Suggestions for global coagulation assays for the assessment of COVID-19 associated hypercoagulability

Intravenous enoxaparin guided by anti-Xa in venovenous extracorporeal membrane oxygenation: A retrospective, single-center study

BACKGROUND

Unfractionated heparin is used as the most common anticoagulation for venovenous extracorporeal membrane oxygenation (VV ECMO) patients. However, it is accompanied by frequent bleeding and thrombotic complications. The aim of the study was to demonstrate the feasibility of Enoxaparin anticoagulation for VV ECMO patients.

METHODS

This study is a retrospective analysis of VV ECMO patients on continuous intravenous Enoxaparin anticoagulation. The primary outcome was the incidence of bleeding, thrombotic, and neurological complications during ECMO support. The secondary outcome was an analysis of secondary and primary hemostasis profiles.

RESULTS

Data from 38 patients were analyzed in this study. The incidence of bleeding complications was 5.3%, for thrombotic complications it was 2.6% and for neurological (bleeding/ischemic events) complications it was 10.5%. The targeted anti-Xa activity of 0.4-0.6 IU/mL was achieved and maintained during whole ECMO period in 28 patients (73.8%), not affecting the hemocoagulation profile represented by APTT-r 1.15 ± 0.2, TT 18.67 ± 3.35 s, PT/INR 1.21 ± 0.19, fibrinogen 5.39 ± 1.49 g/L, antithrombin, and platelet count. Primary hemostasis pathology was diagnosed in all patients by PFA 200 tests Col/EPI 279 ± 38 s and Col/ADP 249 ± 66 s. The running time of ECMO was 7.8 ± 3.4 days.

CONCLUSIONS

Enoxaparin anticoagulation appears to be feasible for VV ECMO patients without an increase in adverse events. Further larger-sampled and comparative studies are needed in the future to support our findings.

Rotational thromboelastometry as a biomarker for mortality - The Maastricht Intensive Care COVID cohort

BACKGROUND

Patients with severe coronavirus disease 2019 (COVID-19) present with persisting hypercoagulability, hypofibrinolysis and prolonged clot initiation as measured with viscoelastic assays. The objective of this study was to investigate the trajectories of traditional assays of hemostasis, routine and tissue plasminogen activator (tPA) rotational thromboelastometry (ROTEM) in COVID-19 patients and to study their association with mortality.

METHODS

Patients enrolled within the Maastricht Intensive Care COVID (MaastrICCht) cohort were included. Traditional assays of hemostasis (prothrombin time; PT, fibrinogen and D-dimer) were measured daily and ROTEM EXTEM, FIBTEM and tPA assays were performed weekly. Trajectories of these biomarkers were analyzed over time for survivors and non-survivors using linear mixed-effects models. Additional Fine and Gray competing risk survival analysis was performed for the first available measurement after intubation.

RESULTS

Of the 138 included patients, 57 (41 %) died in the intensive care unit (ICU). Over 450, 400 and 1900 individual measurements were available for analysis of routine, tPA ROTEM and traditional assays of hemostasis, respectively, with a median [IQR] follow-up of 15 [8-24] days. Non-survivors on average had prolonged CT (clotting time) and increased fibrinogen compared to survivors. MCF (maximum clot firmness), LOT (lysis onset time), LT (lysis time) and PT measurements increased more over time in non-survivors compared to survivors. Associations persisted after adjustment for demographics and disease severity. EXTEM and FIBTEM CT at intubation were associated with increased 45-day ICU mortality.

CONCLUSIONS

ROTEM measurements demonstrate a further increase of hypercoagulability and (hypo)fibrinolysis parameters in non-survivors throughout ICU admission. Furthermore, prolonged CT at intubation was associated with higher 45-day ICU mortality.

Whole blood viscoelastic testing profile and mortality in patients hospitalized with acute COVID-19 pneumonia: A systematic review and meta-analysis

BACKGROUND

Several studies have evaluated the possible association between whole blood viscoelastic testing (VET) parameters in patients hospitalized for acute Coronavirus disease 2019 (COVID-19) pneumonia and mortality. A few studies found no significant differences between survivors and non-survivors, though other studies identified potential predictors of COVID-19-related mortality. We conducted a systematic review and meta-analysis of the literature to evaluate the possible association between standard thromboelastometry/graphy parameters and mortality in patients hospitalized for acute COVID-19 pneumonia.

