Red Cell Damage During Extracorporeal Life Support.

Sublethal damage to red blood cells (RBCs) during extracorporeal life support (ECLS) may lead to RBC loss. Using flow cytometry, phosphatidylserine-positive (PhS+) RBCs and RBC extracellular vesicles were quantified as measures of sublethal RBC injury in 41 pediatric ECLS runs, stored RBC units, and normal adult subjects. We estimated the clearance half-life of PhS+ RBCs and compared the rates of RBC loss during pediatric ECLS due to phlebotomy, intravascular hemolysis, and extravascular clearance of PhS+ RBCs. Extracorporeal life support patients had 0.9% PhS+ RBCs, sixfold higher than normal subjects (p < 0.0001). Phosphatidylserine-positive RBCs were increased in stored RBC units (twofold in whole blood derived units, p = 0.0013; 12-fold in apheresis RBC units, p < 0.0001). Phosphatidylserine-positive RBCs were cleared with an average half-life of 15 hours. During ECLS, PhS+ RBC clearance accounted for 7% of RBC loss (1-60%), phlebotomy 12%, and intravascular hemolysis 12%. Increasing PhS+ RBCs occurred in 40% of patients that died on ECLS. Red blood cell extracellular vesicles, another marker of red cell injury/activation, were elevated fivefold during ECLS. Phosphatidylserine exposure on RBCs is increased during ECLS, marking these cells for extravascular clearance with a half-life of ~15 hours and accounting for ~7% of RBC loss.

Causes of red blood cell loss during extracorporeal membrane oxygenation.

BACKGROUND: Pediatric patients on extracorporeal membrane oxygenation (ECMO) often receive repeated red blood cell (RBC) transfusions. This study aims to quantify and characterize causes of RBC loss on ECMO. METHODS: This retrospective, single-center, observational study includes 91 ECMO patients (age 1 day-20 years). An RBC loss index (RLI), equal to ml RBCs lost per liter of patient + circuit volume per hour, was calculated from the changes in hematocrit and transfused RBCs. To measure the contribution of RBC injury/activation, RBC extracellular vesicle (REV) generation was measured by flow cytometry. RESULTS: Median RLI on ECMO was 1.9 ml/L/h, 13-fold higher than normal RBC production rate (0.15 ml/L/h) and equivalent to a 4.6 drop in hematocrit/day. Median RBC loss was higher in patients who died (2.95 ml/L/h) versus survived (1.70 ml/L/h, p = .0008). RLI correlated with transfusion rate (r2  = 0.71); however, transfusion rate (ml/kg) underestimated RBC loss in patients with large changes in hematocrit and over-estimated RBC loss in neonates where the circuit volume is greater than the patient blood volume. In non-bleeding patients, intravascular hemolysis represented 16% of total RBC loss and diagnostic phlebotomy 24%, suggesting that ~60% of RBC loss was due to other causes. REV generation was increased sevenfold to ninefold during ECMO. DISCUSSION: RLI (ml/L/h) is a more reliable quantitative indicator of RBC loss than transfusion rate (ml/kg) for pediatric patients on ECMO. Phlebotomy and intravascular hemolysis only account for 40% of RBC loss in non-bleeding ECMO patients. High REV generation suggests sublethal damage and extravascular clearance may be a cause of RBC loss on ECMO.

Monitoring Direct Thrombin Inhibitors With Calibrated Diluted Thrombin Time vs Activated Partial Thromboplastin Time in Pediatric Patients.

OBJECTIVES: Activated partial thromboplastin time (aPTT) is the primary test used to monitor intravenous (IV) direct thrombin inhibitors (DTIs) but has many limitations. The plasma diluted thrombin time (dTT) has shown better correlation with DTI levels than aPTT. This study compared dose-response curves for dTT and aPTT in pediatric patients receiving argatroban and bivalirudin. METHODS: A retrospective review of pediatric patients treated with argatroban (n = 45) or bivalirudin (n = 14) monitored with dTT and aPTT. RESULTS: The dTT assay was calibrated to report DTI concentrations in µg/mL for argatroban and bivalirudin with good analytic sensitivity and specificity. The dTT was fivefold more likely to show a stable dose-response slope than the aPTT (P < .0002; odds ratio, 4.9). For patients in whom both dTT and aPTT showed a significant correlation between dose and assay results, dTT had a higher average correlation factor compared with aPTT (P = .007). Argatroban dose-response slopes showed more inter- and intrapatient variation than bivalirudin (dose-response slope coefficient of variation, 132% vs 52%). CONCLUSIONS: The dTT assay was more likely to show a stable dose response and have a stronger correlation with DTI dose than aPTT. Argatroban shows more variation in dose response than bivalirudin.

