Thromboelastometry
Thromboelastometry, also known as rotational thromboelastometry (ROTEM), is a point-of-care viscoelastic testing method that assesses the dynamics of clot formation, strength, and lysis in whole blood samples under low shear stress conditions.[1] It provides a graphical trace and numerical parameters representing key phases of hemostasis, including reaction time, clot formation time, alpha angle, maximum clot firmness, and lysis indices, using specific assays such as EXTEM (extrinsic pathway), INTEM (intrinsic pathway), and FIBTEM (fibrinogen contribution).[2] Developed in the 1990s[3] as an advancement over traditional thromboelastography (TEG), ROTEM employs a rotating pin within a stationary cup of citrated whole blood activated by reagents, differing from TEG's stationary pin and oscillating cup mechanism, which allows for more standardized and automated analysis.[4] This technology enables rapid, real-time evaluation of coagulation factors, platelets, fibrinogen, and fibrinolysis, making it particularly valuable in high-bleeding-risk scenarios.[2] Clinically, thromboelastometry guides targeted transfusion therapy in trauma, cardiac surgery, liver transplantation, and postpartum hemorrhage by identifying specific coagulopathies—such as hyperfibrinolysis or fibrinogen deficiency—and reducing unnecessary blood product administration compared to conventional tests like prothrombin time or activated partial thromboplastin time.[4] Studies have demonstrated its utility in decreasing transfusion volumes and improving outcomes in perioperative settings, with results available within 5–10 minutes versus 30–60 minutes for standard laboratory assays.[2] Additionally, it supports monitoring of anticoagulant effects and prediction of bleeding risks in critical care.[4]Introduction
Definition and Principles
Thromboelastometry, commonly referred to as rotational thromboelastometry (ROTEM), is a point-of-care viscoelastic hemostatic assay designed to measure the kinetics of clot formation, strength, and fibrinolysis in whole blood. This method provides a graphical and numerical representation of the dynamic changes in blood's viscoelastic properties during coagulation, capturing the interplay of plasma factors, platelets, and fibrin. Developed as a variant of thromboelastography, which was invented by Helmut Hartert in 1948, ROTEM employs rotational mechanics to assess hemostasis under low shear stress conditions.[4][2][5] At its core, thromboelastometry evaluates the viscoelastic alterations in blood as it transitions from a liquid to a gel-like state, primarily driven by fibrin polymerization and platelet-fibrin interactions that enhance clot elasticity. The test utilizes small volumes of citrated whole blood, approximately 300 μL, maintained at a physiological temperature of 37°C to mimic in vivo conditions and ensure reproducible results. Unlike conventional static coagulation tests, such as prothrombin time (PT/INR) or activated partial thromboplastin time (aPTT), which analyze isolated factors in platelet-poor plasma and offer snapshots of specific pathways, thromboelastometry delivers a holistic, real-time profile of the entire hemostatic process, including cellular contributions.[6][7][8] In the basic setup, a sample of whole blood is placed in a cylindrical cup, with a pin suspended above it; the cup remains stationary while the pin oscillates, and as fibrin strands form between them, the increasing torque on the pin is optically detected and plotted over time to produce a TEMogram trace. This trace visually represents the progression from initial clot initiation through maximum firmness to potential lysis, enabling point-of-care analysis within minutes. The primary purpose of thromboelastometry is to rapidly detect abnormalities such as hypercoagulability, hypocoagulability, or excessive fibrinolysis, thereby informing precise transfusion strategies and hemostatic therapies in clinical settings.[4][6][2]Historical Development
