Keywords

Introduction

Hemorrhage is the leading cause of preventable death in combat trauma (Eastridge et al. 2012) and secondary cause of death in civilian trauma (U. S. Burden of Disease Collaborators 2013). Coagulopathic bleeding is frequently present (at least one quarter of civilian trauma patients and one third of military trauma patients present with a laboratory-defined coagulopathy) early after major trauma (Chang et al. 2016) and causes a three- to five-fold increase in mortality (Hess et al. 2008; Davenport and Brohi 2015). Nearly one third of severe trauma patients present with trauma-inducted coagulopathy (TIC) which carries a 50% mortality rate (Maegele et al. 2012; Simmons et al. 2014). The degree of TIC worsens as the injury severity increases (Cohen and West 2011; Frith et al. 2010). Therefore, early diagnosis, prevention, and treatment of TIC in the prehospital setting and at admission are of major interest in trauma patients.

Viscoelastic Hemostatic Tests

Thrombelastography (TEG; Haemonetics Corporation, Haemoscope Division, Nile, Illinois, USA) and rotational thromboelastometry (ROTEM; Tem Innovations GmbH, Munich, Germany succeeded by Instrumentation Laboratory, Bedford, Massachusetts, USA) are two point-of-care systems for hemostatic tests in whole blood (Whiting and DiNardo 2014). Both provide a global measure of hemostasis by quantitatively measuring the elasticity of blood from the beginning of coagulation to the ending with fibrinolysis. This includes the onset of clot formation, its progress, maximum clot strength, and clot stability, which provides important information about coagulation, fibrinolysis, and platelet function (Luddington 2005). TEG and ROTEM can also identify the relative contributions of clotting factors, such as fibrinogen and platelets, to the overall coagulation process (Whiting and DiNardo 2014).

TEG and ROTEM have been increasingly used in various clinical settings involving bleeding patients to diagnose and treat TIC (Hartmann et al. 2020a) including fibrinogen deficiency (Schlimp and Schöchl 2014), predict the risk of bleeding and mortality, and guide fibrinogen transfusion in trauma (Figueiredo et al. 2016), cardiac surgery (Görlinger et al. 2013), liver transplantation (Goerlinger 2006), and postpartum bleeding (Ranucci et al. 2016). A randomized clinical trial has concluded that TEG-guided massive transfusion protocol for severe trauma improves survival compared with that guided by conventional coagulation tests (CCTs, e.g., prothrombin time [PT]/international normalized ratio [INR], fibrinogen, and D-dimer) and utilizes less plasma and platelet transfusion during the early phase of resuscitation (Gonzalez et al. 2016). However, the latest multicenter randomized controlled trial (iTACTIC; NCT02593877) involving nearly 400 patients comparing TEG- or ROTEM-guided transfusion therapy with CCT-guided transfusion showed that the 28-day mortality was strongly reduced by TEG- or ROTEM-guided transfusion in patients with major hemorrhage who had a severe traumatic brain injury; however, there was no difference in the proportion of patients who were alive and did not require massive transfusion at 24 h after injury (Baksaas-Aasen et al. 2021). Although the evidence on the benefit of TEG and ROTEM over CCTs in trauma is limited at this time, substantial evidence from elective cardiac and liver transplant surgery studies provides further support for the use of TEG and ROTEM (Dias et al. 2019).

Fibrinogen

Fibrinogen plays a central role in both primary and secondary hemostasis (Levy et al. 2012) and TIC (Schlimp and Schöchl 2014). Upon major trauma, fibrinogen reaches levels critically below the physiological level of 2–4 g/L, earlier than those of other routine coagulation parameters and before patients meet the criteria for massive blood transfusion (Schlimp and Schochl 2014; Hayakawa et al. 2015). Low fibrinogen levels are associated with increased bleeding and coagulopathy and, as a result, poor clinical outcomes (Schlimp and Schochl 2014). The fibrinogen level is an independent predictor of mortality in major trauma patients and of the requirement for massive transfusion in patients with pelvic fractures (McQuilten et al. 2017a; Notani et al. 2020).

Different cutoff values of fibrinogen concentrations ranging from 1 to 1.8 g/L were used to define hypofibrinogenemia (Peng et al. 2019; Rourke et al. 2012). The current guidelines recommend fibrinogen supplementation (with fibrinogen concentrate (FC) or cryoprecipitate) in a bleeding patient with fibrinogen levels <1.5 g/L (Kozek-Langenecker et al. 2017; Rossaint et al. 2016) or equivalent by viscoelastic testing (Černý et al. 2022).

There is, however, a paucity of evidence to support the early replacement of fibrinogen in severely injured trauma patients. A systematic review and meta-analysis of the use of FC for trauma-related bleeding found no statistically significant difference in mortality between the groups, with 22% and 23.4% in the FC and comparator arms, respectively: risk ratio 1.00 [95% confidence internal 0.39–2.56], p = 0.99. Additionally, there was no statistical difference between FC and control in packed red blood cells (RBC), fresh frozen plasma (FFP), platelet transfusion requirements, and thromboembolic events (Stabler et al. 2020). On the other hand, another recent review of 21 major randomized controlled trials assessing FC use in perioperative settings found that approximately 60% of the studies in which FC was used to treat clinically relevant bleeding showed decreased bleeding tendency and decreased transfusion requirements versus comparative treatment (Cushing and Haas 2019).

Viscoelastic functional fibrinogen tests in particular ROTEM FIBTEM have been widely used for assessment of fibrinogen deficiency, prediction for transfusion requirement, and guided fibrinogen replacement, while studies focusing on TEG functional fibrinogen (FF) are limited (Peng and Nascimento 2018). A retrospective observational study showed that the incorporation of TEG FF into TEG-based coagulation management and FC administration reduced the need for transfusion in patients undergoing liver transplantation, with no impact on survival (Kozek-Langenecker et al. 2017; Peng and Nascimento 2018).

