Thromboelastography in the Perioperative Period: A Literature Review

Assessing coagulation status is essential for prompt intervention to reduce morbidity and mortality related to bleeding and thrombotic complications during the perioperative period. Traditional coagulation tests such as platelet count, activated partial thromboplastin time (aPTT), prothrombin time (PT), international normalized ratio (INR), and activated clotting time (ACT) provide only static evaluation. These tests are not designed for assessment of dynamically changing coagulation conditions during the perioperative time. However, viscoelastic coagulation testing such as thromboelastography (TEG) produces a rapid numerical and graphical representation that helps to detect and direct targeted hemostatic therapy. Searching the literature through PubMed, Medline, Ovid, CINAHL, and ClinicalTrials.gov we retrieved 210 studies, which represent the use of TEG in the perioperative period. The included studies were categorized under various settings such as trauma, obstetrics, orthopedics, intensive care unit (ICU), cardiovascular, transplant, and miscellaneous scenarios. TEG showed promising results in trauma surgeries in predicting mortality, hypercoagulability, and bleeding even when it was compared to conventional methods. TEG was also useful in monitoring anticoagulant therapy in orthopedic and obstetric surgeries; however, its role in predicting thrombotic events, hypercoagulability, or complications was questionable. In ICU patients, it showed promising results, especially in the prediction or improvement of sepsis, coagulopathy, thrombotic events, ICU duration, hospital stay, and ventilator duration. TEG parameters effectively predicted hypercoagulation in transplant surgeries. Regarding cardiovascular surgeries, they were effective in the prediction of the need for blood products, coagulopathy, thrombotic events, and monitoring anticoagulation therapy. More randomized clinical trials comparing TEG parameters with standardized tools are needed to produce robust results to standardize its use in different perioperative settings.


Introduction And Background
Monitoring of blood coagulation status during the perioperative period is crucial for prompt intervention as bleeding and thrombotic complications related to surgery can significantly affect morbidity and mortality. Assessing the coagulation status comprehensively is a challenge since the coagulation cascade is dynamic and depends on the interaction of several factors including primary hemostasis, platelet clot formation, secondary hemostasis, thrombin generation, and fibrinolysis [1]. Traditional coagulation tests such as platelet count, activated partial thromboplastin time (aPTT), prothrombin time (PT), international normalized ratio (INR), activated clotting time (ACT), and plasma fibrinogen levels provide only static evaluation of the patient and are not designed for assessment of dynamically changing coagulation conditions during perioperative time; thus, they lack the ability to direct targeted hemostatic therapy [2]. However, viscoelastic coagulation testing such as thromboelastography (TEG) is devised for a quick global assessment of hemostasis more like in vivo hemostasis by continuously monitoring the clotting process from its steps of initiation, amplification, propagation, and termination through fibrinolysis. They produce a rapid numerical and graphical representation that helps clinicians with the early management of goaldirected hemostatic resuscitation and anticoagulation effects [3][4][5][6]. Our goal is to systemically search and summarize the existing evidence from studies that have reported the utility of viscoelastic coagulation testing and its impact on clinical outcomes during the perioperative period.

Thromboelastography (TEG)
TEG is a whole blood-based assay that runs at 37°C to mimic natural blood clotting in vivo [7]. The instrument consists of a pin immersed into a cup containing whole blood that begins to clot when a constant rotational force is applied to it. As the viscosity of blood increases, the pin becomes cross-linked to the cup via fibrin and platelet interactions. Now there is a torque between the cup and the pin, and the movement of the pin produces an electrical signal that is traced as a curve over time. As the clot breaks down and torque decreases, the tracing fades. The signals are then interpreted by TEG software where changes in amplitude are plotted, and different parameters of the curve are measured to assess coagulation status [8]. The parameters include reaction (R) time, coagulation (K) time, alpha (α) angle, maximum amplitude (MA), and lysis at 30 minutes (LY30). The tracing and results are available in real-time, enabling prompt interpretation for goal-directed therapy [9]. While TEG is favored in North America, there are other viscoelastic tests such as rotational thromboelastometry (ROTEM) that are favored in Europe. Both the tests are equivalent with interchangeable results and interpretations, yet characteristics and nomenclature differences exist, and they are illustrated in Tables 1-2

Reaction Time (R-Time)
Reaction time is the first measurement of the coagulation cascade. Its measurement is related to coagulation factor activation. This value is similar to extrinsic and intrinsic clotting pathway measurements by PT and aPTT respectively. The R-time largely reflects the adequacy of coagulation factors and is the most sensitive parameter to measure the effects of heparin therapy including low molecular weight heparin (LMWH) [10,11]. An elevated or prolonged R-value (more than eight minutes) can signify a deficiency in clotting factors, hemodilution, or the presence of heparin. Therefore, indicating a need for transfusion of fresh frozen plasma. On the other hand, a shortened R-time (less than four minutes) can indicate hypercoagulopathy requiring the use of anticoagulation.