Thromboelastography (TEG), the foundational technology for thromboelastometry, was invented in 1948 by German physician Helmut Hartert at the University of Heidelberg as a manual viscoelastic method to assess blood coagulation dynamics in real time, particularly during surgical procedures where rapid monitoring of hemostasis was needed.[4] Hartert's device used a rotating cup containing a blood sample with a suspended pin connected to a torsion wire, recording the mechanical resistance as the clot formed, providing a graphical trace of clot initiation, formation, and lysis.[9] Initially limited to research settings due to its manual nature and sensitivity to vibrations, TEG saw early clinical exploration in Europe but faced challenges in reproducibility from inconsistent pin oscillation.[10] In the early 1990s, rotational thromboelastometry (ROTEM) was developed in Munich, Germany, by TEM International GmbH (now part of Werfen) to address TEG's limitations by introducing a rotating pin and stationary cup, enhancing stability and automation for better reproducibility in point-of-care settings.[11] This innovation shifted the focus toward automated, computerized systems, driven by the demand for rapid hemostasis assessment in high-bleeding-risk surgeries like cardiac and liver transplantation, where traditional lab tests delayed transfusion decisions.[12] ROTEM was first commercialized in Europe around 1995, enabling broader adoption in perioperative monitoring.[3] Key milestones included U.S. FDA clearance for the ROTEM delta system in 2010 as a substantially equivalent device to TEG, facilitating its integration into American clinical practice.[13] By the early 2000s, clinical trials demonstrated ROTEM's utility in guiding transfusions, notably in hepatic surgery where TEG had gained traction since the 1980s through studies like Kang et al.'s 1985 work showing reduced blood product use via real-time monitoring during liver transplants.[14] Standardization efforts in the 2000s established reference ranges and assays such as EXTEM (extrinsically activated) and INTEM (intrinsically activated), supported by multicenter investigations that improved inter-laboratory consistency.[15] ROTEM's incorporation into guidelines, such as the European Society of Anaesthesiology's 2013 recommendations for severe perioperative bleeding management, further solidified its role in algorithm-based transfusion strategies.Methodology
Instrumentation
Thromboelastometry devices, such as the ROTEM analyzer, feature a stationary disposable cup that holds the blood sample, with an oscillating pin suspended within it to detect clot formation dynamics. The pin rotates through a limited arc of 4°45' every 6 seconds. The oscillation of the pin is monitored by an optical detector system that measures the impedance to motion as fibrin strands form and link the pin to the cup. This setup is connected to a computer interface that processes the signal to generate a real-time graphical trace of coagulation parameters.[2][16] The standard ROTEM sigma represents a modern multi-channel variant, supporting up to four simultaneous tests with fully automated cartridge-based sampling that eliminates manual pipetting and incorporates embedded reagents for standardized assays. Technical specifications include a typical sample volume of 300 μL of citrated whole blood, precise temperature control at 37°C to mimic physiological conditions, run times of 30-60 minutes depending on the assay, and disposable cartridges combining cups and pins to prevent cross-contamination. Earlier models like the ROTEM delta use electronic pipetting for semi-automated operation, while the sigma's compact design facilitates point-of-care deployment in various clinical settings.[17][18][19] Calibration involves daily internal electronic checks to verify torque sensitivity and system integrity, ensuring consistent optical detection and mechanical oscillation. Quality assurance is further supported by periodic runs using control plasmas, such as ROTROL, to validate assay performance against reference ranges. Safety features include a closed cartridge system that minimizes aerosol generation and biohazard exposure, along with integration capabilities via ROTEM Integrated Solutions for secure data transfer to laboratory information systems, adhering to standards like HIPAA/HITECH.[20][18][21]Procedure and Assays