METHODS

Relevant studies were searched through MEDLINE, EMBASE, and Google Scholar from their inception until 15th June 2023. We aimed to identify any study including: i) adults admitted to intensive care units (ICU) or medicine wards (MW) for acute COVID-19 pneumonia; ii) viscoelastic testing; iii) mortality.

RESULTS

We included 13 studies: nine prospective and four retrospective, 231 (30.4 %) non-survivors and 528 (69.6 %) survivors. Mortality rates ranged from 12.8 % to 67.5 %. The studies using the TEG apparatus found a significant difference in K time in the Kaolin test among survivors vs. non-survivors (mean difference [MD] 0.20, 95 % confidence interval [CI] 0.12, 0.28, I2 0%). The studies using the rotational thromboelastometry apparatus found a significant difference in CT-INTEM (MD -17.14, 95 % CI -29.23, -5.06, I2 0%) and LI60-EXTEM (MD -1.00, 95 % CI -1.00, -1.00, I2 0%) assays among survivors vs. non-survivors.

CONCLUSION

We identified no specific hypercoagulable or hypocoagulable profile associated with mortality in patients with COVID-19-related pneumonia. Large prospective studies are needed to explore the possible prognostic role of VET in this subset of patients.

Characteristics of blood clotting on thromboelastography in patients with early burns

BACKGROUND

Thromboelastography (TEG) is a viscoelastic test that may be used to evaluate the hemostatic function of whole blood, and it may be useful for burn patients with multiple hemostatic defects.

METHODS

We retrospectively recruited patients with burns between January 2019 and July 2021. Blood samples were drawn on admission and subjected to coagulation parameter assessment, including conventional coagulation tests and TEG assessment. Receiver operating characteristic (ROC) analysis was performed to predict the occurrence of complications in patients with early burns.

RESULTS

Ninety-three patients with early burns met the inclusion criteria. Patients with minor, moderate, severe, and extremely severe burns accounted for 19.4 %, 36.6 %, 16.1 %, and 27.9 % of all patients, respectively. Compared with the healthy controls, patients with early burns showed significant reductions in the R and K values, and significant elevation in the maximum amplitude (MA), coagulation index (CI), and alpha angle. Compared with minor and moderate burn patients, patients with severe and extremely severe burns had lower K values and thrombin time and higher alpha angle, CI, prothrombin time, international normalized ratio, D-dimer, and fibrin degradation products. Patients with hypercoagulation had lower R and K values, longer MA, longer CI, and greater alpha angle. After ROC analysis, the areas under the ROC curve for acute lung injury, acute kidney injury, and bleeding were 0.789, 0.802, and 0.900, respectively.

CONCLUSION

TEG provides insight into the hemostatic state of patients with early burns, and can predict complications in early burn patients when combined with conventional coagulation tests.

Applications of rotational thromboelastometry in heparin monitoring in critical COVID-19 disease: Observations in the Maastricht Intensive Care COVID cohort

Background

Critically ill COVID-19 patients are at risk for venous thromboembolism (VTE). Therefore, they receive thromboprophylaxis and, when appropriate, therapeutic unfractionated heparin (UFH) or low-molecular-weight heparin (LMWH). To monitor heparins in COVID-19 disease, whole-blood rotational thromboelastometry (ROTEM) may be a promising alternative to the aPTT and anti-Xa assays.

Objective

To evaluate the ROTEM INTEM/HEPTEM ratios in mechanically ventilated COVID-19 patients treated with UFH and therapeutic LMWH.

Material and methods

A subcohort of mechanically ventilated COVID-19 patients of the prospective Maastricht Intensive Care Covid (MaastrICCht) cohort was studied. Anti-Xa, aPTT, and ROTEM measurements following treatment with UFH or therapeutic dose of LMWH (nadroparin) were evaluated using uni- and multivariable linear regression analysis and receiver operating characteristics.

Results

A total of 98 patients were included, of which 82 were treated with UFH and 16 with therapeutic LMWH. ROTEM-measured INTEM/HEPTEM CT ratio was higher in patients using UFH (1.4 [1.3-1.4]) compared to patients treated with LMWH (1.0 [1.0-1.1], p < 0.001). Both the aPTT and anti-Xa were associated with the CT ratio. However, the β-regression coefficient (95%CI) was significantly higher in patients on UFH (0.31 (0.001-0.62)) compared to therapeutic LMWH (0.09 (0.05-0.13)) for comparison with the anti-Xa assay. Furthermore, ROC analysis demonstrated an area under the curve for detecting UFH of 0.936(0.849-1.00), 0.851(0.702-1.000), and 0.645(0.465-0.826) for the CT ratio, aPTT, and anti-Xa, respectively.