K-Time
It is a measurement of the time interval between R and time to reach 20 mm clot amplitude. K-time and α angle are both related to coagulation factor amplification. Therefore, their values correlate, and they both indicate a deficiency in clot growth kinetics. A low value can indicate a deficiency in fibrinogen and may reveal a need for cryoprecipitate. A high value is similar to the R-time, which represents a hypercoagulable state, and an anticoagulant may be required.
α Angle It is a measurement of the line tangent to the slope of the curve during clot formation. The computer software calculates the angle based on the slope and time. A number of factors including thrombin generation and fibrinogen levels determine the angle. It identifies states of hyper-or hypo-coagulopathies.

MA Value
It is the maximum amplitude that represents the distance traveled by the cross-linked cup/pin. It is a measurement of maximum clot strength and provides information on both fibrinogen and platelet function. A high MA value may indicate hypercoagulation and the need for an anticoagulant. A low MA value indicates low clot strength, which can be caused by decreased fibrinogen levels, low platelet counts, or decreased platelet function. If a low MA is combined with a decreased K value, this is an indication of cryoprecipitate therapy. MA value is very important when paired with a platelet count because a low platelet count and a normal MA value indicate a patient has a normal platelet function and therefore does not require platelet transfusion. Conversely, treatment with platelets may be indicated for patients with a low MA value, low platelet function, and normal platelet count [12].

LY30
It is clot lysis at 30 minutes. It is the last major TEG parameter and measures the percent of the decrease in area under the curve over 30 minutes. Therefore, it reflects fibrinolysis after maximum amplitude is reached. This measurement is most useful for patients undergoing thrombolytic therapy or during disseminated intravascular coagulation (DIC). A high LY30 percentage indicates hyperfibrinolysis and patients may require antifibrinolytic agents including tranexamic acid, aprotinin, and aminocaproic acid.

Review Methods
A review of the literature was conducted to identify qualifying publications. The search was conducted in the following databases: PubMed, Medline, Ovid, CINAHL, and ClinicalTrials.gov. Search criteria were defined using the string (thromboelastography or TEG) and (perioperative or postoperative or preoperative or operative) in all search fields. Inclusion criteria for the systematic review included articles that represented original research including as a focal outcome evaluation of TEG procedures; in one or more perioperative settings (pre, intra, or postoperative); in a human population; have been published in a peer-reviewed source and in English. Excluded items included theses or dissertations, conference abstracts, and proceedings, theoretical papers, comments or letters to the editor, or previous reviews. We abstracted data from selected studies that include patient samples, perioperative settings where TEG was utilized, TEG parameters that were assessed, and clinical outcomes that were reported. Because of clinical and methodologic heterogeneity among studies, we expected to report results qualitatively rather than conducting a meta-analysis.
The initial database search described in the methods section yielded 8,200 unique articles after duplicates were removed. Among them, 6,156 reports were included and assessed for eligibility after excluding records without data (N = 75), not in English (N = 425), that were non-peer-reviewed (e.g., conference abstracts) (N = 1,012), that were reviewed (N = 526) and that were not retrievable (N = 6). After further automated and manual screening of assessed reports for eligibility, 210 articles were found to be eligible and included in the review ( Figure 1). Reasons for rejection of assessed articles included studies that were ineligible (N = 281), were reviews (N = 249), in which TEG was mentioned but not evaluated (N = 4,959), did not include patient outcomes (N=175), studies not in perioperative settings (N = 71). Also, articles in languages other than English (N = 127), studies with veterinary samples (N = 73), retracted studies (N = 9), and the use of the abbreviation TEG not referring to thromboelastography (N = 2) were excluded. The included 210 studies were categorized under various surgical settings. Studies in the cardiovascular settings were the maximum with 64 studies while those based in the surgical intensive care unit (ICU) setting were the least with only one study.

