The role of viscoelastic testing in assessing peri-interventional platelet function and coagulation

Abstract We carried out a literature search in MEDLINE (PubMed) and EMBASE literature databases to provide a concise review of the role of viscoelastic testing in assessing peri-interventional platelet function and coagulation. The search identified 130 articles that were relevant for the review, covering the basic science of VHA and VHA in clinical settings including cardiac surgery, cardiology, neurology, trauma, non-cardiac surgery, obstetrics, liver disease, and COVID-19. Evidence from these articles is used to describe the important role of VHAs and platelet function testing in various peri-interventional setups. VHAs can help us to comprehensively assess the contribution of platelets and coagulation dynamics to clotting at the site-of-care much faster than standard laboratory measures. In addition to standard coagulation tests, VHAs are beneficial in reducing allogeneic transfusion requirements and bleeding, in predicting ischemic events, and improving outcomes in several peri-interventional care settings. Further focused studies are needed to confirm their utility in the peri-interventional case.


Introduction
A close interplay between platelets and coagulation is essential for the generation of a platelet-fibrin clot during hemostasis. In patients presenting with acute coronary syndromes, stroke, venous thromboembolism in the presence of existing endothelial dysfunction, systemic prothrombotic phenotype (hypercoagulability) results in a strong and occlusive platelet-fibrin clot [1]. Conversely, impaired hemostasis due to low platelet count or low coagulation proteins, and drug-induced inhibition of platelets or coagulation carries an increased risk for spontaneous and periinterventional bleeding [2][3][4][5][6]. Laboratory tests such as blood count, international normalized ratio/prothrombin time (INR/ PTI) and partial thromboplastin time, and Clauss fibrinogen are the current standard methods to assess platelet count and coagulation proteins. However, these tests have a long turnaround time, reflect a single aspect of coagulation, and therefore are suboptimal whenever urgent therapeutic decisions are requested perioperatively, peri-interventional, or during trauma management [7].
Currently available whole blood testing devices or viscoelastic hemostatic assays (VHA) are Thromboelastography (TEG® analyzer), Rotational thromboelastometry (ROTEM®), Quantra®, and Sonoclot® (Table 1). VHAs provide a holistic view of hemostasis with detailed information on dynamic changes in clot characteristics from initiation of clot formation to plateletfibrin clot generation, stability, and lysis ( Figure 1, Table 2). These characteristics are used to assess the relative contribution of coagulation proteins and platelets to clot formation, fibrinolysis, and to estimate hyper-or hypocoagulability and to assess response to antiplatelet or anticoagulant agents (Table 1, Figure 2) [8,9]. Briefly, anticoagulants influence clot initiation, clot kinetics and clot strength whereas, fibrinolytics influence clot strength and lysis. In the TEG6s Platelet Mapping assay, ADPand AA-induced platelet fibrin clot strength is influenced by P2Y 12 receptor blockers and aspirin respectively, whereas both ADP-and AA-induced platelet-fibrin clot strength is influenced by glycoprotein (GP)IIB/IIIa inhibitors. Moreover, VHAs can also assess functional fibrinogen levels thereby enabling targeted administration of platelets and fibrinogen in the bleeding patient.
R/ACT = reaction time/activated clotting time, K = clot kinetics, A = angle, MA-maximum amplitude, LY30 = lysis at 30 ADP = adenosine diphosphate, AA = arachidonic acid In this literature review, we discuss the currently available evidence on how the assessment of platelet-coagulation interaction and response to antiplatelet agents can help us in guiding the administration of blood products, and antiplatelet treatment in the peri-interventional period in patients including trauma, cardiovascular surgery, cardiology, neurology, obstetrics, and COVID-19 to mitigate the risks of ischemia and bleeding. A systematic literature review was conducted on MEDLINE (PubMed) and EMBASE to identify English language articles of adult humans or human samples published between January 1999 and September 2019. Predefined search terms for platelet inhibition and function (e.g., platelet inhibition, platelet function, antiplatelet, platelet reactivity, platelet mapping, clot-forming, clopidogrel) and thromboelastometry (e.g., thromboelastometry, thromboelastography, TEG, ROTEM, Quantra, Sonoclot, Viscoelastic) were used and articles were screened for inclusion against predefined criteria. Key exclusion criteria included: non-English language articles; pediatric, non-human studies; reviews, editorials, case reports, editorials; and articles with no relevant platelet data or no viscoelastic testing reported. As COVID-19 papers were published after the September 2019 search, additional relevant publications about COVID −19 proposed by the authors were also included.