The review is structured into four main sections. The first section describes the principles of the two systems and various commercially available tests employing them, with an emphasis on FF tests. The similarities and differences of the two systems along with new viscoelastic testing systems are discussed as well. The second section reviews the use of TEG and ROTEM for diagnosis of TIC including TEG FF and ROTEM FIBTEM for hypofibrinogenemia. The third section depicts the use of TEG and ROTEM to assess FF levels and the hemostatic effect of fibrinogen replacement. The fourth section discusses TEG- and ROTEM-guided fibrinogen replacement.

Principles of TEG and ROTEM FF Tests

Figure 1a and b show the testing principles of the two most commonly used systems for FF tests: the TEG 5000 Hemostasis Analyzer and the ROTEM delta system. Both systems measure the viscoelastic properties of blood as it clots under low shear stress, but there are primary hardware differences between the two, as detailed by Peng et al. (2018). Briefly, the hardware differences include the mechanisms for cup/pin rotation and the detection of the rotation, cup materials, and interior surface properties.

Fig. 1
figure 1

Principles of and instruments for two viscoelastic testing systems. Schematic illustration of the mechanism and photograph of each instrument: (a) TEG 5000 (Haemonetics Corp., Niles, IL, USA), (b) ROTEM delta (Instrumentation Laboratory, Bedford, MA, USA), and representative TEG/ROTEM tracing showing the relationship between qualitative tracing and quantitative parameters (c). Panels A and B: courtesy of Haemonetics Corp. and TEM Systems Inc.

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For both systems, measurement is graphically represented as a characteristic shape profile over time (Fig. 1c). From this graph, the following parameters can be derived for TEG: (1) the reaction time R, which is related to plasma clotting factors and circulating anticoagulants; (2) the kinetic time K, which is associated with the activities of the clotting factors, fibrinogen, and platelets; (3) the rate of clot polymerization, represented by the angle α, which is a main function of the platelets, fibrinogen, and plasma components residing on the platelet surface; (4) the maximum amplitude (MA) or maximum clot strength, which is a direct function of the maximum dynamic properties of fibrin and platelet number and functions; and (5) fibrinolysis at 30 min or the rate of amplitude reduction 30 min after MA, LY30/CL30, which is related to plasma levels and activities of tissue plasminogen activator and its inhibitors. For the TEG FF test, the FF level (FLEV) in mg/dL or g/L can be calculated from MA using analytical software (Agarwal et al. 2014). For rapid TEG, in which both intrinsic and extrinsic activators are used, the activated clotting time (ACT) is calculated from the R value using the TEG software and may provide a better measure of initial clot formation than R itself (Blaine and Steurer 2019).

Similar parameters, as shown in Fig. 1c (e.g., coagulation time (CT), clot formation time (CFT), angle α, MCF, and clot lysis index LI30), are measured by ROTEM. In addition, clot amplitudes (CAs) at 5 and 10 min after CT (i.e., CA5 and CA10) have been reported for ROTEM.

Comparison of TEG and ROTEM FF Tests

In addition to instrumental differences, the two abovementioned viscoelastometric systems use different reagents, as summarized in Table 1 (Whiting and DiNardo 2014; Schöchl et al. 2013a; Carroll et al. 2009; Blaine and Steurer 2019). Specifically, the FF reagent for TEG is composed of lyophilized tissue factor and a platelet inhibitor (abciximab) that binds to glycoprotein-IIb/glycoprotein-IIIa receptors to inhibit platelet aggregation and exclude platelet contribution to clot strength (Solomon et al. 2012). For the TEG FF test, 0.5 mL of citrated or native blood is activated with a mixture of tissue factor and a monoclonal glycoprotein IIb/IIIa receptor antagonist, and then 340 μL of the activated blood is added to a TEG cup preloaded with 20 μL of 0.2 M CaCl2 (Ferrante et al. 2016). For the ROTEM FF test (FIBTEM), 20 μL of ex-TEM, 20 μL of fib-TEM solution, and 300 μL of citrated blood are mixed directly in a ROTEM cup (Solomon et al. 2012). The ex-TEM solution contains a combination of recombinant tissue factor and phospholipids that activates the extrinsic pathway of the coagulation system, whereas the fib-TEM solution contains CaCl2 as a recalcification reagent and a platelet inhibitor (cytochalasin D) that inhibits the actin/myosin system. Studies comparing different TEG and ROTEM FF tests have shown that the platelet inhibitor abciximab in TEG FF is less effective in eliminating the platelet contribution to clot strength than cytochalasin D in ROTEM FIBTEM; this results in a larger MA in TEG FF than MCF in ROTEM FIBTEM, affecting their dependence on the fibrinogen level (Solomon et al. 2012; Schlimp et al. 2014). Moreover, the TEG FF reagent does not contain heparinase or polybrene and therefore cannot be used for heparinized patients, unless a heparinase TEG cup is used. In contrast, the ROTEM FIBTEM reagent ex-TEM contains the heparin inhibitor polybrene. A new reagent, fib-TEM PLUS, containing two platelet inhibitors, cytochalasin D and tirofiban, provided the most accurate assessment of clot strength ascribed to fibrinogen function (Solomon et al. 2013a).

Table 1 Summary of TEG and ROTEM tests, their corresponding activators and inhibitors, and their applications (Whiting and DiNardo 2014; Schöchl et al. 2013a; Carroll et al. 2009; Blaine and Steurer 2019)
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TEG and ROTEM have yielded different results for diagnosing coagulopathy and guiding transfusion (Sankarankutty et al. 2012), and different transfusion algorithms have been developed for each system (Coakley et al. 2006; Enriquez and Shore-Lesserson 2009). For example, ROTEM-based algorithms tended to recommend the use of FC (Schöchl et al. 2010b) or cryoprecipitate (Tanaka et al. 2012), whereas TEG-based algorithms tended to recommend the use of plasma (Coakley et al. 2006); however, both systems recommended lower transfusion than standard laboratory measures of coagulation. These differences may be due to the tests rather than the instruments, as most ROTEM-guided transfusions involve FIBTEM, which is a specific test for fibrinogen level and function (Görlinger et al. 2012), while the TEG FF test is less involved in TEG-guided transfusions (Sawyer et al. 2012) and FC administration (Levy et al. 2014; Spahn et al. 2016).