Trauma
TEG finds its clinical application critically useful in trauma; the American College of Surgeons recommends it to be available at all level I and level II trauma centers. Complications from trauma-related surgery such as hemorrhage and thrombosis remain the leading causes of preventable death. Hemorrhage exacerbation is associated with trauma-induced coagulopathy (TIC) and has been shown to be present in more than 25% of severely injured patients upon arrival to the emergency department. TIC is a lethal, unbalanced, and abnormal process. Its early stages are characterized by hypercoagulability and bleeding whereas the later stages are characterized by hypercoagulability with venous thromboembolism and multiple organ failure. In such a scenario, comprehensive information about coagulation status is essential. TEG by analyzing various contributors of both hemostasis and clot dissolution provide extensive information that has been shown to predict mortality as well as positively impact it during TIC [13][14][15]. TEG parameters such as MA and R-time detect platelet function and coagulation factor deficiency with a high degree of specificity that guide individualized therapy for patients [16,17]. It is accurate in diagnosing hypofibrinogenemia as well [18]. Their ability to reliably detail the hypercoagulable states in cancer patients and the distorted coagulation status in alcoholic patients during trauma is well demonstrated [19,20]. Overall, they reflect coagulation status better than traditional coagulation tests [21].
Since TIC is associated with uncontrolled bleeding, TEG's ability to provide insight into both depletion coagulopathy and hyperfibrinolysis allows it to guide massive transfusion protocol (MTP); and predict the associated mortality [22][23][24][25]. TEG-guided resuscitation has demonstrated lower blood product usage, shorter ICU and hospital stay, and lower overall costs especially when compared to conventional coagulation tests that come with limitations such as its time-consuming nature, failure to delineate the complex nature of TIC, and unclear value in guiding transfusion [26][27][28][29]. This has been shown to improve mortality outcomes, especially in pelvic fractures, penetrating as well as blunt trauma patients, and burn patients [30][31][32][33]. While the ability of TEG to predict transfusion has been replicated in the general population it was not the case in the polytrauma population [34]. Using TEG protocols that are directed to reduce blood product usage and improve survival [35,36], transfusion has become more patient-specific with an average transfusion ratio of 2.5:1:2.9 (red blood cells: plasma: platelets), different from the current 1:1:1 guideline [37]. TEG has been shown to be valid during MTP and results in different patterns of blood transfusion based on individual patient requirements as well as a reduction in overall hemorrhage-related deaths during trauma [38,39].
When it comes to MTP-related blood product usage, TEG does not differ from conventional testing and ROTEM [40,41]. During trauma surgery involving the liver and spleen, interestingly, TEG guidance has demonstrated less as well as increased blood product usage but shorter surgery time [42,43]. With the success of TEG in assessing coagulopathic parameters in trauma patients, TEG has been investigated for detecting and reversing anticoagulants only with limited success, and conventional tests (e.g., PT, INR, PTT) that have shown better results comparatively have been recommended [44][45][46][47]. TEG finds its utility in the pediatric trauma population as well where it has been shown to accurately predict MTP requirement, thromboembolism, and mortality [48, 49, while outcomes such as blood product use, ventilator duration, and length of ICU stay were found to be worse with TEG use there was no change in mortality [50] ( Table 3).

Obstetric
Pregnancy is uniquely a hypercoagulable state. This usually results in thromboembolic complications that can affect the pregnancy, necessitating the need for anticoagulants. Under such circumstances, TEG parameters have been found to be useful in guiding anticoagulation therapy [51]. But its sensitivity has not been found to be adequate to monitor the progress of anticoagulation [10]. On the other hand, pregnancyrelated complications such as pre-eclampsia and eclampsia reverse the blood coagulability into the hypercoagulable state as well as hemolysis that can be exacerbated during surgery. Although TEG relates such coagulopathic scenarios during pregnancy with the risk profiles preoperatively [52], they have yet been found to be inferior when compared to conventional coagulation tests in predicting intraoperative coagulopathy and blood loss [53]. They still have been demonstrated to reduce blood product use, costs, risks of ICU admission, and the need for emergency hysterectomy [54] ( Table 4).