Conclusion

The ROTEM INTEM/HEPTEM CT ratio appears a promising tool to guide anticoagulant therapy in ICU patients with COVID-19 disease, but associations with clinical endpoints are currently lacking.

Investigation of differences in coagulation characteristics between hospitalized patients with SARS-CoV-2 Alpha, Delta, and Omicron variant infection using rotational thromboelastometry (ROTEM): A single-center, retrospective, observational study

BACKGROUND

The severe acute respiratory syndrome coronavirus 2 Omicron variant has a low rate of serious illness, is highly contagious, and has spread rapidly since January 2022. The number of severe cases and deaths remains problematic. Here, we aimed to elucidate the coagulation pathology of Omicron-infected patients using rotational thromboelastometry.

METHODS

Patients with coronavirus disease 2019, hospitalized and treated from January 2021 to April 2022, were included. The Alpha-Delta and Omicron groups were defined during admission. Blood tests, clinical course, and rotational thromboelastometry measurements were compared using a propensity score-matched cohort.

RESULTS

Both groups had 21 patients each. Lactate dehydrogenase (Alpha-Delta group [interquartile range] vs. Omicron group [interquartile range]; 449 [368-518] U/L vs. 241 [196-398] U/L, p = 0.01) and ferritin (1428 [1145-3061] ng/dl vs. 481 [188-881] ng/dl, p = 0.0002) levels were significantly lower in the Omicron group. In rotational thromboelastometry, the thrombus hardness indexes FIBTEM A5 (29 [23-34] mm vs. 23 [18-28] mm, p = 0.034) and maximum clot firmness (34 [27-40] mm vs. 26 [21-33] mm, p = 0.021) were significantly lower in the Omicron group, whereas the fibrinolysis index FIBTEM LI60 (98 [92-100] % vs. 100 [100-100] %, p = 0.0082) was higher.

CONCLUSION

Severe coagulation abnormalities may be less likely in Omicron-infected patients than in those infected with the previous Alpha and Delta variants.

Serial thrombin generation and exploration of alternative anticoagulants in critically ill COVID-19 patients: Observations from Maastricht Intensive Care COVID Cohort

Background

COVID-19 associated coagulopathy (CAC) is associated with an increase in thromboembolic events. Current guidelines recommend prophylactic heparins in the management of CAC. However, the efficacy of this strategy in the intensive care population remains uncertain.

Objective

We aimed to measure thrombin generation (TG) to assess CAC in intensive care unit (ICU) patients receiving thromboprophylaxis with low molecular weight heparin (LMWH) or unfractionated heparin (UFH). In addition, we performed statistical modeling to link TG parameters to patient characteristics and clinical parameters. Lastly, we studied the potency of different anticoagulants as an alternative to LMWH treatment in ex vivo COVID-19 plasma.

Patients/Methods

We included 33 patients with confirmed COVID-19 admitted at the ICU. TG was measured at least twice over the course of 6 weeks after admission. Thrombin generation parameters peak height and endogenous thrombin potential (ETP) were compared to healthy controls. Results were subsequently correlated with a patient characteristics and laboratory measurements. In vitro spiking in TG with rivaroxaban, dabigatran, argatroban and orgaran was performed and compared to LMWH.

Results

Anti-Xa levels of all patients remained within the therapeutic range throughout follow-up. At baseline, the mean (SE) endogenous thrombin potential (ETP) was 1,727 (170) nM min and 1,620 (460) nM min for ellagic acid (EA) and tissue factor (TF), respectively. In line with this we found a mean (SE) peak height of 353 (45) nM and 264 (96) nM for EA and TF. Although fluctuating across the weeks of follow-up, TG parameters remained elevated despite thromboprophylaxis. In vitro comparison of LMWHs and direct thrombin inhibitors (e.g., agratroban, dabigatran) revealed a higher efficacy in reducing coagulation potential for direct thrombin inhibition in both ellagic acid (EA) and tissue factor (TF) triggered TG.

Conclusion

In a sub-group of mechanically ventilated, critically ill COVID-19 patients, despite apparent adequate anti-coagulation doses evaluated by anti-Xa levels, thrombin generation potential remained high during ICU admission independent of age, sex, body mass index, APACHE II score, cardiovascular disease, and smoking status. These observations could, only partially, be explained by (anti)coagulation and thrombosis, inflammation, and multi-organ failure. Our in vitro data suggested that direct thrombin inhibition compared with LMWH might offer an alternate, more effective anticoagulant strategy in COVID-19.