Agreements between TEG and ROTEM recommendations to transfuse platelets were fair (kappa coefficient of agreement (κ) = 0.33 between ROTEM INTEM and native heparinase TEG, and κ = 0.28 between ROTEM INTEM and kaolin heparinase TEG) but were low in case of a low MA, suggesting the need for transfusion of either fibrinogen or platelets. There was a moderate agreement between ROTEM INTEM and prothrombin time (κ = 0.42) and a poor agreement between the recommendations of viscoelastic tests to administer FFP (Coakley et al. 2006). ROTEM FIBTEM has been used to assess and guide fibrinogen replacement. In contrast, TEG FF is less used for TEG-guided transfusion (Sawyer et al. 2012). There is a lack of studies directly comparing the utilities of ROTEM FIBTEM and TEG FF for the diagnosis of coagulopathies, including hypofibrinogenemia, and the guidance of transfusions, including fibrinogen replacement, although both have been reported to be useful (Carroll et al. 2009; Rugeri et al. 2007).

Meyer et al. (2014) compared different TEG and ROTEM tests, including TEG FF and FIBTEM, and the Clauss method to detect trauma-induced coagulopathy and goal-directed transfusion therapy. TEG FF and ROTEM FIBTEM early amplitudes (CA5, CA10) and MA/MCF had similar correlations with Clauss fibrinogen levels and could differentiate coagulopathic and transfused patients from non-coagulopathic and non-transfused patients. In a similar study, TEG and ROTEM were compared for FF tests in trauma patients (Meyer et al. 2015). TEG FF MA and ROTEM FIBTEM MCF correlated well with each other (ρ = 0.71, p < 0.001) and with the Clauss fibrinogen level (ρ = 0.64 for both, p < 0.001).

We compared the capabilities of the TEG and ROTEM FF tests to detect coagulation and fibrinolysis changes in response to hemostatic treatment and to predict acute traumatic coagulopathy and transfusion requirements in a randomized, controlled trial for fibrinogen in the initial resuscitation of severe trauma (Peng et al. 2018; Peng et al. 2019). Overall, we found significant differences in TEG FF MA and ROTEM MCF between placebo- and fibrinogen-treated groups over hospitalization time. ROTEM FIBTEM MCF seemed to be more consistent with the duration of the between-group difference, as indicated by fibrinogen levels, than TEG FF MA. There were significant correlations between corresponding parameters of TEG FF and ROTEM FIBTEM, with TEG FF MA and ROTEM FIBTEM MCF showing the strongest correlation (ρ = 0.80, p < 0.001); however, they were not interchangeable, and MA was larger than MCF. In addition, ROTEM CT and LI30 indicated the effect of fibrinogen administration on coagulation time and fibrinolysis. There were discrepancies between TEG and ROTEM in their detection of coagulation abnormalities, hypofibrinogenemia, and hyperfibrinolysis (Peng et al. 2018).

New Viscoelastic Hemostatic Testing Systems

New and fully automated (no pipetting) TEG and ROTEM systems (TEG 6 s [Haemonetics Corp.] and ROTEM sigma [Instrumentation Laboratory]) are now available. Both work with four-channel cartridges but are based on different mechanisms. TEG 6 s uses a new technology termed “coagulation resonance analysis” and microfluidic cartridges containing dried reagents (Gurbel et al. 2016). ROTEM sigma operates on the same pin and cup technology as ROTEM delta but uses cartridges containing lyophilized bead reagents instead of liquid reagents (Görlinger et al. 2016).

TEG 6 s reportedly is highly reliable, with results strongly correlating with those derived from TEG 5000 (linear correlation estimates >0.9) (Neal et al. 2020). ROTEM sigma also has a high precision, with results being strongly correlated with those derived from ROTEM delta (Pearson correlation coefficients ≥0.8) (Schenk et al. 2019). Furthermore, when compared for use in trauma patients, strong to very strong correlations (Spearman correlation coefficients >0.6) were observed between corresponding TEG 6 s and ROTEM sigma parameters, although there were significant differences in absolute values for most measurements (Ziegler et al. 2019).

Furthermore, other viscoelastic hemostatic testing systems are available and emerging (Hartmann et al. 2020b). Sonoclot is a legacy device developed by Sienco, Inc. The Sonoclot device differs from TEG and ROTEM in that it is not a rotational-based system but a linear motion system (Ganter and Hofer 2008). Quantra hemostasis analyzer is a relatively new product developed by HemoSonics based on a proprietary technology that uses ultrasound to measure clot time and clot stiffness from changes in viscoelastic properties of whole blood during coagulation (Ferrante et al. 2016). Multicenter evaluation of the Quantra system in adult patients undergoing major surgical procedures consisting primarily of cardiac and major orthopedic surgeries was conducted, showing that the correlation between ROTEM and Quantra was very strong with correlation coefficients ranging between 0.84 and 0.89. (Groves et al. 2020). Additional receiver operating characteristics analysis indicated sensitivities and specificities in the 80–90% range when Quantra parameters were used to discriminate ROTEM threshold values currently used in goal-directed treatment algorithms. Several emerging technologies are currently in development for point-of-care viscoelastic hemostatic testing, including microfluidics, fluorescent microscopy, electrochemical sensing, photoacoustic detection, and micro−/nano-electromechanical systems (MEMS/NEMS) (Mohammadi Aria et al. 2019).

Applications to Diagnosis of TIC and Hypofibrinogenemia

Historically, TIC is defined by CCTs, such as INR above a threshold of 1.2 (Frith et al. 2010; Meyer et al. 2014; Davenport et al. 2011; Hagemo et al. 2015), 1.3 (Tonglet et al. 2018; Kornblith et al. 2014; Cohen et al. 2013), 1.5 (Niles et al. 2008), and 1.6 (Rugeri et al. 2007), PTT ≥ 35 s (Cohen et al. 2013); by plasma fibrinogen levels, ranging from 1.0 to 2.0 g/L; and by platelet counts below 100 × 109/L (Rossaint et al. 2016). However, this is no sound evidence to support the usefulness of these tests in particular INR, PTT for diagnosis of coagulopathy, or to guide hemostatic therapy (Haas et al. 2015). There are several limiting factors with these assays, such as the time to obtaining results from multiple tests; sole measurement of contribution of plasma proteins to clot formation, without regard for the central role of platelets; and the inability to identify hyperfibrinolysis (Moore et al. 2021). The use of CCTs such as INR in trauma has been severely criticized due to the lack of association with bleeding and blood transfusion. It has been reported that INR overestimated coagulopathy and should not be used to guide blood transfusion in stable trauma and surgical patients (McCully et al. 2013).