Orthopedic
Orthopedic surgery in general involves the release of massive tissue factors triggering a coagulation process that requires anticoagulants for venous thromboembolism prevention and treatment. For joint surgeries, neuraxial and peripheral nerve blocks are mainstay anesthesia choices that need information on the patient's coagulation profile and medications that affect coagulation. So comprehensive information about coagulation status in orthopedic surgery patients is important. In demographic-specific orthopedic surgery patients, TEG has been reported to be a better measure of hypercoagulability compared to conventional measures [55] but has been found not to predict venous thromboembolism risk [56]. In spine surgery, TEG has predicted clotting factor deficiency such as hypofibrinogenemia and was found to be an inferior predictor of coagulation status as a whole compared to traditional laboratory measures [57,58]. However, their sensitivity to sustained coagulation changes i.e., after seven days is superior compared with traditional measures [59]. When it comes to anticoagulation, TEG has been established to differentiate anticoagulated patients as well as monitor their therapy [60][61][62]. TEG-guided anticoagulation prophylaxis has better safety and comparable efficacy to conventional prophylaxis strategy [63]. TEG did not find any significance in detecting specific outcomes related to orthopedic surgery such as bone cement implantation syndrome and infections [64,65] (Table 5).

ICU
Surgical ICU patients commonly have a myriad of coagulation abnormalities such as thrombocytopenia, prolonged global coagulation times, reduced levels of coagulation inhibitors, or high levels of fibrin split products. Additionally, they are at increased risk of venous thromboembolism due to immobilization, pharmacologic paralysis, repeat surgical procedures, sepsis, mechanical ventilation, vasopressor use, and renal dialysis. Identifying the etiology of these coagulation abnormalities is of utmost importance since each coagulation disorder necessitates different therapeutic strategies. Since TEG provides a comprehensive evaluation of the viscoelastic properties of blood compared to standard plasma assays, in surgical ICU patients TEG has been demonstrated to be predictive of ICU duration, ventilator duration, hospital length of stay, and risk of thromboembolic events [66]. The detection of coagulation abnormalities is even more important in sepsis, a well-known comorbidity during ICU admission since consumption of coagulation factors and subsequent coagulopathy occurs. TEG in this sense has been established to detect coagulopathy and distinguish it among those with and without sepsis so that appropriate management can ensue (  The use of TEG in cardiovascular surgeries significantly reduced blood product transfusion compared to clinician-guided practice [67][68][69][70][71][72][73][74][75][76]. However, it was not associated with any change in ICU stay or mortality [69,71,72]. Redfern et al. 2020 found that TEG-guided protocol significantly reduced blood product use, costs, and reoperation rates; however, it did not impact mortality compared to clinician discretion in 1098 US cardiac patients [74]. Sun et al. 2014 found that TEG-guided protocol was associated with lower fresh frozen plasma (FFP) and platelet transfusion volume without any association with plasma transfusion volume or platelet count in 39 Chinese cardiac patients during ventricular assist device placement [76].
On the other hand, a weak relationship between thromboelastography with platelet mapping (TEG-PM) and platelet transfusion volume was observed in 44 US pediatric cardiac patients [77]. In addition, Westbrook et al. 2009 showed no significant difference in blood product usage between the TEG-guided and the clinicianguided groups in 69 Australian cardiac patients [78].
On the other hand, Terada et al. 2019 found that intraoperative use of TEG-MA, TEG R-time, TEG-K, and TEG α-angle was not predictive of blood loss volume in 50 Japanese cardiac patients [103]. Moreover, another five studies showed that these TEG parameters were not predictive of postoperative bleeding [104][105][106][107][108] or even intraoperative bleeding [109,110].

Coagulopathy and Thrombotic Events
Mostly TEG parameters could predict both coagulopathy and thrombotic events. The use of TEG-MA, TEG Rtime, TEG-K, and TEG α-angle in cardiac patients was predictive of both coagulopathy [84,85,[113][114][115][116][117][118][119][120] and even intracranial hemorrhage [120]. Also, they could predict thrombotic events [97,121] and even pump thrombosis risk [122]. They detected also the P2Y12 inhibition nonresponse, allowing earlier intervention for patients receiving preoperative inhibition therapy in 453 US vascular patients [123]. In comparison to conventional indicators, TEG parameters were better at predicting bleeding and clotting complications [124]. Heparinase modification allowed TEG parameters to diagnose covert coagulopathy [125,126]. Only Brothers et al. 1993 found that these parameters were not reliably corresponded to clinical coagulopathy in 10 US cardiac patients [127]. Bhardwaj et al. 2017 found that TEG-MA predicted postoperative thrombocytopenia in 35 Indian cardiac patients [128]. In addition, TEG-MA predicted platelet count in cardiac patients [105,129,130]. On the other hand, it did not predict adverse events in 233 Danish vascular patients [131].

