anti-Xa Measurements Research Articles

Monitoring of heparins in antithrombin-deficient patients

IntroductionHeparins exert their anticoagulant effect through activation of antithrombin. Whether antithrombin deficiency leads to clinically relevantly reduced anti-Xa activity of heparins is unknown. We investigated the relation between antithrombin deficiency and anti-Xa activity measurements of plasma samples spiked with unfractionated heparin (UFH) or low-molecular-weight heparin (LMWH). Materials and methodsPlasma samples from 34 antithrombin-deficient subjects and 17 family controls were spiked with UFH and LMWH (nadroparin) aimed to correspond with an anti-Xa activity of 0.8 IU/mL. Antithrombin, β-antithrombin and anti-Xa activities were measured. ResultsMean anti-Xa activity with LWMH was 0.55 IU/mL (0.30–0.74) (recovery 69%, 38–93%) in antithrombin-deficient subjects and 0.82 (0.71–0.89) IU/mL in controls (recovery 103%, 89–111%). Expected anti-Xa measurements after LMWH spiking were found in 17/17 non-deficient subjects and in 8/34 antithrombin-deficient subjects. Anti-Xa measurements in the expected range (0.6–1.0 IU/mL) after UFH spiking were found in 17/17 non-deficient subjects and in 1/22 antithrombin-deficient subjects. Antithrombin activity correlated with anti-Xa activity of UFH (R = 0.77) and LMWH (R = 0.66). Mixing studies of pooled normal plasma and antithrombin-deficient plasma showed that anti-Xa recovery was linearly reduced with antithrombin activity decreasing below 100%. ConclusionsReduced antithrombin activity causes significantly reduced anti-Xa levels. Standard LWMH- or UFH-doses are likely to lead to under treatment in antithrombin-deficient individuals.

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

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. 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. 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%. PTT/anti-Xa discordance due to baseline PTT prolongation could be reduced using a baseline-corrected PTT.

Pharmacokinetics of Enoxaparin After Renal Transplantation in Pediatric Patients.

Enoxaparin is commonly used in the prevention of renal allograft vascular thrombosis but off-label in children, and no consensus exists regarding the optimal dosing and dose adjustment. In this retrospective study, 444 anti-Xa levels were obtained from 30 pediatric renal transplant recipients in order to investigate enoxaparin population pharmacokinetics. The main results were (1) 25% of children achieved the target anti-Xa activity 36hours after initiation of treatment, (2) anti-Xa time courses were best described by a 1-compartment open model with first-order absorption, (3) body weight but not renal function was the sole covariate influencing clearance and volume of distribution, and (4) large between-subject and between-occasion variabilities in anti-Xa activity were observed. However, creatinine-based estimated glomerular filtration rate in the first post-renal transplantation hours may not reliably reflect the actual renal function of the children. Based on the final population model, a Bayesian-based program was developed in order to estimate the individual pharmacokinetic parameters on a single anti-Xa measurement, allowing determination of the next enoxaparin dose that will quickly achieve an appropriate anti-Xa activity (targeting 0.3-0.5IU/mL) and anticoagulation. Finally, these results should help standardize practices that remain to date largely heterogeneous in pediatric intensive care units.

Comparison between activated clotting time and anti-activated factor X activity for the monitoring of unfractionated heparin therapy in patients with aortic aneurysm undergoing an endovascular procedure

