especially so when the patient undergoes cardiopulmonary bypass. However, surgeons prevent unwanted thrombosis by administering high doses of heparin. Because the effects of clotting activation and subsequent risk of bleeding are more pronounced in cardiovascular surgery, surgeons monitor the effect of heparin during and after such procedures. They also look for evidence of thrombocytopenia and fibrinolysis (Figure 3).
Examples of TEG Tracings for Various Hemorrhage Conditions
A. Decreased coagulation factors/heparin.
B. Thrombocytopenia/decreased platelet function.
TEG in Liver Transplantation
TEG is also useful for monitoring fibrinolysis in liver transplant patients, who are at considerable risk for bleeding owing to the liver being a major source of coagulation factors. Liver disease also causes coagulation factor deficiencies, resulting in dysfibrinogemia and enhanced fibrinolysis. Furthermore, patients’ platelet counts are frequently low in severe liver disease, stemming from hypersplenism or disseminated intravascular coagulation (DIC). Fibrinolysis increases during surgery after the diseased liver is removed as concentrations of tissue plasminogen activator (tPA) increase and alpha-2-antiplasmin decrease. TEG results of patients with fibrinolysis will show a decrease in MA on the tracing (Figure 3).
Reperfusion of the grafted liver causes endothelial injury and increases tPA levels, thereby inducing fibrinolysis. This situation, coupled with circulating heparin, reduces the transplant recipient’s ability to form clots. As graft function improves, however, the coagulopathy resolves. Evaluation of these patients by TEG with heparinase also allows clinicians to determine the effect of circulating heparin on hemostasis.
TEG in Cardiac Surgery
Patients who undergo cardiopulmonary bypass surgery typically require extracorporeal circulation, which affects the hemostatic and fibrinolytic systems by causing platelet dysfunction, coagulation factor activation, and factor depletion. Cardiac surgery patients also receive high doses of heparin to reduce clot formation. The extracorporeal devices, however, are composed of either plastic or metal surfaces that fibrinogen and the vWF can adhere to. Platelets adhere to the attached fibrinogen and vWF, become activated, and release their contents, thereby further propagating thrombus formation. Furthermore, phospholipids in the activated platelets provide a surface for the coagulation cascade, and the blood-gas interface also may contribute to platelet activation.
aPTT is not suitable for monitoring coagulation status of cardiac surgery patients, because the doses of unfractionated heparin they receive exceed the capability of the assay. While surgeons may monitor patients using the activated clotting time (ACT), this assay provides only a rough estimate of anticoagulation and its reversal by protamine sulfate, which interferes with heparin’s coagulant activity.
TEG monitoring of cardiac surgery patients also has other benefits. In moderate- to high-risk patients, physicians can use TEG results to reduce the amount of blood required for transfusion, and even the number of patients who receive transfusions. Cardiopulmonary bypass also causes release of kallikrein by the action of activated factor XII on prekallikrein, which is synthesized by the liver and bound to high- molecular weight kininogen in the plasma. Kallikrein stimulates endothelial cells to convert kininogen to bradykinin. This series of reactions ultimately results in release of tPA. Consequently, soon after the start of the procedure, tPA levels rise, plasminogen activator inhibitor-1 (PAI-1) levels fall, and plasmin concentration increases, which is also affected by kallikrein. Plasmin lyses fibrin, the polymer formed as the final end product of the coagulation cascade, and the ensuing accelerated fibrinolysis breaks up the clots, potentially putting the patient at risk for hemorrhage. These two activated processes—excessive clot formation and accelerated fibrinolysis—expose patients to thrombosis and hemorrhage, thereby putting them at risk of serious complications.
Besides the extracorporeal circulation, the surgical procedure itself disrupts the endothelium of the vasculature, exposing tissue factor, which activates the extrinsic portion of the coagulation cascade. In addition to the exposure of tissue factor, the complement system becomes activated and inflammation ensues, activating leukocytes and platelets.
For these reasons, abnormal TEG values and MA are more sensitive and specific predictors of abnormal bleeding during cardiac surgery than PT, aPTT, or platelet counts. Overall, platelet dysfunction represents the primary cause of post-cardiopulmonary bypass bleeding.
TEG and Anti-Platelet Therapy
To prevent myocardial infarction (MI), many patients with coronary artery disease receive prophylactic anti-platelet therapy. Although most cardiac surgeons would prefer that patients discontinue anti-platelet therapy 5–10 days prior to cardiac surgery, doing so actually increases the risk of MI. For example, in one study of 59 patients undergoing coronary artery bypass graft (CABG) procedures, researchers used platelet mapping to predict surgical bleeding. Overall, 25 patients received aspirin therapy, while 34 received clopidogrel in addition to aspirin prior to surgery. In the first 24 hours following surgery, nine of 59 patients bled excessively, eight of whom had received clopidogrel prior to surgery. Platelet mapping successfully identified the patients who bled, with a sensitivity of 78% and specificity of 84%. Furthermore, 85% of patients did not respond to a standard dose of clopidogrel, while 44% did not respond to a standard dose of aspirin. TEG results effectively predicted which patients on anti-platelet therapy would develop post-operative hemorrhage after CABG.
While excessive bleeding during surgery is a major concern, surgery can also induce a hypercoagulable state that has been attributed to surgical trauma, systemic inflammation, exposure of tissue factor, platelet activation, and volume expanders, such as crystalloids. Hypercoagulation is associated with postoperative thrombotic complications, including MI.
Some studies have shown that the TEG MA is useful for following patients at risk of hypercoagulation during surgery. For example, in one study involving 240 surgical patients, six experienced MI, two developed deep-vein thrombosis, two had complications from pulmonary emboli, and two had cerebrovascular accidents. Overall, 95 patients showed an increased TEG MA, while 145 did not. Particularly notable is the fact that all the patients who experienced MI had an increase in the TEG MA (100% sensitivity, 61% specificity). For all thrombotic events, the sensitivity of TEG MA was 80%, and the specificity was 62%. The predictive value of a positive test was 6% in the case of MI, and 8% for all thrombotic events. However, the negative predictive values for MI and all thrombotic events were 100%, and 99%, respectively.
TEG also detects known or established hypercoagulable states induced by surgery (Figure 4). In patients without a previous clinical history of a hypercoagulable state, abnormal laboratory results for Factor V Leiden mutation, protein C, protein S, and antithrombin III deficiencies are rare. In these patients, TEG results are predictive of hypercoagulable state in orthopedic, abdominal, and vascular surgery, in addition to that observed in cardiac surgery.
Examples of TEG Tracings in Hypercoagulation
A. Platelet hypercoagulability.
B. Coagulation cascade hypercoagulability.
A Tool for Coag Monitoring
Hemostasis requires the combined activity of vascular, platelet, and plasma factors. Although traditional lab tests adequately monitor these separate processes, TEG is a valuable tool for monitoring hemostasis as a whole dynamic process. The assay can separate disturbances arising from either the coagulation cascade or platelet dysfunction, making it a useful tool during and after surgery.
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Martin Kroll, MD, is chief of Laboratory Medicine at Boston Medical Center and professor of Pathology and Laboratory Medicine at Boston University School of Medicine.