Anticoagulant Efficacy Prediction
Intraoperative use of TEG-MA, TEG R-time, TEG-K, and TEG α-angle was effective for monitoring anticoagulant therapy [134][135][136]. Postoperatively too they were effective for assessing anticoagulation status [137]. TEG-K was found to be effective in monitoring heparin efficacy intraoperatively in 31 US cardiac patients [138]. They also were useful in monitoring anticoagulation reversal in 40 Singaporean vascular patients [106].
TEG-guided intraoperative anticoagulant therapy was effective in 31 US intracranial aneurysm patients [139]; however, when it was compared to traditional methods, no difference was observed in terms of protamine usage or heparin reversal efficacy [140]. TEG-MA was comparable to ROTEM-EXTEM in terms of guiding anticoagulation reversal in 52 UK cardiac patients [141] ( Table 7).

Transplant
Perioperative TEG is used in organ transplantation surgeries such as liver, kidney, pancreas-kidney, or bowel because of their abilities in the prediction of coagulopathy and thrombotic events. While Abuelkasem et al. found that TEG-R could not predict coagulopathy in liver transplant surgeries as effectively as ROTEM [142], other studies have demonstrated that TEG parameters like TEG-MA, TEG R-Time, TEG-K, and TEG α-angle could predict or be related to coagulopathy [143][144][145][146][147][148][149]. Despite the relation of TEG parameters to coagulopathy, they were not related to bleeding time [144] which was supported by Sujka et al. who compared TEG-directed transfusion protocol and the clinician-directed transfusion system and found no difference between both methods in decreasing the blood loss amount [150].
Also, TEG parameters predicted hypercoagulable status and thrombotic events [149,[151][152][153] even in comparison to the conditional laboratory tests [154] while Krzanicki et al. found that they could predict hypercoagulable status only without thrombotic events in liver transplant patients [155]. On the other hand, Sujka et al. found that TEG-directed blood transfusion increased the thromboembolic events compared to the clinician-directed protocol in liver transplant patients [150].
Regarding the use of blood products, the studies revealed different results. TEG parameters reduced the usage of blood products [156][157][158]; however, in comparison to other conventional tests or clinician-directed transfusion system, no differences were observed except for Sujka et al. who found TEG-directed transfusion system reduced only FFP use between other blood products [150,159,160]. Coakley et al. investigated both TEG and ROTEM parameters and found that ROTEM improved clinicians' decisions compared to TEG usage [161].
In postoperative outcomes like survival, graft function, and hospital stay, controversial results were observed in the studies. Sam et al. found that TEG did not relate to renal graft function while Walker et al. found that it is an indicator of graft function [146,162]. This controversy was seen also in the prediction of liver cirrhosis [148,163]. TEG's usage was not associated with mortality or survival rates [156,160]. On the other hand, it decreased hospital stay length and reoperation needs [147,160] (

Miscellaneous
TEG is used in many other sites involving neurological, gastrointestinal, general, cardiopulmonary, plastic, urological, and oncological procedures.