Causes of platelet loss during extracorporeal life support.

BACKGROUND: Most pediatric patients show a decline in platelet counts while on extracorporeal life support (ECLS) and require multiple platelet transfusions. To better understand platelet loss during ECLS, this study estimated platelet loss rates due to diagnostic phlebotomy, platelet activation, bleeding and other causes. METHODS: We collected data on 91 patients (1d-20y, 50 M, 41F). Platelet losses were estimated based on changes in platelet count, patient+circuit blood volume, and transfused platelet volumes. Platelet extracellular vesicles were measured by flow cytometry. RESULTS: Median platelet loss was 2.8 × 109 /L/hr, more than twice the normal rate of platelet removal and equivalent to a 67 000/µl decrease in platelet count per day. While platelet loss was correlated with platelet transfusion (r2  = 0.51), transfusion underestimated platelet loss in patients with large decreases in platelet count and over-estimated platelet loss in neonates where the circuit volume > patient blood volume. Patients with disseminated intravascular coagulation before or significant bleeding during ECLS have double the rate of platelet loss. Platelet activation accounted for ~32% of total platelet loss, bleeding ~36% and phlebotomy 4%, with the remaining one-third due to other causes. Annexin-negative platelet extracellular vesicle release, a measure of platelet damage, was increased 9-fold during ECLS. CONCLUSION: Our study is the first to quantitate total, phlebotomy and activation related platelet loss during ECLS. Platelet activation accounts for ~32% of total platelet loss, while bleeding doubles the platelet loss rate. The etiology of the remaining platelet loss is unknown.

Coagulation activation during extracorporeal membrane oxygenation (ECMO).

INTRODUCTION: Extracorporeal membrane oxygenation (ECMO) can be life-saving, but suffers from thrombus formation in the circuit with associated risks of oxygenator occlusion, hemolysis and arterial embolism. The formation of thrombin is the key step to thrombus formation and two factors are needed for sustained thrombin generation, a coagulation activator to initiate the process and a procoagulant phospholipid surface for the coagulation system to assemble on. MATERIALS AND METHODS: The purpose of this study was to use thrombin generation potential (TGP) and other assays to determine the specific coagulation activators and sources of procoagulant phospholipid that are present in ECMO patient plasma. Samples were collected from 60 patients on ECMO (age 1d-19y) followed evaluation of native and stimulated TGP, measurement of factor II levels and determination of procoagulant extracellular vesicle levels by flow cytometry. RESULTS: During ECMO, native (unstimulated) TGP was increased, followed by a decrease back towards normal after ECMO ended. The main activator of TGP in ECMO plasma was increased FXIa (100% of samples tested), while increased tissue factor activity was present in 7%. Procoagulant phospholipids were present in plasma from ECMO patients in the form of circulating platelet and red cell extracellular vesicles, which were increased 2 to 7-fold compared to normal. Procoagulant extracellular vesicle levels correlated with increased plasma native TGP activity. CONCLUSIONS: ECMO activates coagulation in plasma primarily through activation of the contact system and formation of activated factor XIa and generation of circulating procoagulant extracellular vesicles through platelet and red cell activation.

Thrombosis in Extracorporeal Membrane Oxygenation (ECMO) Circuits.