ObjectiveCurrent guidelines recommend administration of unfractionated heparin (UFH) and measurement of activated clotting time (ACT) during endovascular procedures. The aim of this study was to compare ACT and anti-activated factor X (anti-Xa) measurements for monitoring of UFH therapy during an aortic endograft procedure and to assess the association of peak ACT and peak anti-Xa activity with periprocedural bleeding. MethodsWe retrospectively studied 104 patients with aortic aneurysm undergoing endovascular procedures with repeated coagulation measurements. After a UFH bolus, further UFH doses were given according to ACT (target range, ≥250 seconds) in clinical routine, and in parallel to each ACT (Hemochron; Accriva Diagnostics, Newport Beach, Calif) measurement, we determined anti-Xa activity (HemosIL Liquid anti-Xa; Instrumentation Laboratory, Bedford, Mass). UFH redosing was solely based on the ACT measurements. We defined periprocedural bleeding as a drop in hemoglobin level ≥3 g/dL or red blood cell transfusion within 24 hours. ResultsAfter the initial UFH bolus (median, 67 IU/kg body weight), ACT and anti-Xa measurements showed a weak correlation (rs, 0.46; P < .001). Median ACT was 233 seconds (range, 127-374 seconds; interquartile range [IQR], 204-257 seconds); median anti-Xa activity was 1.0 IU/mL (range, 0.5-2.0 IU/mL; IQR, 0.9-1.2 IU/mL). Only 31% of the patients had an ACT value ≥250 seconds, whereas all patients had an anti-Xa activity ≥0.5 IU/mL. Accordingly, ACT triggered redosing of UFH frequently. Consequently, we saw a median total UFH use of 90 IU/kg during the procedure, a median peak ACT of 255 seconds (IQR, 234-273 seconds), and a median peak anti-Xa activity of 1.2 IU/mL (IQR, 1.0-1.4 IU/mL). Periprocedural bleeding occurred in 40 (38%) patients. Peak ACT ≥250 seconds was not associated with bleeding (odds ratio, 1.05; 95% confidence interval, 0.41-2.70; P = .952), whereas peak anti-Xa activity ≥1.2 IU/mL was independently associated with bleeding (odds ratio, 4.95; 95% confidence interval, 1.82-13.48; P = .002). Moreover, no periprocedural thromboembolic event occurred. ConclusionsIn this retrospective study of patients with aortic aneurysm undergoing an endovascular procedure, ACT and anti-Xa measurements showed poor correlation; only increased peak anti-Xa activity was independently associated with periprocedural bleeding, not increased ACT. Our findings also suggest that monitoring of UFH therapy with anti-Xa during aortic endograft procedures may reduce total UFH use. We further speculate that this approach could reduce periprocedural bleeding.

Thrombogenesis in Thrombophilic Pregnancy: Evaluation of Low-Molecular-Weight Heparin Prophylaxis

The aim of this study is to investigate thrombogenesis and the hypercoagulable changes in pregnant women affected by thrombophilia who received low-molecular-weight heparin (LWMH) prophylaxis. We included 21 pregnant women affected by thrombophilia treated with LWMH and 20 nontreated normal pregnant women as the control group. The sample group of thrombophilic pregnant women included different conditions (factor V Leiden mutation, protein C deficiency, protein S deficiency, antiphospholipid antibodies syndrome, and combined defects). Three blood samples were collected during pregnancy (i.e., at 16, 20, and 24 weeks) and tested for activated partial thromboplastin time and prothrombin fragment F1 + 2 (F1 + 2); anti-FXa activity was tested only in treated thrombophilic pregnant women. F1 + 2 levels progressively increased during pregnancy in both study groups. However, the F1 + 2 increase in women exposed to heparin prophylaxis was significantly lower than that in normal pregnant women in all 3 measurements carried out during gestation (p < 0.05); a statistically significant inverse correlation between F1 + 2 levels and anti-Xa activity (R = -0.8575, p < 0.05) was observed in treated women during pregnancy. Our findings suggest that F1 + 2 in addition to anti-Xa measurement could be used to adjust LWMH prophylaxis, at least in high-risk pregnant women.

Adjusted value of thromboprophylaxis in hospitalized obese patients: A comparative study of two regimens of enoxaparin: The ITOHENOX study