Neurological
TEG parameters showed an evitable role in improving hematological outcomes in people who underwent neurosurgeries whether they were adults [164][165][166][167][168] or children [169]. TEG parameters like TEG-R, TEG-MA, TEG-K, and TEG α-angle could predict hypercoagulation or thrombotic complications [164,166,168,169]; however, compared to control treatment, no difference was observed [170]. In addition, these parameters predicted bleeding and hypo-coagulation status whether intraoperative or postoperative [166,169,171,172] besides using them could decrease bleeding complications risk compared to other conventional labs [170]. TEG-guided transfusion was effective to decrease the transfusion of blood products compared to the clinician-guided protocol [172]. Also, TEG-guided use of intraoperative antiplatelet therapy succeeded to prevent major complications [167] while only TEG-R was rare to be associated with postoperative complications [164].
Only TEG-PM could not predict thrombotic events or even bleeding complications through neurosurgical procedures [173]; however, it showed good ability in the prediction of platelet inhibition in comparison to other modalities [165].
Gastrointestinal TEG parameters showed promising results in gastroenterology surgeries [174]. Using TEG in bariatric surgeries could predict hypercoagulability conditions [175][176][177] and this ability especially increased in females and older patients [175]. However, in liver-related surgeries, TEG efficacy was controversial as Oo 2020 et al. and Vieira da Rocha 2009 et al. showed that the essential TEG parameters were not predictive of ulcerative bleeding risk or hemostasis variation [178,179]. On the other hand, Okida 1991 et al. and Zanetto 2021 et al. showed the efficacy of these parameters in the prediction of coagulopathy and perioperative bleeding [180,181]. Moreover, compared to clinician-guided transfusion, TEG-guided transfusion decreased the usage of blood products; however, it was not different to reduce the complications rate [182]. TEG usage could not predict postoperative sepsis in oesophagectomy surgeries [183]. In patients with obstructive jaundice, TEG parameters also could not predict coagulopathy or platelet function during their surgeries for drainage of obstructive jaundice [184] while they predicted bleeding and coagulopathy in cystectomy operations [185,186]. Also, they could predict deep venous thrombosis risk in gastric cancer patients comorbid with portal hypertension [187].

General
Few studies investigated the role of TEG among pediatric patients undergoing general surgical procedures and they found that applying TEG or ROTEM in pediatric patients increased coagulopathy risk and blood products use [188] while in neonates, TEG parameters predicted sepsis early [189]. Also, TEG-guided transfusion decreased blood products use compared to clinician-guided transfusion while in mortality and morbidity risks, no differences were detected [190]. The use of TEG among adults undergoing general surgical procedures was better described in the literature. They were effective in the prediction of bleeding [191]. Using TEG-PM in monitoring platelet inhibition in patients on clopidogrel was useful in decreasing unneeded treatment cancellations besides the patient risk [192]. However, comparing the conventional transfusion protocol to TEG-guided transfusion revealed no significant difference in detecting bleeding [193]. The conventional TEG parameters with the celite-activated ones were predictive or associated with hypercoagulability or thrombotic events [194][195][196]. Coagulopathy prediction was achieved also by TEGguided transfusion compared to the use of conventional methods [193]. Also, they showed better prediction values of survival rates compared to other conventional methods [197]. On the other hand, TEG-guided transfusion was not different to the conventional protocol in the prediction of mortality [193]. They could predict the blood products use [191] and using TEG-guided transfusion was effective in reducing the need for blood products [193]. Moreover, they resembled a good option to guide the optimal treatment, especially in patients comorbid with Gaucher disease who undergoing general surgeries [198]. In flap operations, the TEG parameters could not predict the flap loss risks [199]; however, they were predictive of coagulopathy and thrombotic events [200]. Also, in maxillary surgeries, they could predict both bleeding and platelet count [201].

Extracorporeal Membrane Oxygenation (ECMO)
Applying TEG in surgical procedures in patients on ECMO was controversial in the literature in both adults and pediatric patients. In pediatric patients, TEG-guided anticoagulation protocol significantly reduced blood products usage, decreased complications, and increased ECMO circuit life compared to the clinicianguided system [202,203] which was supported by Moynihan 2017 et al. who found that they were useful in monitoring intraoperative anticoagulation [204]. Moreover, TEG-R significantly predicted thrombotic events [205]. On the other hand, the bleeding complications predictive value of conventional TEG parameters was controversial as Saini et al. showed that they could not predict bleeding [206] while Sleeper et al. found that these parameters predicted bleeding [207]. Also, TEG kaolin and heparinase had a poor indication ability of aPTT and an acceptable indication of platelet count which recommended the usage of conventional laboratory tests [208]. Regarding their use in adult patients on ECMO, TEG-R, ROTEM-INTEM, and conventional methods had the same efficacy in anticoagulation monitoring [209]. Also, the TEG flat line reading had no relation to the perioperative bleeding [210]. However, other studies showed that the conventional TEG parameters were effective to monitor anticoagulation [211] and to predict coagulopathy in adults on ECMO patients [212].
Others TEG was also used in monitoring hematological outcomes in urological procedures such as prostatectomy [213,214] and renal biopsy [215] or even in nephrotic syndrome patients [216]. However, its efficacy was questionable as in prostatectomy procedures, TEG clot lysis correlated with bleeding [214] while other parameters like TEG-LY30 and TEG-LY40 were not able to predict postoperative coagulopathy [213]. Also, during the renal biopsy, TEG-MA was not effective to predict bleeding time [215]. However, TEG parameters like TEG-MA, TEG-R, TEG-K, and TEG α-angle were associated with coagulopathy complications and could distinguish different renal pathologies in 713 Chinese nephrotic syndrome patients [216]. TEG parameters were predictive in oncology patients regarding platelet count, hypercoagulability, tumor type, resection success, and postoperative complications [217][218][219]. Also, they were useful in monitoring the anticoagulation status in patients who underwent thoracic surgeries [220] and patients on mechanical circulatory support devices [221] ( Table 9).