Hypercoagulability in COVID-19: from an unknown beginning to future therapies

Coronavirus disease 2019 (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is a global public health concern and is characterized by an exaggerated inflammatory response that can lead to a large variety of clinical manifestations such as respiratory distress, sepsis, coagulopathy, and death. While it was initially considered primarily a respiratory illness, different data suggests that COVID-19 can lead to a pro-inflammatory milieu and a hypercoagulable state. Several mechanisms attempt to explain the pro-coagulant state seen in COVID-19 patients, including increased fibrinogen concentration, different receptor binding, exhausted fibrinolysis, cytokine storm, and endothelial dysfunction. Some hematological parameters, such as elevated D-dimers and other fibrinolytic products, indicate that the essence of coagulopathy is massive fibrin formation. Moreover, elevated D-dimer levels have emerged as an independent risk factor for a worse outcome, including death, indicating a potential risk for deep vein thrombosis and pulmonary thromboembolism. Prophylactic anticoagulation is recommended in all in-patients with COVID-19 to reduce the incidence of thrombosis. Those with elevated D-dimer values or with a higher risk of developing thromboembolic events should be treated with higher doses of anticoagulant. Anticoagulation may not be enough in some circumstances, highlighting the need for alternative therapies. An understanding of the complex cross-talk between inflammation and coagulopathy is necessary for developing direct appropriate interventional strategies.

Current and novel biomarkers of thrombotic risk in COVID-19: a Consensus Statement from the International COVID-19 Thrombosis Biomarkers Colloquium

Coronavirus disease 2019 (COVID-19) predisposes patients to thrombotic and thromboembolic events, owing to excessive inflammation, endothelial cell activation and injury, platelet activation and hypercoagulability. Patients with COVID-19 have a prothrombotic or thrombophilic state, with elevations in the levels of several biomarkers of thrombosis, which are associated with disease severity and prognosis. Although some biomarkers of COVID-19-associated coagulopathy, including high levels of fibrinogen and D-dimer, were recognized early during the pandemic, many new biomarkers of thrombotic risk in COVID-19 have emerged. In this Consensus Statement, we delineate the thrombotic signature of COVID-19 and present the latest biomarkers and platforms to assess the risk of thrombosis in these patients, including markers of platelet activation, platelet aggregation, endothelial cell activation or injury, coagulation and fibrinolysis as well as biomarkers of the newly recognized post-vaccine thrombosis with thrombocytopenia syndrome. We then make consensus recommendations for the clinical use of these biomarkers to inform prognosis, assess disease acuity, and predict thrombotic risk and in-hospital mortality. A thorough understanding of these biomarkers might aid risk stratification and prognostication, guide interventions and provide a platform for future research.

The Composition and Physical Properties of Clots in COVID-19 Pathology

Hemostasis is a finely tuned process of which dysregulation can lead either to bleeding or thrombotic complications. The latter is often caused by the hypercoagulable state as it is also seen in patients with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, i.e., in COVID-19 patients. COVID-19 patients requiring hospitalization often suffer from thrombotic events that could not be predicted using routine coagulation assays. Recently, several studies have reported ROtational ThromboElastoMetry (ROTEM) as a promising tool to predict outcomes in COVID-19 patients. In this review we give an overview of ROTEM with a particular focus on the interpretation of the symmetrical clot formation curve in relation to coagulopathy in COVID-19 patients. Furthermore, we have introduced new parameters that might help to better distinguish between COVID-19 patients and outcomes.

The Outcome of Catheter-Directed Thrombolysis in COVID-19-Associated Deep Vein Thrombosis