Thromboprophylaxis is a mainstay of hospital care in patients at high risk of thrombosis. Fixed doses of low-molecular-weight heparin (LMWH) are recommended for thromboprophylaxis in patients admitted to hospital for an acute medical condition. However, the distribution of LMWH is weight-based, and the efficacy of standard doses in obese patients may be decreased. Data for obese patients are mainly available in bariatric surgery with extremely obese patients who are at greater risk of venous thromboembolism than those hospitalized for a medical condition. We conducted a randomized control trial in medically obese inpatients (BMI≥30kg/m2) assessing two regimens of enoxaparin (40mg and 60mg SQ daily) in order to determine whether a stronger dosage would achieve higher anti-Xa level suitable for thromboprophylaxis. Between September 2013 and April 2015, 91 patients were included in the study (mean (±standard deviation) age was 70.4±10.7years, average BMI 37.8±6.4kg/m2). Main indications of thromboprophylaxis were mainly acute infection (50%), acute respiratory failure (10%), acute congestive heart failure (9%) and acute rheumatic disorders (18%).Average anti-Xa activity, measured 4h after the third administration of enoxaparin was 0.25±0.09IU/mL in group 1 (enoxaparin 40mg) and 0.35±0.13IU/mL in group 2 (enoxaparin 60mg) (P<10−3). The proportions of patients with normal anti-Xa activity (between 0.32 and 0.54IU/mL) were 31% (n=11) and 69% (n=24) in group 1 and 2 respectively (P=0.007). The proportions of anti-Xa activity measurement below the normal range were 64% and 36% in group 1 and 2 (P<10−3) respectively. Subgroup analysis focusing on high weight patients (above 100kg, n=45) showed a marked difference in the proportion of patients with normal anti-Xa activity between group 1 (9%) and 2 (44%) (P=0.009). No venous thromboembolism occurred during the study and one patient in group 1 died because of hemorrhagic shock due to a gastric ulcer. Incidence of adverse events was not different between the two groups (P=0.52).In conclusion, the ITOHENOX study shows in medically obese inpatients that thromboprophylaxis with enoxaparin 60mg provides higher control of anti-Xa activity, without more bleeding complications than the standard enoxaparin regimen. This trial is registered with ClinicalTrials.gov, number NCT01707732.

The value and impact of anti-Xa activity monitoring for prophylactic dose adjustment of low-molecular-weight heparin during pregnancy: a retrospective study.

: Low-molecular-weight heparin (LMWH) is the anticoagulant drug class of choice for the prevention of thrombosis during pregnancy and often used for prevention of severe obstetrical complications, yet the optimal dosage is still a matter of debate. Although several studies have evaluated LMWH dose adjustment methods based on anti-Xa levels, no definitive conclusion as yet exists. This extensive retrospective study sought to address the rationale of regular anti-Xa activity monitoring in pregnant patients given LMWH prophylaxis. We evaluated pregnant patients receiving LMWH prophylaxis with three or more anti-Xa measurements. The initial LMWH dose was fixed and adjusted for obesity. The desired anti-Xa activity was 0.2-0.6 IU/ml and LMWH dose was increased for anti-Xa levels less than 0.3 IU/ml. Of the 173 enrolled women, 79% required LMWH dose adjustment (56%: one dose increase; 20%: two; 3%: three). In the vast majority of patients, LMWH dose was increased before substantial body weight gain. The number of dose adjustments positively correlated with BMI, though not with body weight gain. Based on our study findings, we recommend conducting LMWH dose adjustments based on anti-Xa levels, monitored regularly during pregnancy to keep the anti-Xa activity within the desired range.

Serial Trough Anti-Xa Levels to Assess Low Molecular Weight Heparin Accumulation in Patients with Chronic Kidney Disease: Analysis of Crcl

Serial Trough Anti-Xa Levels to Assess Low Molecular Weight Heparin Accumulation in Patients with Chronic Kidney Disease: Analysis of Crcl <30 Ml/Min from the Trivet Study

Establishing the heparin therapeutic range using aPTT and anti-Xa measurements for monitoring unfractionated heparin therapy.

BackgroundUnfractionated heparin (UFH) has unstable pharmacokinetics and requires close monitoring. The activated partial thromboplastin time (aPTT) test has been used to monitor UFH therapy for decades in Korea, but its results can be affected by numerous variables. We established an aPTT heparin therapeutic range (HTR) corresponding to therapeutic anti-Xa levels for continuous intravenous UFH administration, and used appropriate monitoring to determine if an adequate dose of UFH was applied.MethodsA total of 134 ex vivo samples were obtained from 71 patients with a variety of thromboembolisms. All patients received intravenous UFH therapy and were enrolled from June to September 2015 at Gyeongsang National University Hospital. All laboratory protocols were in accordance with the Clinical and Laboratory Standards Institute guidelines and the College of American Pathologist requirements for aPTT HTR.ResultsAn aPTT range of 87.1 sec to 128.7 sec corresponded to anti-Xa levels of 0.3 IU/mL to 0.7 IU/mL for HTR under our laboratory conditions. Based on their anti-Xa levels, blood specimen distribution were as follows: less than 0.3 IU/mL, 65.7%; 0.3–0.7 IU/mL (therapeutic range), 33.6%; and more than 0.7 IU/mL, 0.7%. No evidence of recurring thromboembolism was observed.ConclusionUsing the conventional aPTT target range may lead to inappropriate dosing of UFH. Transitioning from the aPTT test to the anti-Xa assay is required to avoid the laborious validation of the aPTT HTR test, even though the anti-Xa assay is more expensive.