The procedure for thromboelastometry begins with pre-analytical sample collection and preparation to ensure accurate assessment of whole blood coagulation dynamics. Blood is collected into citrated tubes at a 9:1 blood-to-citrate ratio, typically 1.8 mL per tube, to prevent spontaneous clotting while maintaining physiological calcium levels for later recalcification.[22] Samples remain stable for analysis up to 120 minutes at room temperature (approximately 23°C), after which progressive deterioration in clot formation parameters may occur; refrigeration is avoided as it alters results, and hemodilution from improper mixing or delays should be minimized to preserve native hemostatic components.[23] The sample is pre-warmed to 37°C for 5-10 minutes prior to testing to simulate physiological conditions. The core testing process involves recalcification and activation in a disposable cylindrical cup. Approximately 300 μL of citrated whole blood is pipetted into the cup, followed by the addition of 20 μL of Star-TEM recalcifying agent (0.2 mol/L CaCl₂) to reverse citrate anticoagulation and initiate calcium-dependent coagulation factors.[24] Next, 20 μL of a specific assay reagent (activator or inhibitor) is added, and the contents are gently mixed by aspiration and dispensation once to ensure homogeneity without introducing bubbles. The measuring cell, consisting of the cup and an attached pin connected via a ball-bearing shaft, is inserted into the instrument channel. The system then automatically oscillates the pin at 4.75° every 6 seconds within the stationary cup, with motion detected optically as fibrin strands form between the cup and pin; this generates a continuous thromboelastometric trace recorded for up to 60 minutes or until predefined endpoints like maximum lysis are reached.[24] Multiple channels allow simultaneous running of 2-4 assays per sample. Standard assays in thromboelastometry target specific coagulation pathways using tailored reagents for comprehensive profiling. The INTEM assay activates the intrinsic pathway with ellagic acid and phospholipids, providing a contact activation-based evaluation similar to activated partial thromboplastin time but in whole blood.[25] The EXTEM assay employs low concentrations of recombinant tissue factor and phospholipids to rapidly initiate the extrinsic pathway, mimicking tissue injury and enabling quick assessment of overall clot formation.[25] FIBTEM isolates fibrinogen and fibrin polymerization by adding cytochalasin D, which inhibits platelet glycoprotein IIb/IIIa receptors to eliminate platelet contribution, thus highlighting fibrin-specific defects.[25] HEPTEM neutralizes heparin effects using heparinase I alongside contact activator, allowing differentiation of heparin-induced anticoagulation from intrinsic pathway deficiencies by comparing traces to INTEM.[25] APTEM assesses hyperfibrinolysis by incorporating aprotinin (a plasmin inhibitor) into the EXTEM setup; a stable trace here versus EXTEM indicates plasmin-mediated lysis.[25] Specialized assays extend thromboelastometry's utility for targeted evaluations. Platelet mapping assays, adapted for rotational thromboelastometry, quantify antiplatelet drug effects by measuring aggregation inhibition using activators like arachidonic acid (for aspirin) or adenosine diphosphate (for P2Y12 inhibitors like clopidogrel) in combination with standard reagents; percent inhibition is calculated from amplitude differences, with the method showing good correlation to light transmission aggregometry but longer run times.Parameters and Interpretation
Key Measurement Parameters
Thromboelastometry, also known as rotational thromboelastometry (ROTEM), generates a viscoelastic trace from which several key quantitative parameters are derived to assess the dynamics of clot formation, strength, and degradation. These parameters provide insights into the initiation, kinetics, stability, and lysis of the clot, enabling evaluation of coagulation factor activity, fibrinogen and platelet function, and fibrinolytic processes.[2][4] The Clotting Time (CT) measures the latency from the start of the test until the clot amplitude reaches 2 mm, corresponding to the initial detection of fibrin strands and reflecting the efficiency of coagulation factor activation and enzymatic pathways.[2] Prolonged CT values indicate deficiencies in clotting factors or the presence of anticoagulants, while shortened values may signal hypercoagulability.[4] In assays such as INTEM, which evaluates intrinsic pathway function, representative normal CT ranges from 100 to 240 seconds. For EXTEM (extrinsic pathway), the range is 38 to 79 seconds.