Thrombosis in extracorporeal membrane oxygenation (ECMO) circuits remains a frequent complication. We characterize the location, extent, structure, and clinical implications of thrombi in 53 ECMO circuits from 46 pediatric patients. The tubing, pump, and oxygenator were examined for visible thrombi. Representative samples of thrombi were collected for histologic, immunofluorescence, and immunohistochemical analysis. Thrombi were found in 81% of ECMO circuits. The most clinically significant were inflow oxygenator membrane surface thrombi (11% of circuits), arterial tubing thrombi (30%), and venous tubing (26%) or connector thrombi (26%). Oxygenator membrane surface thrombi resulted in rapidly increasing delta pressure across the oxygenator over 1-2 days, oxygenator failure, and circuit replacement. Oxygenator membrane surface thrombi were associated with intravascular venous thrombosis and bacterial infection before starting ECMO. Arterial cannula/tubing thrombi led in one case to aortic and mesenteric artery thrombosis followed by bowel infarction. In 11% of cases, venous tubing thrombi grew large enough to break off and embolize to the pump, resulting in increased hemolysis. Antifibrinolytic therapy during ECMO was associated with an increased risk of pump thromboembolism. Other less clinically significant thrombi included pump axle thrombi with thrombus fragments trapped in the oxygenator (45%), and deep oxygenator membrane thrombi (15%). Examination of ECMO circuits after removal is a useful quality improvement tool that can elucidate the cause of circuit problems, indicate patients at increased risk of thrombosis, and suggest areas for possible improvements.

Interlaboratory Performance in Measurement of Dabigatran and Rivaroxaban.

CONTEXT.­: Assessing direct oral anticoagulant (DOAC) drug levels by reliable laboratory assays is necessary in a number of clinical scenarios. OBJECTIVE.­: To evaluate the performance of DOAC-specific assays for various concentrations of dabigatran and rivaroxaban, assess the interlaboratory variability in measurement of these DOACs, and investigate the responsiveness of the routine clotting assays to various concentrations of these oral anticoagulants. DESIGN.­: College of American Pathologists proficiency testing survey data from 2013 to 2016 were summarized and analyzed. RESULTS.­: For dabigatran, the interlaboratory coefficient of variation (CV) of ecarin chromogenic assay was broad (ranging from 7.5% to 29.1%, 6.3% to 15.5%, and 6.8% to 9.0% for 100-ng/mL, 200-ng/mL, and 400-ng/mL targeted drug concentrations, respectively). The CV for diluted thrombin time for dabigatran was better overall (ranging from 11.6% to 17.2%, 9.3% to 12.3, and 7.1% to 11.2% for 100 ng/mL, 200 ng/mL, and 400 ng/mL, respectively). The rivaroxaban-calibrated anti-Xa assay CVs also showed variability (ranging from 11.5% to 22.2%, 7.2% to 10.9%, and 6.4% to 8.1% for 50-ng/mL, 200-ng/mL, and 400-ng/mL targeted drug concentrations, respectively). The prothrombin time (PT) and activated partial thromboplastin time (aPTT) showed variable dose- and reagent-dependent responsiveness to DOACs: PT was more responsive to rivaroxaban and aPTT to dabigatran. The undiluted thrombin time showed maximum prolongation across all 3 dabigatran concentrations, making it too sensitive for drug-level monitoring, but supporting its use as a qualitative screening assay. CONCLUSIONS.­: DOAC-specific assays performed reasonably well. While PT and aPTT cannot be used safely to determine DOAC degree of anticoagulation, a normal thrombin time excludes the presence of dabigatran.

Platelet, Red Cell, and Endothelial Activation and Injury During Extracorporeal Membrane Oxygenation.

Extracorporeal membrane oxygenation (ECMO) can be lifesaving but suffers from high rates of bleeding and repeated transfusions. Current monitoring of blood cell damage during ECMO is limited to platelet counts, hematocrit, and plasma hemoglobin levels. Extracelluar vesicles (EV) are small cell fragments released when cells are activated/injured. The objective was to evaluate flow cytometric measurements of EV during ECMO as an indication of platelet, red cell, and endothelial activation/injury. Samples were collected from 55 patients (1 day to 19 years) during 58 ECMO runs. ECMO activated or injured blood cells, but the extent was highly variable and patient dependent. On average platelet activation was increased sevenfold during ECMO with up to 60-fold increased activation during the first 24 hours in some patients. EV associated with platelet and red-cell injury were increased eightfold on average but up to 200-fold in patients with disseminated intravascular coagulation, severe hemolysis, or massive transfusion. Approximately 9% of ECMO patients showed a red-cell and endothelial activation pattern that was associated with poor prognosis. Extracellular vesicles with autofluorescence similar to bilirubin appeared to come from monocytes processing hemoglobin. ECMO is associated with a highly variable, sustained increase in platelet, red-cell, and endothelial activation and injury that is a combination of circuit and transfusion related events, the patients underlying condition and possibly genetic influences on blood cell activation and injury. Extracellular vesicle measurements may improve our understanding of cellular activation and injury during ECMO as we work to improve the biocompatibility of these systems.