Clinical Use of Anti-Xa Monitoring in Malignancy-Associated Thrombosis.

Introduction. Low molecular weight heparin (LMWH) is preferred for malignancy-associated venous thromboembolism (VTE). Many providers monitor LMWH with anti-Xa levels, despite little validation on correspondence with patient outcome. Methods. This is a retrospective, single institution study of anti-Xa measurement in malignancy-associated thrombosis. Cases were identified using the Electronic Data Warehouse, and inclusion was confirmed by two independent reviewers. Malignancy type, thrombotic history, measurement rationale and accuracy, clinical context, and management changes were evaluated. Results. 167 cases met inclusion criteria. There was no clear rationale for anti-Xa testing in 56%. Impaired renal function (10%), documented or suspected recurrent thrombosis despite anticoagulation (9%), and bleeding (6%) were the most common reasons for testing. Incorrect measurement occurred in 44%. Renal impairment was not a significant impetus for testing, as 70% had a GFR > 60. BMI > 30 was present in 40%, and 28% had a BMI < 25. Clinical impact was low, as only 11% of patients had management changes. Conclusions. Provider education in accuracy and rationale for anti-Xa testing is needed. Our study illustrates uncertainty of interpretation and clinical impact of routine anti-Xa testing, as management was affected in few patients. It is not yet clear in which clinical context providers should send anti-Xa levels.

Prospective Evaluation of Weight-Based Prophylactic Enoxaparin Dosing in Critically Ill Trauma Patients: Adequacy of AntiXa Levels is Improved

Venous thromboembolism (VTE) is a leading cause of death in multisystem trauma patients; the importance of VTE prevention is well recognized. Presently, standard dose enoxaparin (30 mg BID) is used as chemical prophylaxis, regardless of weight or physiologic status. However, evidence suggests decreased bioavailability of enoxaparin in critically ill patients. Therefore, we hypothesized that a weight-based enoxaparin dosing regimen would provide more adequate prophylaxis (as indicated by antifactor Xa levels) for patients in our trauma intensive care unit (TICU).These data were prospectively collected in TICU patients admitted over a 5-month period given twice daily 0.6 mg/kg enoxaparin (actual body weight). Patients were compared with a historical cohort receiving standard dosing. Anti-Xa levels were collected at 11.5 hours (trough, goal ≥ 0.1 IU/mL) after each evening administration. Patient demographics, admission weight, dose, and daily anti-Xa levels were recorded. Patients with renal insufficiency or brain, spine, or spinal cord injury were excluded. Data were collected from 26 patients in the standard-dose group and 37 in the weight-based group. Sixty-four trough anti-Xa measurements were taken in the standard dose group and 74 collected in the weight-based group. Evaluating only levels measured after the third dose, the change in dosing of enoxaparin from 30 to 0.6 mg/kg resulted in an increased percentage of patients with goal antifactor Xa levels from 8 per cent to 61 per cent (P < 0.0001). Examining all troughs, the change in dose resulted in an increase in patients with a goal anti-Xa level from 19 to 59 per cent (P < 0.0001). Weight-based dosing of enoxaparin in trauma ICU patients yields superior results with respect to adequate anti-Xa levels when compared with standard dosing. These findings suggest that weight-based dosing may provide superior VTE prophylaxis in TICU patients. Evaluation of the effects of this dosing paradigm on actual VTE rate is ongoing at our institution.

Anti-coagulation assessment with prothrombin time and anti-Xa assays in real-world patients on treatment with rivaroxaban.