Consequently, viscoelastic hemostatic tests in particular TEG and ROTEM have been adopted for the diagnosis of TIC, owing to their assessment of whole blood clot formation and degradation in real time, and rapid availability of the comprehensive information. TEG has been shown to identify additional coagulopathies compared to CCT methods (Sumislawski et al. 2019). TEG and ROTEM can detect different fibrinolysis phenotypes in trauma (Stettler et al. 2019).

Table 2 summarizes the literature on the use of viscoelastic hemostatic tests for diagnosis of TIC according to test done (TEG or ROTEM) and the study design, method of blood sampling, activators used, and parameters studied along with main findings.

Table 2 Viscoelastic hemostatic tests (TEG and ROTEM) for diagnosis of TIC
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TIC was defined by celite- and kaolin-activated TEG, respectively, when ≥2 TEG parameters are abnormal (Kaufmann et al. 1997; Rizoli et al. 2011). TIC was also defined according to rapid TEG if any of the following variables were abnormal (Holcomb et al. 2012; Ostrowski et al. 2017): ACT>128 s, R time > 66 s, K > 150 s, Alpha<56°, MA < 55 mm, and LY30 > 3%. When defined by kaolin TEG, one or several of the TEG parameters could be abnormal to indicate coagulopathy in an algorithm with R ≥ 11 min, MA ≤ 50 mm, angle<52°, or LY30 > 8% (Johansson et al. 2010). Rapid TEG is faster than kaolin TEG and CCTs for providing reliable information on coagulopathy in patients with multiple injuries (Jeger et al. 2009).

TEG and ROTEM could detect a hypercoagulable state, hyperfibrinolysis, and were better tests than PT or PTT (Branco et al. 2014; Park et al. 2009; Schreiber et al. 2005; Spasiano et al. 2022; Watters et al. 2010). However, single TEG R performed worse than INR at identifying vitamin K-dependent coagulation factor deficiency (Nascimento et al. 2012).

TIC was defined by hypocoagulable state on ROTEM which was determined by one of the principle parameters (CT, CFT, MCF, ML) outside the manufacturer’s normal ranges by 20% (i.e., CT ≥ 94 s, CFT ≥ 190 s, MCF ≤ 40 mm, ML ≥ 12%) (Tonglet et al. 2018). EXTEM which includes the platelet contribution to the development of coagulation abnormalities would be more suitable to detect coagulopathy. EXTEM CA5 ≤ 35 mm could predict massive transfusion and was used to define TIC (Rourke et al. 2012). EXTEM CA5 ≤ 35 mm (Davenport et al. 2011) and ≤ 37 mm (Hagemo et al. 2015) threshold values for detection of TIC resulted in a detection rate of 77% and 66.3%, respectively, and FIBTEM CA5 ≤ 8 mm detected TIC in 67.5%, while fibrinogen concentration ≤ 1.6 g/L detected TIC in 73.6% (Hagemo et al. 2015). TIC defined by EXTEM MCF < 40 mm was 39% in combat casualties (Woolley et al. 2013). ROTEM also detected more abnormal coagulation status than CCTs (PT and PTT) in a deployed military setting (Doran et al. 2010) as well as in-hospital emergency department (Spagnolello et al. 2021). EXTEM MCF showed 100% sensitivity and specificity for detection of hyperfibrinolysis defined as a euglobulin lysis time (ELT) < 90 min (Levrat et al. 2008). Combined with INR > 1.2, EXTEM A5 ≤ 35 mm and/or LI30 < 97% on admission classified 15% more patients with TIC and predicted massive transfusion with higher sensitivity (86% vs. 64%) than INR > 1.2 alone in military trauma patients (Cohen et al. 2019).

High-quality studies are need for diagnosis of TIC using viscoelastic devices (Sakamoto et al. 2017). Additionally, a clinical scoring system for assessing TIC, which includes subclassifications for the anatomic location of injury and interventions required for bleeding control, has been proposed (Neal et al. 2015). European trauma experts recommend a grading system comprising three severity levels based on fibrinogen level, INR, and platelet count, to define TIC (Černý et al. 2022).

Table 3 summarizes the predictive accuracy of TEG FF and ROTEM FIBTEM for hypofibrinogenemia in trauma. MA and MCF are the main parameters used for the predictions of hypofibrinogenemia and blood transfusions. The prediction accuracy was evaluated by sensitivity, specificity, and area under the receiver operating characteristic curve (AUC) and variate regression analyses. Different cutoff values of fibrinogen concentrations ranging from 1 to 1.8 g/L were used to define hypofibrinogenemia. Traditionally, a plasma fibrinogen level of 1 g/L was established for fibrinogen replacement in patients with congenital fibrinogen deficiency, whereas the threshold varied from 0.8 to 2.0 g/L in patients with acquired fibrinogen deficiency (Levy et al. 2014). In contrast, a critical fibrinogen concentration of 2.29 g/L was identified in trauma below which a significant increase in mortality occurred (Hagemo et al. 2014). The discrepancy implies that the negative impact of fibrinogen deficiency in trauma may have been underestimated. It should also be noted that hypofibrinogenemia prevalence in major bleeding varies across clinical contexts (McQuilten et al. 2017b).