Viscoelastic testing for hemostasis: current site-of-care analyzers
Currently, TEG® 5000 and TEG® 6s with PlateletMapping® assay, ROTEM®, and Sonoclot® are the main VHA assays being used. The TEG® PlateletMapping® assay has been validated against the current gold-standard light transmission aggregometry and is used assess response to P2Y 12 receptor blockers and aspirin by measuring adenosine diphosphate (ADP) -and arachidonic acid (AA) -induced platelet aggregation, respectively ( Figure 2) [10,11]. TEG®6s is the new site of-care assay that uses resonance-frequency viscoelasticity measurements and a disposable multi-channel microfluidic cartridge to assess hemostasis and has been validated against the earlier TEG®5000 [12]. Rotational thromboelastometry (ROTEM®) has been shown previously to correlate with conventional coagulation assays, with the newer ROTEM® sigma device showing higher precision compared with the ROTEM® delta [12,13]. ROTEM® is able to assess the intrinsic (INTEM) and extrinsic pathways (EXTEM) of coagulation and functional fibrinogen (FIBTEM), however it is not able to measure drug-induced platelet inhibition. The Sonoclot® coagulation analyzer has been compared with both the TEG® and ROTEM® devices in patients undergoing cardiac surgery [14], with TEG® and ROTEM® devices showing a greater correlation with conventional coagulation tests, including fibrinogen and platelet count [14]. A close correlation between well-established ROTEM parameters including FIBTEM and parameters as assessed by the fully automated Quantra Q Plus system ® has been demonstrated recently [15].
Major limitations of VHAs include labor-intensive methodology (TEG 5000 and ROTEM), and complicated data output. Although the cost is higher, VHAs provide detailed information on clot kinetics and response to antiplatelet agents simultaneously    [27,28]. However, further studies are needed to explore the utility of VHAs in these patients to personalize antithrombotic therapy. The coagulopathy of trauma brain injury (TBI) patients differs from other trauma patients. Drug-induced platelet inhibition assessed by VHAs is a prominent early feature of TBI and is linked to the severity of brain injury in patients with isolated head trauma [18]. Impaired platelet P2Y 12 receptor pathway has been associated with increased clot sensitivity, suggesting that P2Y 12 receptor pathway inhibition may be an early step in the pathogenesis of systemic hyperfibrinolysis [29,30]. A significant reduction in platelet response to arachidonic acid was also demonstrated in major TBI patients compared to healthy controls [31]. These observations highlight the importance of early identification of coagulopathy and goal-directed treatment in patients with TBI. TBI monitoring and treatment is a challenging area for further studies to explore and validate platelet dysfunction by VHA.
In trauma patients, VHAs can be used to assess primary and secondary fibrinolysis, thrombocytopenia, clotting factor consumption, and hypercoagulability [32]. Serial monitoring of coagulation status in a trauma patient is critical to guide transfusion and the correction of coagulopathy. These observations are incorporated into the current European and American practice guidelines for the management of trauma [33,34]. In the recently reported randomized, international viscoelastic hemostatic assay augmented protocols for major trauma hemorrhage (ITACTIC) trial of 396 trauma patients, there was no difference in outcomes between patients receiving empiric major hemorrhage protocols augmented by either VHAs or conventional coagulation tests (CCTs)-guided interventions. There are numerous important limitations to this study. Nearly 75% of patients were not coagulopathic by prothrombin time ratio at baseline and very few of these patients subsequently developed a prolonged prothrombin time ratio before hemostasis. The reduction in the primary endpoint was 3% as compared to the estimated 13% reduction indicating an inadequate sample size. In the VHA group, 1.8 times more patients received study interventions than the CCT group indicating that the widespread occurrence of coagulation deficits was not detected by the CCTs. Therefore, further studies are warranted to assess the true utility of VHAs in this setting [35].