Monitoring of anti-coagulation with the direct factor Xa inhibitor rivaroxaban is considered unnecessary in a routine clinical setting. However, assessment of its anti-coagulant effect may be desirable in certain clinical situations. We assessed prothrombin time (PT) reagents and commercially available anti-Xa assays (Biophen) calibrated for rivaroxaban and heparin in comparison to liquid chromatography-mass spectrometry (LC-MS/MS) measurements of rivaroxaban concentration in samples from patients on treatment with rivaroxaban for stroke prevention in atrial fibrillation. Citrate plasma samples were obtained from 30 randomly selected patients on uninterrupted treatment with rivaroxaban for a minimum of 1month. The anti-Xa assays, direct Xa inhibitor (DiXa-I®), and Heparin LRT® were conducted for both wide and low calibrations for rivaroxaban. Measurements were compared to LC-MS/MS using correlation, linear regression, intra-class correlation, and Bland-Altman analysis. In 30 patients (9 female) of median age 71.5years and BMI 26.5kg/m(2), rivaroxaban concentrations between 2.4 and 625ng/ml (median 82ng/ml) were measured by LC-MS/MS. PT reagents were poorly correlated with rivaroxaban concentrations (r (2) = 0.52 and 0.09). Anti-Xa assays DiXa-I (r (2) = 0.95) and Heparin LRT (r (2) = 0.97) were correlated with rivaroxaban in all concentrations, but especially in low concentrations with low calibrations (r (2) = 0.97 and 0.98, respectively). The highest agreement occurred between Heparin LRT and low rivaroxaban concentrations with a mean difference of -5.3ng/ml (limits of agreement, 12.9 to 2.4ng/ml). Anti-Xa assays can indirectly determine the concentration of rivaroxaban for a wide range of concentrations in real-world patients. An interpretation of anti-Xa and PT measurements in treatment with rivaroxaban requires knowledge of the local reagents.

Retrospective cohort study comparing activated partial thromboplastin time versus anti‐factor Xa activity nomograms for therapeutic unfractionated heparin monitoring in pediatrics

Unfractionated heparin (UFH) is widely used to treat thromboembolic disease, but monitoring in children is challenging. Both activated partial thromboplastin time (aPTT) and anti-factor Xa activity (anti-Xa) are utilized, but a comparison of dosing nomograms has not been reported in pediatrics. To compare the performance of aPTT and anti-Xa for UFH monitoring in pediatric patients. A retrospective cohort study was conducted in patients ≤21years old treated with UFH at Johns Hopkins Hospital from January 2009 to May 2011. For monitoring, an aPTT nomogram was used for the initial 15months, and an anti-Xa nomogram was used for the subsequent 12months. Clinical characteristics, laboratory data and outcomes were analyzed. Thirty-four patients were monitored with aPTT and 26 patients with anti-Xa. There was no significant difference in median time to therapeutic range (11.6h aPTT, 95%CI=6.0-17.0; 9.9h anti-Xa, 95%CI=7.3-20.7) or per cent of patients achieving therapeutic measurements at 24 (79% aPTT, 95%CI=62-91; 73% anti-Xa, 95%CI=52-88) and 48h (88% aPTT, 95%CI=73-97; 96% anti-Xa, 95%CI=80-100). However, anti-Xa measurements were more frequently therapeutic than aPTT (74% [95%CI=69-78] vs. 54% [95%CI=50-59]). Variance between anti-Xa and aPTT measurements was high (R(2) =0.236). No significant difference was seen in bleeding incidence (9% aPTT, 95%CI=2-24; 15% anti-Xa, 95%CI=4-35). The time to achieve therapeutic measures and bleeding outcomes were not significantly different between anti-Xa and aPTT nomograms. However, a small study size limits the power to detect clinically relevant differences. The results warrant larger prospective studies of UFH monitoring in children with thromboembolic disease.