Table 3 Clinical evaluation of TEG and ROTEM functional fibrinogen tests for diagnosis of hypofibrinogenemia in trauma
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Most clinical studies of hypofibrinogenemia in trauma are prospective observational, while a few are retrospective and randomized controlled. Sample size ranged from 23 to 1077 patients. In contrast with ROTEM, TEG FF has been used less to detect hypofibrinogenemia and predict blood transfusion requirements. Among various clinical settings, ROTEM FIBTEM has been mostly used in trauma, cardiac surgery, and liver transplantation with best predictive power for hypofibrinogenemia (fibrinogen <1.5 g/L) (AUC = 0.99) in cardiac surgery (Bhardwaj et al. 2017). Furthermore, several studies have shown that TEG FF and ROTEM FIBTEM could predict bleeding and transfusion requirements in trauma (Johansson et al. 2013; Schöchl et al. 2011), with various accuracies. It appeared that ROTEM would have better predictive accuracy than TEG because it has greater specificity for some common coagulopathies in cardiac surgery, such as fibrinogen deficiency. The averaged likelihood ratio of TEG FF MA for diagnosis of hypofibrinogenemia is 4.71 ± 2.18 based on a number of studies (Gautam et al. 2017; Meyer et al. 2015; Peng et al. 2018), while the corresponding value of ROTEM FIBTEM MCF is 9.24 ± 2.64 calculated from the literature (Meyer et al. 2015; Peng et al. 2018; Jeong et al. 2015).

Only a few studies demonstrated ROTEM FIBTEM provided faster and better prediction than plasma fibrinogen concentration for massive transfusion (Schöchl et al. 2011) and bleeding (Dötsch et al. 2017), respectively. ROTEM FIBTEM provided early prediction of massive transfusion in trauma similar to the most predictive laboratory parameters (e.g., fibrinogen and hemoglobin concentrations) (Schöchl et al. 2011). A separate study comparing standard fibrinogen measurement methods (i.e., Clauss method and thrombin clotting time) with ROTEM FIBTEM in patients with cirrhosis suggested FIBTEM as a promising alternative to standard plasma fibrinogen measurement in cirrhotic patients, especially in evaluating fibrin polymerization disorders in these patients (Vucelic et al. 2015).

There is insufficient evidence or low-quality evidence for the benefits of TEG and ROTEM for the prediction of bleeding and adverse outcomes beyond that achieved using routinely measured baseline factors or CCTs except for rapidity. ROTEM EXTEM and FIBTEM were no better than routine laboratory tests for detecting differences between surviving and nonsurviving critically ill patients (Larsson et al. 2015). ROTEM FIBTEM was not a good test to predict the presence of acute coagulopathy of trauma defined as an INR > 1.3 or a fibrinogen level < 1.5 g/L unless combined with EXTEM, and either of the tests could predict the need for emergent blood product transfusions (defined as ≥5 units of RBC and ≥ 3 units of plasma within the first 24 h of care) (Tonglet et al. 2018).

Finally, if fibrinogen deficiency has a causal relationship with bleeding and adverse clinical outcomes, it is sensible to suggest that TEG and ROTEM FF tests that improve clinical prediction for fibrinogen-related bleeding may also have the potential to predict adverse clinical outcomes. However, randomized trials are needed to provide high-quality evidence for the role of TEG and ROTEM in diagnosis, management, and monitoring of fibrinogen function and replacement in bleeding patients.

Assessment of the FF Level and Hemostatic Effect of Fibrinogen Replacement

The Clauss test is considered a standard FF test for determining the plasma fibrinogen level, although other methods, such as the prothrombin time-derived method (Blasi et al. 2012) and enzyme-linked immunosorbent assay (ELISA) (Kalina et al. 2008), are also used. However, ELISA does not discriminate between functional and nonfunctional immunoreactive fibrinogen proteins or even some fibrinogen degradation products (Mackie et al. 2002).

The Clauss method is limited to low levels of heparin (which inactivate thrombin through antithrombin III), which is a serious limitation to its use in cardiac surgery. It may be affected by fibrin degradation products and polymerization inhibitors as well as inhibitors of fibrin formation (Koh et al. 1994). Its turnaround time is approximately 40 min (Asmis 2015). In comparison, the TEG and ROTEM FF tests can be completed in 15 min and provide rapid and accurate detection of hyperfibrinolysis (Schöchl et al. 2010a). Another advantage of TEG and ROTEM is that they can be used for fully heparinized patients (Solomon et al. 2012; Gertler et al. 2011).

TEG has been used to study in vitro effects of fibrinogen on coagulation of plasma deficient in coagulation factors and diluted by colloids (Nielsen et al. 2005; Nielsen 2005). It has been used to monitor the effect of a cardiopulmonary bypass system with biocompatible coating on fibrinogen levels (Fluger et al. 2011). ROTEM has been used to determine the usefulness of fibrinogen substitution to reverse dilutional coagulopathy in in vitro (Fries et al. 2006), animal (Fries et al. 2005), and ex vivo models (Fenger-Eriksen et al. 2005). In vitro study showed dose-dependent increase in ROTEM MCF with the amount (0–3 mg/mL) of FC (Haemocomplettan P, CSL Behring GmbH, Marburg, Germany) added to normal human plasma pool, fibrinogen-deficient plasma pool, and individual plasma samples from 17 patients with fibrinogen deficiency (Kalina et al. 2008). All these studies showed that to various extents, fibrinogen improved clot strength (MA or MCF), clot formation (R or CT), and clot propagation (α) as measured by TEG or ROTEM.

In addition, ex vivo ROTEM studies indicated that administration of 6 g FC to samples of coagulopathic trauma patients could correct FIBTEM CA5 and MCF to the level of patients with minor injury (Rourke et al. 2012). In contrast, the ex vivo addition of cryoprecipitate at a standard dose of cryoprecipitate (equivalent to 2.6 g fibrinogen) was unable to reverse the coagulopathy until a high dose (equivalent to 7.8 g).

As summarized in Table 4, a number of clinical studies on TEG and ROTEM tests, especially those on ROTEM FIBTEM, have assessed hemostatic effects of FC administration in major trauma (Peng et al. 2019; Rourke et al. 2012; Ponschab et al. 2015; Schlimp et al. 2013a; Innerhofer et al. 2013, 2017; Schöchl et al. 2010b; Ziegler et al. 2021), including early cryoprecipitate transfusion (Curry et al. 2015), cardiovascular surgery with cardiopulmonary bypass (Schlimp and Schöchl 2014; Gautam et al. 2017; Meyer et al. 2015), liver transplantation (Görlinger et al. 2013; Peng et al. 2018), and orthopedic surgery (Jeong et al. 2015). Unless specified otherwise, the TEG and ROTEM tests were performed using TEG 5000 and ROTEM delta and the reagents and procedures recommended by the respective manufacturers.