Cardiac Surgery. The severity of cardiac surgery-related bleeding has been associated with postoperative morbidity and mortality [36,37]. Preventing perioperative blood loss may be more efficacious in improving outcomes than mere reducing allogenic blood components because restrictive transfusion strategy was not superior to liberal transfusion strategy in patients undergoing cardiac surgery in a recent randomized study [3,38]. Current guidelines suggest the implementation of institutional perioperative treatment algorithms based on VHA tests for the bleeding patient to reduce the number of transfusions [2,39,40]. Recent studies indicated that transfusion algorithms including site-of-care tests to measure the interaction of coagulation and platelets, platelet count, and platelet function, reduce allogeneic transfusion and cardiac surgery-associated bleeding without affecting mortality [41][42][43]. In patients on preoperative dual antiplatelet therapy undergoing cardiac surgery, implementation of preoperative TEG® PlateletMapping® or Multiplate assays in addition to extended intraoperative TEG® assessments allowed targeted post-bypass coagulation management and reduced allogeneic blood transfusion and costs [44]. However, cutoff values of TEG® and ROTEM® used in the RCTs and observational studies included in the metaanalyses differed between studies and ranges rather than specific cutoff values may be more appropriate for guiding transfusion therapy, also considering the multifactorial nature of surgery-related bleeding [45].
Despite conflicting results in individual RCTs targeting fibrinogen administration based on VHA results in heterogeneous cardiac surgical patient populations, a recent metaanalysis demonstrated that intraoperative fibrinogen supplementation reduced the incidence of allogeneic red blood cell transfusion, without however affecting morbidity or mortality [46]. Platelet function tests have been used to schedule elective on-pump and off-pump cardiac surgery in patients on dual antiplatelet therapy, by stratifying preoperative waiting based on the VHA MA-ADP and the Innovance 2Y platelet function test, respectively [47,48]. In the TARGET CABG study, surgery performed within 1 day, 3-5 days, or >5 days using cut off points of MA-ADP >50 mm, 35-50 mm, and <35 mm respectively reduced preoperative waiting by about 50% as compared to the guideline-mandated uniform delay in surgery without any difference in surgery-related bleeding [47].
In order to attenuate the risk of surgery-related bleeding in patients on P2Y 12 receptor inhibitors presenting for non-emergent major surgery, current guidelines recommend a "one size fits all" preoperative discontinuation period of at least three to five days for ticagrelor, five days for clopidogrel, and seven days for prasugrel based largely on results of the large scale clinical studies (IIa recommendation). The recommendation for a standardized preoperative waiting period neither accounts for highly variable pharmacodynamic responsiveness nor for variability in platelet reactivity recovery time for clopidogrel, the most widely used P2Y 12 receptor blocker. Therefore, an individualized approach using an objective measure of residual preoperative platelet function may serve as an alternative (IIb recommendation) although both the safe cutoff and the optimal device are elusive so far [49,50].
The addition of platelet function assays has improved the predictive power of VHA for perioperative bleeding compared with coagulation factors alone [51-53]. Platelet function measurements allowed accurate prediction of transfusion requirements [44,54,55]. The platelet function assays can also be used to monitor and guide personalized antiplatelet therapy for patients with left ventricular assist devices to reduce bleeding [56,57]. Similarly, guidelines suggest the use of platelet function tests to aid with the timing of surgical procedures, and suggest that further evidence should be gathered on their optimal use [50] (supplemental Table 1)

