Chromogenic Anti-factor Xa Assay Research Articles

Dried plasma spot as an innovative approach to therapeutic drug monitoring of apixaban: Development and validation of a novel liquid chromatography-tandem mass spectrometry method.

Apixaban, a direct oral anticoagulant drug (DOAC), typically does not require routine therapeutic drug monitoring (TDM), yet recent guidelines propose its use in specific clinical scenarios. While various antifactor Xa (anti-FXa) chromogenic assays serve as useful proxies for measuring plasma exposure to apixaban in emergencies, they lack specificity compared with chromatographic methods. This research project is intended to the development and validation of a standardized protocol of liquid chromatography-tandem mass spectrometry (LC-MS/MS) in conformity with the ICH guidelines M10 for the measurement of apixaban in both plasma and dried plasma spots (DPSs). Samples preparation included protein precipitation after the addition of a deuterated internal standard (IS), and the chromatographic separation was carried out on a Thermo Scientific™ Accucore™ Polar Premium column (50 mm × 2.1mm, i.d. 2.6 m). The newly developed LC-MS/MS method for apixaban mesurement from both plasma and DPS resulted linear over a wideconcentration range (31.25-500 ng/mL), accurate, and reproducible without matrix effects, allowing for specific and rapid quantification. Stability was assessed on quality controls and a real sample, allowing the setting up of a robust TDM protocol that was applied to five anonymized plasma samples obtained from adult patients undergoing apixaban treatment at steady-state. In conclusion our novel LC-MS/MS method is adequate for accurate apixaban quantitation from both plasma and DPS matrixes, andmay thus facilitate the guidelines suggested implementation of apixaban TDM, even in peripheral hospitals through shipment of DPS at reference laboratories.

Two Cases of Acute Direct Oral Anticoagulant Overdose Without Adverse Effect.

We report 2 pediatric patients who had acute overdoses of the direct oral anticoagulants medications. Both patients were managed conservatively; neither required reversal agents or blood products nor had any major or minor bleeding events. With therapeutic usage of direct oral anticoagulants, routine coagulation studies typically are considered insufficient measures of anticoagulation and the preferred chromogenic anti-Factor Xa assay is recommended but not widely available. Using a routine hybrid heparin anti-Factor Xa assay, 1 patient demonstrated a strong linear correlation up to a serum rivaroxaban concentration of 940 ng/mL.

Correlation of Thromboelastography with Apparent Rivaroxaban Concentration: Has Point-of-Care Testing Improved?

Concern remains over reliable point-of-care testing to guide reversal of rivaroxaban, a commonly used factor Xa inhibitor, in high-acuity settings. Thromboelastography (TEG), a point-of-care viscoelastic assay, may have the ability to detect the anticoagulant effect of rivaroxaban. The authors ascertained the association of apparent rivaroxaban concentration with thromboelastography reaction time, i.e., time elapsed from blood sample placement in analyzer until beginning of clot formation, as measured using TEG and TEG6S instruments (Haemonetics Corporation, USA), hypothesizing that reaction time would correlate to degree of functional factor Xa impairment. The authors prospectively performed a diagnostic accuracy study comparing coagulation assays to apparent (i.e., indirectly assessed) rivaroxaban concentration in trauma patients with and without preinjury rivaroxaban presenting to a single center between April 2016 and July 2018. Blood samples at admission and after reversal or 24 h postadmission underwent TEG, TEG6S, thrombin generation assay, anti-factor Xa chromogenic assay, prothrombin time (PT), and ecarin chromogenic assay testing. The authors determined correlation of kaolin TEG, TEG6S, and prothrombin time to apparent rivaroxaban concentration. Receiver operating characteristic curve compared capacity to distinguish therapeutic rivaroxaban concentration (i.e., greater than or equal to 50 ng/ml) from nontherapeutic concentrations. Eighty rivaroxaban patients were compared to 20 controls. Significant strong correlations existed between rivaroxaban concentration and TEG reaction time (ρ = 0.67; P < 0.001), TEG6S reaction time (ρ = 0.68; P < 0.001), and prothrombin time (ρ = 0.73; P < 0.001), however reaction time remained within the defined normal range for the assay. Rivaroxaban concentration demonstrated strong but not significant association with coagulation assays postreversal (n = 9; TEG reaction time ρ = 0.62; P = 0.101; TEG6S reaction time ρ = 0.57; P = 0.112) and small nonsignificant association for controls (TEG reaction time: ρ = -0.04; P = 0.845; TEG6S reaction time: ρ = -0.09; P = 0.667; PT-neoplastine: ρ = 0.19; P = 0.301). Rivaroxaban concentration (area under the curve, 0.91) and TEG6S reaction time (area under the curve, 0.84) best predicted therapeutic rivaroxaban concentration and exhibited similar receiver operating characteristic curves (P = 0.180). Although TEG6S demonstrates significant strong correlation with rivaroxaban concentration, values within normal range limit clinical utility rendering rivaroxaban concentration the gold standard in measuring anticoagulant effect.