Conversion From Activated Clotting Time to Anti-Xa Heparin Activity Assay for Heparin Monitoring During Extracorporeal Membrane Oxygenation.

OBJECTIVES: Anticoagulation with unfractionated heparin remains the most common therapy used to prevent circuit thrombosis during extracorporeal membrane oxygenation, but no consensus exists on the optimal method or targets for heparin monitoring. From 2015 to 2018, we switched from monitoring heparin during extracorporeal membrane oxygenation using activated clotting times to anti-Xa heparin activity assays. This study describes the transition from activated clotting time to anti-Xa heparin activity assay monitoring and the associated clinical changes. DESIGN: Retrospective analysis at single institution. SETTING: Referral Children's Hospital. PATIENTS: A total of 145 pediatric patients over 152 extracorporeal membrane oxygenation runs using 206 extracorporeal membrane oxygenation circuits. INTERVENTIONS: Anticoagulation protocol quality improvement. MEASUREMENTS AND MAIN RESULTS: From 2015 to 2018, heparin monitoring during extracorporeal membrane oxygenation changed from hourly activated clotting time to anti-Xa heparin activity assay every 6 hours with an associated 75% reduction in the circuit changes per extracorporeal membrane oxygenation day. Over the 4 years, patients with an average anti-Xa heparin activity assay of at least 0.25 U/mL showed a 59% reduction in circuit changes per extracorporeal membrane oxygenation day compared with less than 0.15 U/mL. In addition to its association with reduced circuit changes, anti-Xa heparin activity assay monitoring was also associated with reduced heparin dose changes per day from 11 ± 4 to 2 ± 1 (p < 0.001), smaller heparin dose changes (less variation in dose), and reduced diagnostic phlebotomy volumes from 41 ± 6 to 25 ± 11 mL/day (p < 0.001). The number of patients with reported bleeding decreased from 69% using activated clotting time to 51% (p = 0.03). Transfusion rates did not change. CONCLUSIONS: Over 4 years, we replaced the activated clotting time assay with the anti-Xa heparin activity assay for heparin monitoring during extracorporeal membrane oxygenation. Minimum anti-Xa heparin activity assay levels of 0.25 U/mL were associated with reduced circuit changes. Further studies are needed to determine the optimum anti-Xa heparin activity assay therapeutic range during extracorporeal membrane oxygenation.

Monitoring Hemostasis During Extracorporeal Life Support.

To balance the risk of bleeding versus circuit thrombosis during extracorporeal life support (ECLS), it is important to monitor anticoagulants and hemostasis. We evaluated the prothrombin time (PT), partial thromboplastin time (PTT), activated clotting time (ACT), and antifactor Xa heparin activity (aXa) correlation with changes in coagulation factor and heparin levels using in vitro and in vivo samples. aXa correlated with heparin (r = 0.97) and antithrombin (r = 0.98) but was unaffected by other parameters. PT correlated with coagulation factors (r = 0.88) but was minimally affected by heparin or other parameters. When single parameters were changed, ACT was insensitive to <0.5 U/ml heparin, correlated with coagulation factors (r = 0.99), and was affected by factor XII and platelets. When multiple parameters changed in vitro and in vivo, ACT was not correlated with heparin or coagulation factors. PTT correlated with heparin and coagulation factors individually but had low correlation when multiple parameters changed in vitro and in vivo. In conclusion, aXa is the most specific for heparin levels, and PT is most specific for coagulation factor levels making these assays well suited to monitor anticoagulation and hemostasis for patients on ECLS. PTT is highly variable when multiple parameters are changing in vitro and in vivo, but may be useful when aXa cannot be used because of interference. ACT is too insensitive to heparin, sensitive to too many other factors, and too imprecise to be useful for monitoring hemostasis during ECLS.