Non-cardiac surgery
Platelet function tests provide information on antiplatelet response in addition to that provided by hemostatic assays. The extensive use of antiplatelet agents is associated with a higher risk of bleeding during surgical procedures, and VHAs can be used to assess hemostasis, antiplatelet response and monitor antiplatelet therapy. Clopidogrel-induced platelet inhibition levels showed a good correlation with standard VHA [58]. Patients who were currently or had recently received dual antiplatelet therapy presenting for non-cardiac surgery had higher levels of preoperative platelet inhibition compared with patients who had been off therapy for longer. The latter may have a clinical impact on the decisions for antiplatelet treatment before surgery [59]. Two publications exploring the use of platelet function assays to stratify patient bleeding risk in surgical patients demonstrated that assessing ADP-induced platelet aggregation can identify a statistically significant platelet inhibition during antiplatelet therapy (p < .01) [60] and that 35% inhibition of ADP-induced platelet aggregation may be an useful cutoff for proceeding to surgery [61]. However, a recent meta-analysis including four randomized clinical trials with 229 patients undergoing non-cardiac surgery did not provide any concrete evidence to support the use of VHAs in reducing mortality and transfusion needs as compared to standard monitoring [62].

Interventional cardiology
Hemostatic and coagulation assays with VHA have been shown in both prospective and retrospective analyses to provide important information for the treatment of interventional cardiology patients [63,64]. VHAs are useful in monitoring antiplatelet therapy and other cardiology medications (e.g., amlodipine for blood pressure) [65], giving physicians a site-of-care measure of the effectiveness of the therapy [66]. VHA measurements of platelet function and hemostasis factors are demonstrated as risk predictors of ischemia and bleeding in patients undergoing percutaneous coronary intervention (PCI) [67][68][69], and have shown a correlation with disease severity in patients with cardiovascular diseases [70][71][72][73]. The addition of platelet function assays to treatment algorithms in the presence of aspirin or a P2Y 12 receptor inhibitor further allows clinicians to monitor antiplatelet therapy and individualize both the medication type and the dose [66,[74][75][76][77][78][79][80][81][82][83][84][85]. VHAs have been used to explore the efficacy and safety of antiplatelet therapies, including aspirin [64], clopidogrel [86], cangrelor [87], and the direct oral anticoagulant such as dabigatran [88]. Dabigatran was found to significantly influence thrombin activity and thrombus generation parameters compared with placebo (p < .001), without affecting clot structure, fibrinolysis or measures of platelet reactivity [88]. Expert consensus statements have suggested that platelet function tests including TEG® be used as tools to guide treatment, e.g., for determining appropriate antiplatelet therapy escalation and deescalation of antiplatelet therapy and to assess compliance to treatment [89][90][91] (Supplemental Table 2).

Stroke
Platelet count assays are commonly used in neurology and neurosurgery to monitor hemostasis and assess blood transfusion requirements. The standard global hemostatic VHA showed some predictive value for recurrent ischemic events [92], with coagulation assays able to detect the differences in coagulopathy associated with dual antiplatelet therapy [93]. Platelet function assays have further demonstrated a correlation between platelet dysfunction and subsequent ischemic events following stroke and may be used to predict the risk of future thromboembolic complications [94][95][96][97].
As in cardiology patients, platelet function tests are also beneficial in identifying antiplatelet resistance and guiding antiplatelet therapy to improve outcomes in neurological patients [98,99]. The use of site-of-care VHA to assess thrombogenicity in patients with symptoms of acute stroke showed that clotting time could predict acute ischemic stroke [100]. Poor response to clopidogrel therapy was reported in patients sustaining acute ischemic stroke while treated with long-term aspirin [101], and there is evidence that clopidogrel resistance may be associated with an increased risk for ischemic complications in patients undergoing stenting [95]. VHAs with platelet function assays have shown some success in monitoring patients undergoing neurosurgery to guide platelet transfusions for active bleeding, as they can evaluate preoperative coagulation function alongside platelet function [102,103].

Obstetrics
There are fewer publications regarding the use of VHA in obstetrics compared with other clinical indications, this may be because VHA trials in obstetrics are focused on fibrinogen levels rather than platelet function. In total, three papers were found which used VHAs in an obstetric setting [104][105][106]. The standard VHA hemostatic and fibrinogen assays were shown to be reliable for urgently and emergently assessing patient coagulation status [105] with one suggesting that VHAs should be used to monitor patients for blood transfusion during postpartum hemorrhage [104]. VHAs are also being used to assess the safety of neural analgesia in pregnant patients with possible coagulopathy, such as in thrombocytopenia or preeclampsia [106].