Blutungen unter NOAK

SummaryThe doses of these drugs are barely tested and the potential clinical thromboembolic risk must be taken into account. Despite the widespread use of NOAC (non vitamin-K dependent oral anticoagulants) and recommendations of regulatory agencies and first consensus meeting on handling the bleeding situation under NOAC, especially in hospitals without a large hemostatic focus, uncertainty still exists. In case of mild bleeding from a clinical perspective, the medical care of these patients and the delay of the next dose or discontinuation is advised. A special laboratory analysis is indicated i.e. in case of known higher grade liver and kidney failure, which can cause a prolonged elimination of NOAC. The administration of factor concentrates is not indicated in this situation. In case of moderate to severe bleeding, the primary measures focus on the stabilization of the heart and circulatory function and parallel on the treatment depending on the localization of the bleeding source. According to experience, mostly gastrointestinal bleeding occurs under the NOAC, which should be supplied endoscopically. In life-threatening bleeding in addition to the measures of hemodynamic stabilization usually a special haemostasis management is required, which should be mainly clinically oriented. After the assessment of bleeding predictor, the time of the last dose and the dose of NOAC should be learned, but other causes of bleeding, including Fibrinolysis, should be excluded or treated. Subsequently, routinely promptly rivaroxaban and/ or apixaban sensitive thromboplastin time (Quick’s value) and a thrombin time (thrombin- poor calibrator) for qualitative assessment can be carried out because only very few hospitals have specific tests (anti-Xa measurements, bovine thrombin), which could be promptly done. If there is a significant deviation from the normal range or to present preliminary value of particular patient, an effect of NOAC most likely exists. In life-threatening bleeding the use of factor concentrates (procoagulants) is indicated. The first-line therapy should be PPSB. Only in exceptional cases, especially when dabigatran is taken, the use of aPPSB (FEIBA®) for prompt haemostasis can be considered. The haemostasis should be always clinically estimated and not according to coagulation tests. The use of rFVIIa (Novo Seven®) shows different results in the bleeding therapy (reversal) under Dabigatran. The doses of these drugs are barely tested and the potential clinical thromboembolic risk must be taken into account.The current concepts of the newly developed antidotes are not clinically validated. First prospective, clinical registries have been started.

Heparin thromboprophylaxis in critically ill patients: Is it really changing outcome?

Heparin thromboprophylaxis in critically ill patients: Is it really changing outcome?

Effectiveness of Unfractionated Heparin in Normal Saline versus Dextrose for Achieving and Maintaining Therapeutic Anti-Factor Xa Levels in Patients with Non-ST-Elevation Acute Coronary Syndrome.

Unfractionated heparin (UFH) administered by IV infusion is effective in preventing myocardial infarction and death after non-ST-elevation acute coronary syndrome. At the authors' centre, preparations of UFH in 0.9% sodium chloride (normal saline; UFH-NS) were used during a shortage of commercially available UFH in dextrose 5% in water (UFH-D5W), the usual preparation. Anecdotal observations raised concerns about the effectiveness of the saline-based preparation in achieving minimally therapeutic anticoagulation. To compare the effectiveness of UFH-NS and UFH-D5W for achieving and maintaining therapeutic anti-factor Xa levels in patients with non-ST-elevation acute coronary syndrome. A retrospective cohort study was conducted with 2 groups of 100 consecutive patients who received either UFH-NS or UFH-D5W for a minimum of 24 h after non-ST-elevation acute coronary syndrome in accordance with a weight-based dosing nomogram. A minimally therapeutic level of anti-Xa (≥ 0.31 IU/mL) was achieved within 24 h for 92% of the patients receiving UFH-D5W and 67% of those receiving UFH-NS (absolute risk difference 25%, 95% confidence interval [CI] 13.4%-36.6%; p < 0.001). Infusion of UFH-NS was associated with lower probability of achieving minimally therapeutic anticoagulation (hazard ratio [HR] 2.30, 95% CI 1.68-3.15; p < 0.001) and maintaining therapeutic anticoagulation (HR 2.31, 95% CI 1.69-3.17; p < 0.001) relative to UFH-D5W. Significant differences in the numbers of patients with subtherapeutic and therapeutic anticoagulation, favouring UFH-D5W, were observed at each of the first, second, and third anti-Xa measurements (p < 0.05). Patients receiving UFH-NS required a greater median number of adjustments to the infusion rate during the first 48 h (1.0 v. 0.5 adjustment per day, p < 0.001). There was no difference between groups in terms of major reductions in hemoglobin. Infusion of UFH-NS was inferior to UFH-D5W for achieving and maintaining therapeutic anticoagulation in patients with non-ST-elevation acute coronary syndrome. Until further study, saline-based heparin infusions should be used with caution, and patients should be monitored closely to ensure timely achievement and maintenance of therapeutic anticoagulation.