Rivaroxaban concentrations in acute stroke patients with different dosage forms.

BackgroundThe crushed-tablet rivaroxaban concentration has been previously reported to be lower than the non-crushed concentration. However, the rivaroxaban concentration of fine granules has not yet been investigated. The anticoagulation intensity of rivaroxaban with fine granules, tablets, and crushed tablets was compared in acute stroke patients to assess the efficacy of each form.Methods and findingsHospitalized patients over 75 years old with acute stroke who started taking rivaroxaban from April 2012 to September 2017 were included. Blood samples were drawn just before and 4 hours after taking rivaroxaban on a median of 5 days after treatment initiation for concentration measurements (C0h, C4h) based on an anti-factor Xa chromogenic assay. Of 114 patients (49 female, 83±5 years old), 97 had ischemic strokes, 9 had transient ischemic attacks, and 8 had intracerebral hemorrhages. Rivaroxaban was administered a median of 7 days after onset. Of these, 38 patients were given the 15 mg dose, and 76 were given the 10 mg dose. In the 15 mg dose group, C0h was significantly higher in the fine granule group than in the crushed tablet group, with no significant difference compared to the tablet group [C0h: 27.6±6.8 vs 4.0±4.1 (P = 0.01) vs. 33.3±25.2 ng/ml, (P = 0.51), respectively], as was C4h [223.0±66.6 vs 103.0±79.5 (P = 0.02) vs. 229.5±121.6 ng/ml (P = 0.88)]. In the 10 mg dose group, C0h was significantly higher in the fine granule group than in the crushed tablet group and comparable to that in the tablet group [23.2±7.9 vs 7.5±6.2 (P<0.01) vs 19.0±15.8 ng/ml, (P = 0.35)], as was C4h [150.7±85.4 vs 85.1±46.8 (P<0.01) vs 189.8±92.7 ng/ml (P = 0.18)].ConclusionsThe rivaroxaban concentration with fine granules was consistent with that in the tablet group and higher than that in the crushed tablet group.

Evaluation of global laboratory methods and establishing on-therapy ranges for monitoring apixaban and rivaroxaban: Experience at a single institution.