Strengths and limitations
To our knowledge, this is one of the reviews that addressed the application of TEG usage in monitoring the hematological outcomes in the perioperative periods including nearly all surgical procedures. Therefore, this review opens the doors for clinicians to reach out to recent evidence about TEG applications on patients having any surgery or procedure to enhance transfusion and coagulation-related management. In addition, we searched many databases and screened the relevant records in detail to include all relevant studies, which provide the recent updates in TEG applications in multiple surgeries.
The limitation is that this is only a literature review that summarizes existing research on TEG. It does not include other viscoelastic tests such as ROTEM. Most of the studies lacked comparison groups. While comparing with standardized laboratory tests, a controversy was observed between the related studies in the literature. In addition, lacking direct statistical analysis including all related studies made it difficult to solve the controversy about the efficacy of TEG usage in some surgeries.

Summary
TEG showed promising results in detecting and improving hematological outcomes in patients who underwent major surgeries and procedures or who were critically ill; however, more comparative studies are needed to establish this efficacy. These promising results were observed in trauma surgeries regarding predicting mortality, hypercoagulability, and bleeding even when it was compared to conventional methods; however, its role to guide blood product transfusion was questionable.
TEG was useful in monitoring anticoagulant therapy in orthopedics operations; however, its roles in predicting thrombotic events, hypercoagulability, or complications were questionable among the studies. The same controversy was observed in obstetric operations; however, it showed promising results in ICU patients, especially in the prediction or improvement of sepsis, coagulopathy, thrombotic events, ICU duration, hospital stay, and ventilator duration.
In transplant surgeries, they effectively predicted hypercoagulation; however, their roles in predicting bleeding, blood product needs, and thrombotic events were still questionable. Regarding cardiovascular surgeries, they were effective in the prediction of the need for blood products, coagulopathy, and thrombotic events and they were effective in monitoring anticoagulation therapy.
TEG parameters were useful in predicting coagulation and bleeding, preventing complications, and decreasing blood product transfusion in neurological surgeries; however, compared to the conventional tools, they were better in all these outcomes except for hypercoagulation, which had the same results. In abdominal surgeries, TEG was effective in bariatric, cystectomy, and gastric cancer surgeries; however, their results were controversial in hepatic, esophagectomy, and obstructive jaundice surgeries. The efficacy of TEG usage was also controversial in patients on ECMO whether they were adults or pediatrics. However, in general surgeries, a controversy was observed in pediatric patients while a promising efficacy was observed in adults regarding predicting hypercoagulation, thrombotic events, and blood product transfusion.

Conclusions
Based on the evidence reviewed here we conclude that TEG can be used in a wide range of perioperative settings to guide transfusion and coagulation management and thereby influence certain outcomes. Because of some limitations addressed in this review, we recommend performing more randomized clinical trials comparing TEG parameters with standardized tools and performing meta-analyses to pool all related studies' data to solve the controversy between studies. More clinical trials also are needed to investigate the usage of TEG in critically ill patients, especially in cardiothoracic, obstetric and oncology surgeries as well as patients on ECMO; geriatric and pediatric patients, and patients with renal disease.

Conflicts of interest:
In compliance with the ICMJE uniform disclosure form, all authors declare the following: Payment/services info: All authors have declared that no financial support was received from any organization for the submitted work. Financial relationships: All authors have declared that they have no financial relationships at present or within the previous three years with any organizations that might have an interest in the submitted work. Other relationships: All authors have declared that there are no other relationships or activities that could appear to have influenced the submitted work.