[2] This parameter is analogous to the reaction time (R-time) in thromboelastography (TEG).[4] The Clot Formation Time (CFT) quantifies the time elapsed from the end of CT until the amplitude reaches 20 mm, assessing the speed of clot development primarily influenced by fibrinogen concentration and platelet contribution to fibrin polymerization.[2] Extended CFT suggests impaired clot kinetics, often due to low fibrinogen levels or platelet dysfunction.[4] For INTEM assays, a typical normal range is 30 to 110 seconds. For EXTEM, it is 34 to 159 seconds.[2] It corresponds to the K-time in TEG.[4] The alpha angle represents the angle formed by the tangent to the trace curve at the 20 mm amplitude point relative to the horizontal baseline, serving as an indicator of the rate of fibrin cross-linking and overall clot-building kinetics.[2] A reduced alpha angle points to slower polymerization, commonly associated with fibrinogen deficiencies.[4] Normal values are 70 to 83 degrees for INTEM and 63 to 83 degrees for EXTEM.[2] This metric is directly comparable to the alpha angle in TEG.[4] Maximum Clot Firmness (MCF) denotes the peak amplitude of the trace, equivalent to the maximum rotational torque exerted by the formed clot in millimeters, which integrates the contributions of platelets, fibrinogen, and factor XIII to overall clot mechanical strength.[2] Diminished MCF reflects hypofibrinogenemia or thrombocytopenia, guiding targeted interventions like fibrinogen replacement.[4] In EXTEM assays, which assess extrinsic pathway activation, normal MCF typically falls between 50 and 72 mm.[2] It is equivalent to the maximum amplitude (MA) in TEG.[4] Lysis parameters evaluate clot stability post-formation. The Lysis Index at 30 minutes (LI30) is calculated as the amplitude at 30 minutes after CT as a percentage of MCF, indicating the degree of clot degradation over time.[2][26] Reduced LI30 values signify ongoing fibrinolysis, potentially requiring antifibrinolytic therapy.[26] The Maximum Lysis (ML) measures the maximum percentage decrease in amplitude from MCF, capturing the extent of hyperfibrinolysis.[2] Elevated ML, such as greater than 15%, highlights significant clot breakdown and bleeding risk.[26] These indices are similar to the LY30 in TEG.[4] Parameter values can vary across assays like EXTEM or INTEM due to differences in activators, influencing interpretation for specific hemostatic pathways.[2]Trace Analysis and Reference Values
The thromboelastometry trace, or TEMogram, provides a visual representation of the viscoelastic changes in whole blood during coagulation and fibrinolysis. It initiates with a flat baseline denoting the reaction or clotting time (CT), the latency period before detectable clot formation begins. This transitions into a rapid upward curve signifying the clot formation phase, followed by a plateau at maximum clot firmness (MCF), which reflects peak clot stability. The trace may subsequently decline due to enzymatic lysis, with the extent quantified by maximum lysis (ML). A persistently flat trace, lacking significant rise or amplitude, indicates severe hypocoagulability, such as profound factor or fibrinogen deficiency.[27][2] Abnormal TEMogram patterns highlight specific hemostatic derangements. Prolonged CT manifests as an extended flat baseline, often due to coagulation factor deficiencies or anticoagulant effects. Reduced MCF appears as a lowered plateau, suggestive of hypofibrinogenemia or impaired platelet contribution to clot strength. Hyperfibrinolysis is characterized by a steep post-plateau decline with elevated ML, indicating excessive clot breakdown.[27][2] Reference ranges for TEMogram parameters are derived from multi-center studies in healthy adults and vary slightly by assay reagent and population. The following table summarizes standard ranges for commonly used assays (ROTEM delta device, adults):| Assay | Parameter | Reference Range (Adults) |
|---|---|---|
| EXTEM | CT | 38–79 s |
| EXTEM | CFT | 34–159 s |
| EXTEM | Alpha angle | 63–83° |
| EXTEM | MCF | 50–72 mm |
| INTEM | CT | 100–240 s |
| INTEM | CFT | 30–110 s |
| INTEM | Alpha angle | 70–83° |
| INTEM | MCF | 50–72 mm |
| FIBTEM | MCF | 9–25 mm |
| EXTEM | ML | <15% |
| INTEM | ML | <15% |