Use Of Fondaparinux For Venous Thromboembolism Prophylaxis In Critically Ill Adult Patients With Moderate Renal Insufficiency

Introduction Fondaparinux, a selective activated Factor Xa inhibitor, is a parenteral anticoagulant primarily excreted by the kidneys. Renal insufficiency (RI), commonly encountered in the critically ill, has been shown to portend an increased risk of bleeding than in normal healthy adults. In comparison to the low molecular weight heparin dalteparin, fondaparinux has been shown to be superior for the prevention of post-surgical venous thromboembolism (VTE), however an increased rate of nonfatal bleeding has been observed. Recently, data has emerged supporting empiric dose reductions in patients with moderate RI, defined as a creatinine clearance (CrCl) between 30 and 50 mL/min. Overall, the data supporting appropriate use of fondaparinux in patients with RI is lacking. This retrospective, single center study describes our experience using prophylactic fondaparinux in critically ill patients with moderate RI. Methods We report our experience utilizing fondaparinux in a mixed medical and surgical, critically ill adult population with moderate RI. The site examined was UF Health; an 852-bed sized tertiary care hospital in Gainesville, Florida between 2006 and 2012. Moderate RI was defined as a CrCl between 30 and 50 mL/min calculated by the Cockcroft/Gault equation. Only patients treated with fondaparinux for prophylaxis of VTE were included, patients treated for other indications were excluded. Doses, dose frequency and dose adjustments were documented. If patients had anti-Xa level monitoring these values were recorded. Timing of anti-Xa levels drawn with respect to fondaparinux dosing was also noted. The primary end point of the study was the rate of clinically significant bleeding events in patients receiving at least 72 hours of fondaparinux. Clinically significant bleeding was defined as transfusion of at least two units of packed red blood cells within 48 hours of a documented bleeding event or the workup of bleeding with endoscopy or imaging. Bleeding events were screened by ICD-9 codes and validated via chart review, incidence of VTE was also noted. Results Sixty-fourpatients met inclusion criteria between October 2006 and November 2012. Two patients (3.1%) experienced clinically significant bleeding events as per our definition, both in the form of gastrointestinal bleeding. Empiric dose reductions occurred in 8 (12.5%) patients, this occurred in the form of every other day administration or dose reduction using fondaparinux 1.25mg daily. Nine (14%) patients were monitored with anti-Xa levels, none of these patients encountered bleeding events. The median anti-Xa level was 0.5 mcg/mL, which is in the target prophylactic range for in our lab. The median time interval after last dose of fondaparinux to anti-Xa level measurement was 4 hours. Seven (10.9%) patients had VTE diagnoses prior to the initiation of prophylactic fondaparinux. No patients suffered a VTE event after treatment with prophylactic fondaparinux in our data set. Conclusion We report a cohort of patients with moderate RI receiving fondaparinux for the prevention of VTE. The incidence of clinically significant bleeding was 3.1%. The risk of VTE was low, no patients suffered from a VTE after treatment with fondaparinux in our data set. The rate of bleeding is slightly higher compared to patients with normal renal function. Our experience over this 6 year period supports the notion that RI portends an increased risk of bleeding with the use of fondaparinux. Further prospective studies are needed to determine the safety and efficacy of empiric fondaparinux dose reductions in patients with RI and end stage renal disease. Anti-Xa monitoring may help reduce the risk of bleeding in this patient population by identifying those patients in whom a dose reduction may be beneficial. Disclosures: No relevant conflicts of interest to declare.

Lack Of Anti-Xa Assay Standardization Results In Significant Dose Variation In Neonates and Children Receiving Enoxaparin