Table 4 Hemostatic effects of fibrinogen replacement as measured by TEG and ROTEM
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Most of these clinical studies were randomized and controlled (Peng et al. 2019; Innerhofer et al. 2017; Curry et al. 2015; Ziegler et al. 2021; Nascimento et al. 2016), whereas a few were prospective, observational, or retrospective (Ponschab et al. 2015; Schlimp et al. 2013a; Innerhofer et al. 2013; Schöchl et al. 2010b; Seebold et al. 2019). Fibrinogen replacement was conducted preemptively or was guided by ROTEM or TEG. ROTEM FIBTEM has been well used in trauma, showing a dose-dependent increase in MCF immediately after fibrinogen administration. The hemostatic effect could last from 4 to 48 h (Peng et al. 2019; Wikkelsø et al. 2015). Furthermore, several studies have shown that the TEG FF- and ROTEM FIBTEM-measured hemostatic effect mirrored plasma fibrinogen profiles in response to fibrinogen replacement (Peng et al. 2019; Curry et al. 2015).

Some of these studies also used ROTEM to guide FC administration (Ponschab et al. 2015; Schlimp et al. 2013a; Innerhofer et al. 2013, 2017; Schöchl et al. 2010b). Few studies on the effects of FC administration on TEG FF have been reported (Peng et al. 2019), although some have shown a correlation between TEG FF MA and the Clauss fibrinogen level (Kornblith et al. 2014; Harr et al. 2013). Alternatively, TEG FF has been used to measure the effect of fibrinogen levels on heparin resistance/thromboprophylactic treatment in trauma (Harr et al. 2014).

Fibrinogen is not the only contributor to TEG FF and ROTEM FIBTEM CAs, which may limit their utility for the assessment of the hemostatic effect of fibrinogen replacement. Activated factor XIII and hematocrit levels may affect clot firmness as well (Schlimp et al. 2013b; Solomon et al. 2013b; Nielsen et al. 2004; Ogawa et al. 2012; Thomas et al. 2016). In one study, postoperative factor XIII levels correlated with FIBTEM MCF more significantly than fibrinogen levels in patients undergoing major upper gastrointestinal surgery (Thomas et al. 2016). The same study showed a significant correlation between platelet count and ROTEM FIBTEM MCF (r = 0.55, p < 0.01), which implied that the test might be profoundly impaired by the incomplete inhibition of platelet contribution to clot strength. Factor XIII levels and platelet count might also affect TEG FF (Gautam et al. 2017; Nielsen et al. 2004). The correlation between FIBTEM CA10 and Clauss fibrinogen became weaker as the hemoglobin level increased, suggesting that the hemoglobin level could influence the measurement of fibrinogen by FIBTEM (Mace et al. 2016). The correlation could also be weakened by fibrinogen replacement in trauma patients (David et al. 2016).

Hemostatic effects, as measured by TEG and ROTEM, can be affected by resuscitation fluids. Fenger-Eriksen et al. (2010) assessed fibrinogen levels in plasma diluted in vitro with different fluids (isotonic saline, hydroxyethyl starch, human albumin), using an antigen determination method, three photo-optical Clauss methods, one mechanical Clauss method, a prothrombin-derived method, and viscoelastic measurement through ROTEM. Fibrinogen levels were overestimated by the photo-optical Clauss methods due to dilution with hydroxyethyl starch. In contrast, ROTEM FIBTEM MCF was reduced by dilution with hydroxyethyl starch and, to a lesser extent, by dilution with human albumin; the former effect was ascribed to an unexplained interference with the optical source by hydroxyethyl starch, and the latter was due to impairment of fibrin polymerization induced by the fluids. Mittermayr et al. (2007) reported that the magnitude of clot firmness reduction was determined by the type of fluid used in major orthopedic surgery. FIBTEM MCF was most strongly affected by hydroxyethyl starch, followed by gelatin solution and Ringer’s lactate solution.

In addition to MA, other TEG parameters, e.g., estimated FLEV, kinetic time K, and α, and kaolin TEG K and α can be used to assess fibrinogen levels (Kornblith et al. 2014; Harr et al. 2013). Kornblith et al. (2014) confirmed a significant correlation between TEG FF FLEV and Clauss fibrinogen test in trauma patients, similar to results by Harr et al. (2013). However, the correlations were affected by the fibrinogen level; they decreased at low and high FLEVs (Harr et al. 2013).

In contrast, the FLEV estimated using TEG FF was, on average, 1.0 g/L higher than that determined by the Clauss method in both surgical patients and healthy controls (Fries et al. 2006). This is consistent with other reports of higher TEG FF FLEVs than Clauss values in cardiac surgery (Fries et al. 2005) and obstetric patients (Fenger-Eriksen et al. 2005).

Among all the parameters (kaolin TEG K, α, and MA), the strongest correlations have been reported between TEG FF MA/ROTEM FIBTEM MCF and the plasma fibrinogen level (Kornblith et al. 2014; Harr et al. 2013), suggesting that these parameters are the most useful for monitoring the role of fibrinogen in the hemostasis of bleeding patients.

TEG/ROTEM-Guided Fibrinogen Replacement

ROTEM has been widely used to guide FC administration in different perioperative settings, including trauma surgery, cardiovascular surgeries, and liver transplantation obstetric hemorrhage. Retro- and prospective studies of cardiac surgery have shown that FIBTEM-guided fibrinogen replacement generally reduces transfusion (Williams et al. 2017).