IntroductionPublished evidence of venous thrombotic complications of COVID-19 is lacking from India. This case series consists of twenty-nine adult patients who were COVID -19 positive and treated for Deep Vein Thrombosis (DVT) during the second wave of the COVID-19 pandemic, in India. The study was aimed at analyzing patient demographics of patients with DVT and the outcome of Catheter-Directed Thrombolysis (CDT) in COVID positive patients. Material and Methods: Patients who developed DVT while or after being COVID positive were managed between February and April 2021 at the institution of the first two authors and were included in this retrospective study. Demographic, clinical data, laboratory data, and treatment given were analyzed. All patients were followed up for 3 months with a Villalta score. Results: There were a total of 29 patients (12 male and 17 female) included in the study with a mean age of 47 ± 17 years. The average time of presentation from being COVID positive was 17.8 ± 3.6 days and one patient developed DVT after becoming Covid negative. All but one patient had lower limb involvement, with 42.8% having proximal and 57.2% distal DVT. All patients with Iliofemoral and two with Femoropopliteal DVT were treated with catheter-di thrombolysis and the other 15 patients were managed with anticoagulation alone. No re-thrombosis was observed in the thrombolysis group. Overall average Villalta score at 3 months was 10.7 ± 2.1 with a score of 10.58 ± 2.1 in the anticoagulation-only group and 10.85 ± 2.3 in the CDT group. Conclusion: COVID-19 seems to be an additional risk factor in the development of DVT. The outcome of such patients, treated by thrombolysis appears to be similar to non-COVID patients. In this, observational experience of the authors suggests that CDT could be offered to COVID positive patients with symptomatic Iliofemoral DVT with good outcomes and an acceptable post-intervention Villalta score.

Clinical use of thrombin generation assays

Determining patient's coagulation profile, i.e. detecting a bleeding tendency or the opposite, a thrombotic risk, is crucial for clinicians in many situations. Routine coagulation assays and even more specialized tests may not allow a relevant characterization of the hemostatic balance. In contrast, thrombin generation assay (TGA) is a global assay allowing the dynamic continuous and simultaneous recording of the combined effects of both thrombin generation and thrombin inactivation. TGA thus reflects the result of procoagulant and anticoagulant activities in blood and plasma. Because of this unique feature, TGA has been widely used in a wide array of settings from both research, clinical and pharmaceutical perspectives. This includes diagnosis, prognosis, prophylaxis, and treatment of inherited and acquired bleeding and thrombotic disorders. In addition, TGA has been shown to provide relevant information for the diagnosis of coagulopathies induced by infectious diseases, comprising also disturbance of the coagulation system in COVID-19, or for the assessment of early recurrence in breast cancer. This review article aims to document most clinical applications of TGA.

The modification of the thrombin generation assay for the clinical assessment of hypercoagulability in patients receiving heparin therapy

BACKGROUND

Heparin diminishes thrombin generation (TG) because it decreases the survival time of thrombin in plasma. Under heparin therapy, the TG curve therefore does not reflect the true hemostatic status of the patient.

AIM

We investigated how far the in vitro addition of a heparin antagonist can restore the underlying TG capacity.

MATERIALS & METHODS

Five different heparin antagonists were tested: polybrene, protamine sulfate, heparinase type 1, heparinase HEP-TEM, and (Z-GGR)₂ -rhodamine (P2Rho).

RESULTS AND CONCLUSION

Polybrene, P2Rho, and heparinase HEP-TEM effectively neutralized heparin at prophylactic and therapeutical doses of both low molecular weight and unfractionated heparin. The advantages and limits of each molecule and the most favorable combinations of TG-trigger and antagonist are discussed.

Hemostasis and fibrinolysis in COVID-19 survivors 6 months after intensive care unit discharge

BACKGROUND

The prothrombotic phenotype has been extensively described in patients with acute coronavirus disease 2019 (COVID-19). However, potential long-term hemostatic abnormalities are unknown.

OBJECTIVE

To evaluate the changes in routine hemostasis laboratory parameters and tissue-type plasminogen activator (tPA) rotational thromboelastometry (ROTEM) 6 months after COVID-19 intensive care unit (ICU) discharge in patients with and without venous thromboembolism (VTE) during admission.

METHODS

Patients with COVID-19 of the Maastricht Intensive Care COVID cohort with tPA ROTEM measurement at ICU and 6-month follow-up were included. TPA ROTEM is a whole blood viscoelastic assay that illustrates both clot development and fibrinolysis due to simultaneous addition of tissue factor and tPA. Analyzed ROTEM parameters include clotting time, maximum clot firmness (MCF), lysis onset time (LOT), and lysis time (LT).

RESULTS

Twenty-two patients with COVID-19 were included and showed extensive hemostatic abnormalities before ICU discharge. TPA ROTEM MCF (75 mm [interquartile range, 68-78]-59 mm [49-63]; P ≤ .001), LOT (3690 seconds [2963-4418]-1786 seconds [1465-2650]; P ≤ .001), and LT (7200 seconds [6144-7200]-3138 seconds [2591-4389]; P ≤ .001) normalized 6 months after ICU discharge. Of note, eight and four patients still had elevated fibrinogen and D-dimer concentrations at follow-up, respectively. In general, no difference in median hemostasis parameters at 6-month follow-up was observed between patients with (n=14) and without (n=8) VTE, although fibrinogen appeared to be lower in the VTE group (VTE-, 4.3 g/L [3.7-4.7] vs VTE+, 3.4 g/L [3.2-4.2]; P = .05).