Interference in the anti-Xa heparin activity assay due to hemolysis and icterus during pediatric extracorporeal life support.

Chromogenic anti-Xa assays for unfractionated heparin monitoring (heparin activity) are susceptible to interference from hemolysis and icterus. The purpose of this study was to better understand the effect of hemolysis and icterus on anti-Xa heparin activity and to predict the magnitude of the error. Increasing levels of hemoglobin and unconjugated bilirubin were added to pooled normal plasma or buffer containing known levels of heparin. Increased plasma hemoglobin or bilirubin produced falsely increased residual factor Xa activity as measured by the absorbance change (OD/min) in the Stago heparin activity assay. This increased absorbance change slope resulted in falsely lower estimates of heparin activity. The falsely lower heparin activity measurement occurred even when heparin was not present, indicating it was not due to heparin neutralization. In a sample containing 0.62 ± 0.06 U/mL heparin and 228 mg/dL hemoglobin, the measured heparin activity was 0.41 ± 0.03 U/mL, underestimating heparin activity by 0.21 ± 0.07 U/mL. Interference occurred if plasma hemoglobin was above 70 mg/dL or bilirubin was above 16 mg/dL, which happened in 16%-26% of samples from pediatric patients on extracorporeal life support (ECLS). In conclusion, hemolysis and icterus were common in ECLS patients, leading to underestimates of unfractionated heparin activity and potentially higher doses of heparin than intended. The magnitude of the heparin activity measurement error could be predicted based on plasma hemoglobin and bilirubin levels until these levels exceeded the technical limits of the assay, ~230 mg/dL hemoglobin and 55 mg/dL bilirubin.

External Quality Assurance of Platelet Function Assays: Results of the College of American Pathologists Proficiency Testing Program.

CONTEXT.­: The College of American Pathologists (CAP) developed proficiency testing for platelet function assays by using blood collected by the participant added to challenge tubes containing either saline (normal) or tirofiban (abnormal). OBJECTIVE.­: To analyze platelet function proficiency testing for Platelet Function Analyzer PFA-100, platelet aggregation, PlateletWorks, and PlateletMapping. DESIGN.­: Proficiency testing data from 2012-2016 were analyzed. RESULTS.­: For PFA-100, a total of 1200 laboratories participated; the coefficient variation (CV) of cartridge closure times was 22% (saline); 44,952 of 45,616 survey responses (99%) provided an interpretation, and 42,934 of 44,952 (96%) were correct. For optical platelet aggregation, 190 laboratories participated; the CV was 17% (saline), 7444 of 7813 survey responses (95%) provided an interpretation, and 7015 of 7444 (94%) were correct. For PlateletWorks, 60 laboratories participated; the CV was 3% to 11% (saline); 2412 of 2454 survey responses (98%) provided an interpretation, and 1207 of 1276 (95%) were correct for adenosine diphosphate (ADP) and 936 of 1136 (82%) for collagen. For PlateletMapping, 200 laboratories participated. For ADP, 1128 of 2697 survey responses (42%) provided an interpretation, but only 927 of 1128 (82%) were correct. For arachidonic acid, 1139 of 2604 survey responses (44%) provided an interpretation and 964 of 1139 (85%) were correct. CONCLUSIONS.­: CAP is the first to provide proficiency testing for platelet aggregation, PlateletWorks, and PlateletMapping. Platelet aggregation, PFA-100, and PlateletWorks using ADP as an agonist performed well with more than 90% of laboratories providing an interpretation and a similar number providing correct results. PlateletWorks using collagen and PlateletMapping showed worse interpretive accuracy than the other methods.

Discordant Partial Thromboplastin Time (PTT) vs Anti-Xa Heparin Activity.