TEG and ROTEM have been mostly implemented during active bleeding situations in the emergency room and during surgery. As summarized in Table 5, case reports (Schöchl et al. 2010c, d; Ziegler et al. 2013; Brenni et al. 2010; Grassetto et al. 2012), retrospective (Ponschab et al. 2015; Schlimp et al. 2013a, 2016; Schöchl et al. 2010b, 2011; Seebold et al. 2019) and prospective clinical studies (Innerhofer et al. 2013; Schöchl et al. 2014), and randomized controlled trials (Innerhofer et al. 2017) have demonstrated that ROTEM FIBTEM has been successfully used to guide fibrinogen administration in trauma, leading to reduced allogeneic blood transfusion (Innerhofer et al. 2013; Ziegler et al. 2013; Schöchl et al. 2010d).

Table 5 Summary of TEG/ROTEM-guided fibrinogen replacement in trauma
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In contrast, there are few studies on TEG-guided fibrinogen replacement across various clinical settings, with most focusing on trauma. Kaolin-activated TEG (Johansson and Stensballe 2009; Tapia et al. 2013; Walsh et al. 2011) and rapid TEG (Gonzalez et al. 2016; Holcomb et al. 2012; Sawyer et al. 2012; Pezold et al. 2012) rather than TEG FF were used to guide fibrinogen supplementation in these studies, and TEG α was used to guide fibrinogen supplementation, whereas MA was used to guide platelet transfusion. Some of these studies used FFP (Johansson and Stensballe 2009) and cryoprecipitate transfusion guided by TEG (Walsh et al. 2011; Pezold et al. 2012) instead of FC. Disadvantages of FFP and cryoprecipitate include the requirement for cold storage and time for thawing (17 min on average) (Curry et al. 2015), risk of viral transmission, and large administration volume. FFP contains a low fibrinogen level, which can vary greatly between batches, and, when administered in large volumes, may dilute plasma fibrinogen (McNamara et al. 2015).

The clinically best-studied FCs in the USA and Canada are Haemocomplettan P and RiaSTAP (CSL Behring GmbH, Marburg, Germany); other commercially available FC products include Clottagen (LFB Biomédicaments, Les Ulis, France) (Roullet et al. 2015), Fibrinogen HT (Benesis, Osaka, Japan), and FibroRAAS (Shanghai RAAS, Shanghai, China) (Franchini and Lippi 2012). Fibryga (Octapharma, Lachen, Switzerland) is a new, highly purified, lyophilized FC (Schulz et al. 2018). In vitro and clinical studies have shown a higher factor XIII level (10.1 IU/mL vs. 7.2 IU/mL) (Haas et al. 2018), slower clearance (0.665 mL/h/kg vs. 0.804 mL/h/kg), and a larger volume of distribution (70.158 mL/kg vs. 76.631 mL/kg) for Fibryga than for RiaSTAP (Ross et al. 2018). Another clinical study reported an even lower clearance (0.53 mL/h/kg) and lower distribution volume (50.7 mL/kg) for Clottafact (Djambas Khayat et al. 2019).

While FC is generally administered by bolus intravenous injection, one study showed potential advantages of using continuous infusion, as it allows rapid adjustments in the delivery rate in response to changing plasma levels (Morrison et al. 2012). It avoids or reduces peaks and troughs in the plasma fibrinogen level and allows the maintenance of satisfactory hemostasis during surgery.

Different critical fibrinogen levels and cutoff values for TEG and ROTEM have been used to guide fibrinogen replacement therapy in trauma (Theusinger et al. 2014; Mengoli et al. 2017; Nardi et al. 2015; Schöchl et al. 2012, 2013b; Fries et al. 2009; Lier et al. 2013; Görlinger et al. 2012) (Table 6). Most of these thresholds are parts of the ROTEM- or TEG-guided transfusion algorithms for different blood products (RBC, FFP, platelets) (Johansson et al. 2013; Stensballe et al. 2014; Johansson et al. 2014). Fibrinogen supplementation has been recommended for a plasma fibrinogen level below 1 g/L (Miceli et al. 2016), which approximately corresponds to a TEG FF MA of 16 mm and ROTEM FIBTEM MCF of 8 mm (Peng et al. 2019). The abovementioned values of TEG FF MA and ROTEM FIBTEM MCF are both higher than the lower thresholds of the normal ranges for the TEG FF (11–24 mm) and ROTEM FIBTEM (7–24 mm) tests recommended by the respective manufacturers. This agrees with a report that the frequently recommended threshold for fibrinogen substitution of 9 mm MCF in FIBTEM does not match the recommended threshold of ≤1.0 g/L plasma fibrinogen measured by the Clauss method, although there was a strong correlation between FIBTEM MCF and Claus fibrinogen (r > 0.8) (Requena et al. 2011). These discrepancies should be considered carefully when developing goal-guided fibrinogen replacement using TEG and ROTEM.

Table 6 Summary of threshold values for TEG- and ROTEM-guided fibrinogen replacement in trauma
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Fibrinogen levels of 0.8–2.0 g/L have been recommended as transfusion triggers in trauma and massive hemorrhage (Levy et al. 2014; Kaufner et al. 2016), with a level of 1 g/L being reported in most guidelines for fibrinogen replacement (McQuilten et al. 2017b). Accordingly, a range of CA10 and MCF values in FIBTEM, including CA10 < 7 mm (target FIBTEM CA10: 10–12 mm) (Ponschab et al. 2015; Schöchl et al. 2012) or MCF <7 mm in trauma (Innerhofer et al. 2013), CA < 8 mm in cardiac surgery (Weber et al. 2014), and MCF < 8 mm in liver transplantation (Goerlinger 2006), have been used to trigger fibrinogen replacement. Moreover, FIBTEM CA10 or MCF can be used to determine the FC dosage. For example, 2–4 g FC was required in trauma patients if FIBTEM CA10 was 4–6 mm; and 6 g FC was required if FIBTEM CA10 was 0–3 mm (Schlimp et al. 2013a). FC administration has also been based on the plasma fibrinogen level, with varying thresholds (Weiss et al. 2011; Danés et al. 2008). Specifically, the fibrinogen dosage can be calculated based on the desired increment in fibrinogen level, as follows (Lier et al. 2013):

$$ \textrm{Fibrinogen}\kern0.5em \textrm{dosage}\kern0.5em \left(\textrm{g}\right)=0.05\times \textrm{desired}\kern0.5em \textrm{increment}\kern0.5em \left(\frac{g}{L}\right)\times \textrm{body}\kern0.5em \textrm{weight}\kern0.5em \left(\textrm{kg}\right) $$