Liver disease/surgery
The literature search identified only two publications using VHA for exploring hemostasis, coagulation factors, and platelet function in patients with cirrhosis [107,108]. Studies have also shown that including platelet function with platelet count assays can reduce the need for blood transfusions in patients undergoing liver surgery and reduces the incidence of bleeding complications [109,110].
Coronavirus Disease −19. COVID-19 is primarily associated with elevated levels of fibrinogen and D-dimer indicating a systemic hypercoagulability state that may contribute to the observed thromboembolic events and to mortality by triggering cardiovascular events [111]. However, studies with mostly limited number of patients have demonstrated abnormalities in hemostasis evaluated by VHAs. Moreover, limited data available exploring the utility of VHAs guided therapy in patients with ARDS, or sepsis that have similar inflammation and hemostasis abnormalities as COVID-19. VHAs based studies in COVID patients reported fibrinolysis shutdown based on the absence of clot lysis, hypercoagulability with highly elevated levels of platelet-fibrin clot strength that can be attributed to high levels of fibrinogen, and an attenuated response to anticoagulant therapy [112][113][114][115][116][117][118]. There are opportunities for VHAs in the early identification of COVID-19 patients and management of antithrombotic therapy to improve clinical outcomes [119]. These observations support the guidance published by the United States Food and Drug Administration (FDA) highlighting the importance of measurement of whole blood viscoelastic properties to facilitate management in patients with COVID-19 [120].
Major limitations of VHAs include labor-intensive methodology (TEG 5000 and ROTEM delta), and complicated data output. Although the cost is higher, VHAs provide detailed information on clot kinetics and response to antiplatelet agents simultaneously (with TEG 6s PlateletMapping Assay). Moreover, the cost may be recovered by saving of blood products. Other important limitations are the need for significant training to get through the learning curve of the new way of assessing coagulation, and standardization of treatment protocols to provide a consistent methodology of utilizing the information within an institution. Finally, VHAs do not assess the contribution of shear force and endothelial function during clot generation. The latter factors may be a limitation of VHAs to assess some coagulation disorders like Von Willebrand disease.

Discussion
It is vital to monitor platelet function along with coagulation dynamics quickly and efficiently at the site-of-care for optimal management of patients during the peri-operative and periintervention period, and trauma management. Evidence from a range of studies shows that currently available VHA correlate well with standard hemostatic tests [121,122]. The TEG® PlateletMapping® assay was shown to correlate well with LTA and compared with other whole blood platelet function tests such as Multiplate® and PFA-100 [123].
Various studies and meta-analyses have shown that monitoring platelet-fibrin clot formation with VHA can improve diagnosis, trigger goal-directed transfusion and antiplatelet therapy, and decrease surgery-related bleeding [42,124,125] In addition, platelet function testing and viscoelastic platelet function assays, when added to standard coagulation assays, demonstrated clinical importance in both surgical settings for assessing coagulopathic bleeding [41,42] cardiology settings for monitoring response to antiplatelet therapy [76], and the prediction of future ischemic events [71]. The addition of the AA-and ADP-based VHA allows platelet function to be assessed alongside coagulation dynamics, so that clinicians can determine response to therapy, and to assess the relative contribution of platelet function and coagulation dynamics, and individually schedule major surgery without increased bleeding risk [47,126]. While TEG® and ROTEM® devices performed similarly for standard platelet count assays, the addition of platelet function assays may enhance the utility of management algorithms in surgical patients [44,92,127].
In other clinical indications such as obstetrics and TBI, while VHAs are commonly used to measure coagulation dynamics [105], platelet function tests are less widely used [26]. Reduced platelet function following ER admission for trauma can be a sign of coagulopathy associated with increased mortality [128], and VHAs have been used to assess platelet dysfunction immediately after isolated blunt TBI [129]. Further data on platelet function tests in these settings may help us to understand the nature of coagulopathy, and to assess patient hemostasis.
Current European and American guidelines suggest implementing an institution-based VHA algorithm in cardiac surgery to decrease surgery-related bleeding [2,40]. Finally, the goal of VHA testing is to optimize transfusion regimes of red cells/ platelets, clotting factors, fibrinogen, fresh frozen plasma or cryoprecipitate, and administration of antifibrinolytic drugs. This approach is aimed at improving clinical outcomes and reducing costs.