OBJECTIVES: To determine the relationship between baseline variations in the partial thromboplastin time (PTT) and the discordance between the PTT and anti-Xa heparin activity (anti-Xa) during heparin therapy. METHODS: The baseline PTT on heparin was determined using automated heparin neutralization with protamine (prPTT). The prPTT was used to calculate a baseline-corrected PTT on heparin to reduce discordance with anti-Xa measurements. RESULTS: The prPTT removed up to 1 U/mL of heparin, returning baseline values for normal, factor-deficient, and lupus inhibitor plasmas. A prolonged prPTT was seen in 97 (53%) of 182 samples from heparinized patients. The heparinized PTT was discordant compared with anti-Xa in 64 (35%) of 182 samples and 43 (67%) of 64 discordant samples, and 46% of concordant samples showed a prolonged prPTT. A baseline-corrected PTT reduced discordance with anti-Xa measurements by 64%. CONCLUSIONS: PTT/anti-Xa discordance due to baseline PTT prolongation could be reduced using a baseline-corrected PTT.

Measurement of microvesicle levels in human blood using flow cytometry.

Microvesicles are fragments of cells released when the cells are activated, injured, or apoptotic. Analysis of microvesicle levels in blood has the potential to shed new light on the pathophysiology of many diseases. Flow cytometry is currently the only method that can simultaneously separate true lipid microvesicles from other microparticles in blood, determine the cell of origin and other microvesicle characteristics, and handle large numbers of clinical samples with a reasonable effort, but expanded use of flow cytometric measurement of microvesicle levels as a clinical and research tool requires improved, standardized assays. The goal of this review is to aid investigators in applying current best practices to microvesicle measurements. First pre-analytical factors are evaluated and data summarized for anticoagulant effects, sample transport and centrifugation. Next flow cytometer optimization is reviewed including interference from background in buffers and reagents, accurate microvesicle counting, swarm interference, and other types of coincidence errors, size calibration, and detection limits using light scattering, impedance and fluorescence. Finally current progress on method standardization is discussed and a summary of current best practices provided. © 2016 Clinical Cytometry Society.

Evaluation of prothrombin time and activated partial thromboplastin time mixing studies using an estimated factor correction method.

Mixing studies for prolonged prothrombin time (PT)/activated partial thromboplastin time (aPTT) are used to estimate whether the prolongation is due to an inhibitor or factor deficiency. We propose a new method of mixing study interpretation based on estimation of average factor level changes. Factor level vs. PT/aPTT curves were prepared for single factor, vitamin K-dependent factor, and all factor deficiencies. These curves were used to predict the factor level in the sample and the correction needed to differentiate deficiencies from inhibitors. We compared this estimated factor correction (EFC) method to normal range, percentage correction, and Rosner index. For a given factor level, multiple factor deficiencies prolonged the PT/aPTT more than single factor deficiency, necessitating different thresholds for defining correction on mixing studies. The EFC method was superior to other the correction methods, correctly identifying 38 of 39 known inhibitors, single and multiple factor deficiencies, and correctly identifying inhibitor vs. deficiency in 50 of 59 patient samples. In 99 adult patient mixing studies over 18 months, 30% showed deficiency only, 30% inhibitor only, whereas 40% showed evidence of both. The EFC method for PT/aPTT mixing study interpretation was more accurate than the comparison methods at determining deficiency versus inhibitor.

In-vitro thrombogenicity assessment of flow diversion and aneurysm bridging devices.

Endoluminal devices such as metallic flow diversion (FD) and aneurysm bridging (AB) stents are used for treatment of intracranial aneurysms. Treatments are associated with thrombogenic events mandating the use of dual antiplatelet therapy in all cases. In the current in vitro study, we utilize a slow binding fluorogenic thrombin specific substrate to measure the thrombin generation potential of six devices: four FD devices (Pipeline™ Flex embolization device, Pipeline™ Flex embolization device with Shield Technology™, SILK+, FRED™) and two AB devices (Solitaire™ AB, LEO+). We show that the Pipeline™ Flex embolization device with Shield Technology™ has significantly lower peak thrombin and takes significantly longer time to achieve peak thrombin (time to peak) compared to the other three FD devices (p < 0.05), with statistically similar results to the less thrombogenic AB devices. We conclude that surface modification of endoluminal stents could be an effective method to mitigate thrombogenic complications.