▪ IntroductionDue to favorable pharmacokinetic properties, enoxaparin is the most frequently used anticoagulant in children. Unlike adults, a developing hemostatic system provides rationale for therapeutic monitoring with an anti-Xa goal of 0.5-1 units/ml. However, accepted therapeutic ranges are not well correlated with clinical outcomes (i.e. thrombosis or hemorrhage) and there is no assay standardization. In 2011 the coagulation laboratory at the Children's Hospital of Philadelphia (CHOP) changed anti-Xa laboratory assays resulting in anti-Xa levels that were on average, 33% higher than the prior assay (Figure 1). We describe how this change in assay influenced therapeutic enoxaparin dose (mg/kg) at our center. MethodsThe coagulation laboratory performed simultaneous anti-Xa measurements using the Stachrom (old) assay and Rotachrom (new) assay on 19 subjects from split plasma samples. The new assay was then implemented into clinical practice. Information on anti-Xa values and enoxaparin dosing during the initiation of anticoagulation was retrospectively compiled for the 6 months pre and post assay change. Data on enoxaparin dosing and anti-Xa values was collected on 109 children in the old assay phase and 104 children in the new assay phase. Therapeutic enoxaparin dose was defined as the first dose resulting in an anti-Xa value 0.5- 1 unit/ml. All subjects initiated on anticoagulation during this 12 month period were included. Anti-Xa levels were obtained per CHOP standard clinical monitoring: until therapeutic, after dose adjustment, and every other week while inpatient with some subjects more frequently evaluated per provider discretion. Cumulative number of inpatient anti-Xa values obtained was determined for each subject. Enoxaparin dosing and cumulative anti-Xa values pre and post assay changes in each cohort were analyzed using a t-test. ResultsUsing 19 split samples there was strong correlation between the two assays (r=0.96), however, the means between the two assays differed significantly (p <0.0001) with new assay findings being, on average, 33% higher (Figure 1). There was a significant decrease in the new assay mean therapeutic enoxaparin dose (mg/kg) in subjects <3 months (p=0.01) and 3 months–2 years (p<0.0001), but not in subjects >2 years (p=0.18) (Table 1). However, the mean first therapeutic anti-Xa value was significantly higher in subjects > 2 yrs post-assay change (p=0.001). Among all 213 subjects, a total of 1,061 inpatient anti-Xa values were obtained, with a median of 3 (range: 1-32) per subject. Median number of anti-Xa values obtained to achieve therapeutic range was 2 (range: 1-6) prior to assay change and 1 (range: 1-7) post assay change with a significant difference (p=0.004) observed between the groups. [Display omitted] ConclusionsUsing split patient samples, the Rotachrom assay demonstrated significantly higher anti-Xa values than the Stachrom assay. Once the Rotachrom assay was clinically implemented, there was a significant difference in weight based enoxaparin dosing in subjects < 2 years, and higher anti-Xa levels in subjects > 2 yrs. These observations demonstrate enoxaparin dose titration to achieve therapeutic anti-Xa is vulnerable to assay variability. Since these assays are not well standardized, this practice is likely to provide false reassurance of efficacy and safety. The large number of anti-Xa levels obtained may represent unnecessary venipuncture as well as misappropriation of time and resources. It is possible the majority of neonates and children may be safely and effectively treated with weight based dosing without monitoring. These data support a randomized controlled clinical trial in neonates and children comparing safety and efficacy of enoxaparin weight based dosing to dosing titrated to achieve therapeutic anti-Xa. Disclosures:No relevant conflicts of interest to declare.

Low-molecular-weight heparin pharmacokinetics: a dual absorption model approach

The aim of this study was to develop a novel population pharmacokinetic (PPK) model of dalteparin after subcutaneous (s.c.) injection, to describe the impact of the "flip-flop" phenomenon and to demonstrate any ethnic difference between Asian and Caucasian subjects. The PPK model was constructed based on data collected from Asian (Japanese) and Caucasian (French) subjects with a total of 931 plasma anti-Xa activity measurements. After s.c. injection, the apparent elimination half-life of the dalteparin was about 4 hours, longer than that reported after intravenous (i.v.) injection, indicating a "flip-flop" phenomenon. In addition, following the mono-exponential decline profile after s.c. injection, a longer secondary phase was apparently observed in 70% of subjects. To investigate the phenomenon, we applied a dual absorption model including fast first-order and slow zero-order inputs as the structural model. The PPK model for s.c. injection provided the half-life consistent with that of i.v. injection and could account for the observed bi-phasic profile. Body weight and gender for clearance and body weight for volume of distribution were identified as covariates. Due to lower body weight in Asian subjects, an ethnic difference might occur but it would not be reflected by per kg body weight injection. Dalteparin PK profiles after s.c. injection were described reasonably by the novel PPK model based on flip-flop pharmacokinetics and a dual absorption process.

PF.59 What is the Optimal Dose of Low Molecular Weight Heparin in Pregnant Women with Raised BMI?

IntroductionObesity is a risk factor for thromboembolism in pregnancy. Recent RCOG guidelines suggested a non-evidence based weight-dependent protocol in which monitoring of anti-Xa levels was not required. This contrasted with...