There are fewer studies on TEG-guided fibrinogen transfusion in trauma (Gonzalez et al. 2016; Holcomb et al. 2012; Johansson and Stensballe 2009; Tapia et al. 2013; Walsh et al. 2011; Pezold et al. 2012). Compared with ROTEM FIBTEM, TEG FF, which uses a platelet inhibitor, has been less employed to measure fibrinogen levels and guide its administration. TEG FF MA < 14 mm has been used to trigger fibrinogen supplementation in patients with massive hemorrhage (Johansson et al. 2014), and MA ≤ 7 mm has been used in liver transplantation (De Pietri et al. 2016). Kaolin or rapid TEG K and α has been used to guide fibrinogen supplementation with cryoprecipitate in trauma (Schöchl et al. 2013a; Tapia et al. 2013; Brazzel 2013; Gonzalez et al. 2010; Stahel et al. 2009; Kashuk et al. 2009, 2012), but may not be as good as TEG FF MA, which is a more direct measure of the plasma fibrinogen level (Harr et al. 2013; Solomon et al. 2015).

Compared with ROTEM MCF, TEG α, in particular, kaolin-activated TEG α, is most commonly used to guide fibrinogen replacement (mostly using a cryoprecipitate), whereas TEG MA is generally used to guide platelet transfusion (Solomon et al. 2015). However, TEG MA could not distinguish fibrinogen from platelet deficiency when a single TEG test was conducted without platelet inhibitors; thus, its use in guiding fibrinogen transfusion may be limited (Kashuk et al. 2012).

These results underline the necessity to implement different individual triggers for fibrinogen supplementation, depending on the viscoelastic hemostatic tests used and the clinical settings. For example, in bleeding trauma patients, a FIBTEM CA10 ≤ 7 mm may trigger FC administration, with a target MCF of 10–12 mm. In contrast, when using TEG FF, MA < 14 mm is recommended as a trigger (Schochl et al. 2016).

Cutoff values of kaolin TEG K > 2.4 min, α < 60.6°, and MA < 51.2 mm have been recommended for the diagnosis and treatment of severe hypofibrinogenemia (fibrinogen <1 g/L) in trauma patients, whereas K could be used to guide early cryoprecipitate or FC transfusion (Chow et al. 2019).

One study suggested that the CA 10 min after R or CT reflects a more dynamic phase of the hemostatic process than MA/MCF and may lead to earlier goal-directed transfusion therapy (Meyer et al. 2014). FIBTEM and APTEM have been used in combination with EXTEM to guide platelet transfusion and the treatment of hyperfibrinolysis with tranexamic acid (Smith et al. 2020; Juffermans et al. 2019), respectively.

Mini-Dictionary of Terms

  • Trauma-induced coagulopathy (TIC). TIC normally refers to acute traumatic coagulopathy which consists of two core components: (1) trauma itself, tissue damage- and hypoperfusion-induced endogenous TIC and (2) resuscitation-associated exogenous TIC involving hypothermia, acidosis, and hemodilution.

  • Viscoelastic hemostatic tests. These tests measure changes in viscoelastic properties of whole blood during clot formation, buildup, and degradation. The most commonly used devices are thrombelastography (TEG 5000) and rotational thromboelastometry (ROTEM delta).

  • Conventional coagulation tests (CCTs). These tests also refer to standard laboratory tests typically including prothrombin time (PT) and activated partial thromboplastin time (aPTT), Clauss fibrinogen test, and platelet count.

  • Fibrinogen concentrate. It is plasma-derived, highly purified concentrate of lyophilized human fibrinogen and needs to be reconstituted with sterile water for infusion.

  • Hypofibrinogenemia. It is normally defined as plasma fibrinogen concentration below 1.5 g/L.

  • Fibrinogen replacement. It is treatment of fibrinogen deficiency with exogenous fibrinogen via infusion of fibrinogen concentrate or cryoprecipitate.

Key Facts of Trauma

  • Trauma is a major global public health issue, causing nearly six million deaths worldwide each year.

  • It is the leading cause of death in people aged 18–39 years.

  • Hemorrhage is the most common cause of preventable deaths after trauma.

Summary Points

  • TEG and ROTEM tests play important roles in early diagnosis of TIC and its phenotypes, assessment, and guidance of fibrinogen replacement. Their potential clinical benefits are often inferred from trauma and cardiac surgery literature.

  • ROTEM FIBTEM MCF has been mostly used to discriminate fibrinogen deficiencies and assess hemostatic effects of fibrinogen replacement compared to kaolin and rapid TEG, and TEG FF parameters including K, α, and MA.

  • When using TEG FF and ROTEM FIBTEM to diagnose fibrinogen deficiency and guide fibrinogen administration, other variables, such as hematocrit, factor XIII levels, resuscitation fluids, and fibrinogen level ranges, should be considered.

  • Since TEG FF and ROTEM FIBTEM test results have shown the strongest correlation with plasma fibrinogen level and provided the greatest discrimination of fibrinogen deficiencies, these tests are recommended for guiding fibrinogen replacement and monitoring its hemostatic effects.

  • Studies comparing TEG FF and ROTEM FIBTEM suggest a stronger correlation of the latter with the plasma fibrinogen level, likely owing to its more effective elimination of the platelet contribution to clot strength.

  • Studies supporting the use of TEG FF and ROTEM FIBTEM are limited to trauma and surgical bleeding patients. Even without robust clinical data, TEG and ROTEM are likely to remain popular for the hemostatic management of bleeding patients.

  • Future studies comparing the different intervention thresholds for TEG and ROTEM and the therapeutic effects of predefined thresholds for fibrinogen augmentation are required to optimize fibrinogen administration (i.e., dosage and time of fibrinogen administration) to improve its efficacy and patient safety and to reduce costs in various clinical settings. Studies comparing preemptive and guided fibrinogen replacement are also warranted.