Evaluation and Pharmacokinetics of Treatment Dose Enoxaparin in Hospitalized Patients With Morbid Obesity.

BACKGROUND: The pharmacokinetic properties of enoxaparin may lead to supratherapeutic antifactor Xa (anti-Xa) levels and increased bleeding when standard treatment doses are used in patients with morbid obesity. OBJECTIVE: To evaluate the dose of enoxaparin needed to achieve therapeutic anti-Xa levels in a prospective, masked observational cohort of heterogeneous inpatients with morbid obesity and to determine whether patients with morbid obesity treated with 1 mg/kg of enoxaparin are at increased risk of supratherapeutic levels and bleeding events compared to patients receiving lower doses. METHODS: Hospitalized patients with a body mass index ≥40 kg/m(2) or actual body weight ≥140 kg and prescribed treatment doses of enoxaparin >60 mg per day were enrolled and consented to phlebotomy for determination of anti-Xa levels. RESULTS: Forty-one patients were included for data analysis. The dose of enoxaparin that resulted in therapeutic and supratherapeutic anti-Xa levels at steady state was 0.83 mg/kg and 0.98 mg/kg (-0.11; 95% confidence interval [CI] -0.20 to -0.01, P = .02), respectively. Enoxaparin dose as mg/kg of actual body weight was an independent predictor of having a supratherapeutic anti-Xa level. Patients with doses <0.95 mg/kg versus ≥0.95 mg/kg were less likely to have supratherapeutic levels (odds ratio 0.21 [95% CI 0.05-0.84], P = .02) and had similar rates of subtherapeutic levels. Doses <0.95 mg/kg and ≥0.95 mg/kg resulted in similar bleeding rates of 17.9% and 22.2% (P = .71), respectively. CONCLUSION: Patients with morbid obesity required less than the recommended 1 mg/kg enoxaparin dose to achieve therapeutic peak anti-Xa levels; therefore, initiation with lower dosages is prudent and anti-Xa monitoring should guide dosage adjustments.

Patients with cirrhosis show a relative increase in thrombin generation that is correlated with lower antithrombin levels.

Predicting the risk of bleeding or thrombosis in cirrhotic patients is difficult due to reduced levels and dysregulation of both procoagulant and anticoagulant factors. We utilized thrombin generation and microvesicle analysis to better understand the regulation of haemostasis in cirrhotic patients. We studied 24 patients with cirrhosis vs. 21 healthy controls. Cirrhotic patients had reduced prothrombin activity (40 ±â€Š9 vs. 112 ±â€Š15), protein C activity (36 ±â€Š10 vs. 114 ±â€Š19) and antithrombin activity (43 ±â€Š14 vs. 109 ±â€Š10). Peak thrombin generation was reduced in cirrhotic patients (165 ±â€Š47 vs. 232 ±â€Š101), but the ratio of peak thrombin generation to prothrombin activity was increased in cirrhotic patients (4.2 ±â€Š1.0 vs. 2.1 ±â€Š0.9) indicating a relative increase in thrombin generation in cirrhosis. The termination time ratio was increased in cirrhotic patients (7.2 ±â€Š1.9 vs. 3.1 ±â€Š0.7) and correlated with reduced antithrombin levels, indicating that cirrhotic patients took longer to stop thrombin generation than controls. Cirrhotic patients showed reduced procoagulant microvesicles from platelets (39 500 ±â€Š24 800 vs. 107 700 ±â€Š74 200) and other cells, but levels overlapped with controls. Cirrhotic patients showed a wide range of procoagulant and anticoagulant levels leading to variability in the regulation of thrombin generation. In conclusion, compared with healthy controls, patients with cirrhosis have lower antithrombin levels that lead to slower downregulation of thrombin generation and more overall thrombin being produced for a given procoagulant level in blood, but also low normal levels of procoagulant microvesicles that would slow initiation of thrombin generation. Whether an individual cirrhosis patient is at a greater risk of bleeding vs. thrombosis may depend on their specific imbalance in procoagulants vs. anticoagulants.