CONCLUSIONS

Six months after COVID-19 ICU discharge, no persisting hypercoagulable or hypofibrinolytic profile was detected by tPA ROTEM. Nevertheless, increased D-dimer and fibrinogen concentrations persist up to 6 months in some patients, warranting further exploration of the role of hemostasis in long-term morbidity after hospital discharge.

Serial EXTEM, FIBTEM, and tPA Rotational Thromboelastometry Observations in the Maastricht Intensive Care COVID Cohort-Persistence of Hypercoagulability and Hypofibrinolysis Despite Anticoagulation

Background: Coronavirus Disease 2019 (COVID-19) patients often present with thromboembolic events. In COVID-19 patients, routine hemostatic assays cannot correctly identify patients at risk for thromboembolic events. Viscoelastic testing with rotational thromboelastometry (ROTEM) might improve the characterization of COVID-19-associated coagulopathy.

Objective: To unravel underlying coagulopathy and fibrinolysis over time as measured by serial assessment heparin-independent (FIBTEM and EXTEM) and fibrinolysis illustrating (tissue plasminogen activator; tPA) ROTEM assays.

Patients/Methods: Between April 23 and June 12, consecutive adult patients enrolled within the Maastricht Intensive Care COVID (MaastrICCht) cohort were included, and a comprehensive set of clinical, physiological, pharmaceutical, and laboratory variables were collected daily. Twice per week, EXTEM, FIBTEM, and tPA ROTEM were performed. Clotting time (CT), clot formation time (CFT), maximum clot firmness (MCF), lysis onset time (LOT), and lysis time (LT) were determined to assess clot development and breakdown and were compared to routine hemostatic assays.

Results: In 36 patients, 96 EXTEM/FIBTEM and 87 tPA ROTEM tests were performed during a 6-week follow-up. CT prolongation was present in 54% of EXTEM measurements, which were not matched by prothrombin time (PT) in 37%. Respectively, 81 and 99% of all EXTEM and FIBTEM MCF values were above the reference range, and median MCF remained elevated during follow-up. The ROTEM fibrinolysis parameters remained prolonged with median LOT consequently >49 min and unmeasurable LT in 56% of measurements, suggesting a severe hypofibrinolytic phenotype.

Conclusion: ROTEM tests in COVID-19 ICU patients show hypercoagulability and severe hypofibrinolysis persisting over at least 6 weeks.

Viscoelastometric Testing to Assess Hemostasis of COVID-19: A Systematic Review

Infection by SARS-CoV-2 is associated with a high risk of thrombosis. The laboratory documentation of hypercoagulability and impaired fibrinolysis remains a challenge. Our aim was to assess the potential usefulness of viscoelastometric testing (VET) to predict thrombotic events in COVID-19 patients according to the literature. We also (i) analyzed the impact of anticoagulation and the methods used to neutralize heparin, (ii) analyzed whether maximal clot mechanical strength brings more information than Clauss fibrinogen, and (iii) critically scrutinized the diagnosis of hypofibrinolysis. We performed a systematic search in PubMed and Scopus databases until 31st December 2020. VET methods and parameters, and patients' features and outcomes were extracted. VET was performed for 1063 patients (893 intensive care unit (ICU) and 170 non-ICU, 44 studies). There was extensive heterogeneity concerning study design, VET device used (ROTEM, TEG, Quantra and ClotPro) and reagents (with non-systematic use of heparin neutralization), timing of assay, and definition of hypercoagulable state. Notably, only 4 out of 25 studies using ROTEM reported data with heparinase (HEPTEM). The common findings were increased clot mechanical strength mainly due to excessive fibrinogen component and impaired to absent fibrinolysis, more conspicuous in the presence of an added plasminogen activator. Only 4 studies out of the 16 that addressed the point found an association of VETs with thrombotic events. So-called functional fibrinogen assessed by VETs showed a variable correlation with Clauss fibrinogen. Abnormal VET pattern, often evidenced despite standard prophylactic anticoagulation, tended to normalize after increased dosing. VET studies reported heterogeneity, and small sample sizes do not support an association between the poorly defined prothrombotic phenotype of COVID-19 and thrombotic events.