BackgroundApixaban and rivaroxaban are approved for the prevention and treatment of deep vein thrombosis (DVT), pulmonary embolism (PE), and embolic stroke in atrial fibrillation (AF) patients. The aim of this study was to find appropriate methods of monitoring the anticoagulant effects of are direct oral anticoagulants (DOACs) and establish on‐therapy ranges using conventional tests.MethodsA total of 184 samples were collected from 91 patients receiving DOACs. Concentrations of apixaban and rivaroxaban in plasma were accessed by an anti‐factor Xa chromogenic assay. PT, APTT, antithrombin, D‐dimer, dRVVT screen/confirm, FDP, and fibrinogen levels were measured. On‐therapy ranges were calculated by substituting previously reported trough plasma concentrations of DOACs.ResultsAnti‐factor Xa chromogenic assay‐based DOACs levels were 26.0‐279.5 (115.9 ± 56.5) ng/mL for apixaban at 2.5 mg BID, 19.9‐565.1 (205.3 ± 162.4) ng/mL for apixaban at 5 mg BID, 2.3‐395.3 (205.3 ± 162.4) ng/mL for rivaroxaban at 15 mg OD, 3.6‐494.8 (119.6 ± 95.1) ng/mL for rivaroxaban at 20 mg OD, and 9.6‐431.4 (140.8 ± 113.6) ng/mL for rivaroxaban at 15 mg BID. PT (%), antithrombin, and dRVVT confirm tests showed good correlation with plasma apixaban levels. Plasma rivaroxaban concentrations were correlated well with PT (sec), PT (%),and dRVVT confirm results. On‐therapy ranges established for dRVVT confirm test by linear regression were as follows: 1.32‐1.52 for apixaban 2.5 mg BID, 1.12‐1.75 for apixaban 5 mg BID, 1.11‐1.78 for rivaroxaban 15 mg OD, 1.09‐1.64 for rivaroxaban 20 mg OD, and 1.22‐1.81 for rivaroxaban 20 mg BID.ConclusionsApixaban concentrations were well correlated with PT (%), antithrombin, and dRVVT confirm test. Rivaroxaban concentrations showed good correlation with PT (sec), PT (%), and dRVVT confirm test.

Determination of edoxaban equivalent concentrations in human plasma by an automated anti-factor Xa chromogenic assay

IntroductionThis phase I, open-label, multiple-dose, two-treatment study assessed the relationship between edoxaban equivalent concentration derived from an anti-FXa assay with the summed concentration of edoxaban and its active metabolite, M-4, as assessed by liquid chromatography coupled with tandem mass spectrometry (LC/MS/MS). This study also assessed the relationship between edoxaban plasma concentrations assessed by LC/MS/MS in sodium citrate and lithium heparin tubes. Materials and methodsHealthy volunteers were randomized to receive once-daily edoxaban 60mg or 90mg for 5days (15 participants per treatment group). Serial blood samples were collected for analysis by LC/MS/MS and by the anti-FXa assay. Edoxaban equivalent levels were assessed using a commercially available anti-FXa activity assay with an edoxaban-specific setup. Results and conclusionsThe day 5 concentration estimates were significantly correlated between the 2 assays (P<0.0001 for both edoxaban doses). The geometric least squares mean (GLSM) ratio (90% confidence interval) for edoxaban equivalent concentrations vs edoxaban + M-4 concentrations was 114.3% (108.2–120.8) for edoxaban 60mg (P<0.0001) and 113.0% (107.1–119.2) for edoxaban 90mg (P=0.0002). The GLSM ratio for edoxaban concentrations in sodium citrate vs lithium heparin tubes for 60-mg and 90-mg edoxaban doses were 82.8% (78.5–87.3) and 83.9% (79.1–89.0), respectively. In this study, an anti-FXa chromogenic assay with edoxaban-specific calibrators and controls demonstrated good accuracy in estimating edoxaban concentrations across a wide range of concentrations relative to LC/MS/MS at steady state following the administration of once-daily edoxaban for 5days.

A fluorescent probe assay (Heparin Red) for direct detection of heparins in human plasma.

Heparins are widely used anticoagulant drugs. The current monitoring practice for heparin in plasma, such as the chromogenic anti-factor Xa assay, relies on heparin-triggered activation of antithrombin, an inhibitor of coagulation proteases. Such assays are not applicable to the detection of non-anticoagulant heparins, an emerging class of drug candidates for therapeutic applications unrelated to anticlotting activity. This study describes the application of a commercially available fluorescent probe assay (Heparin Red) for the direct and sensitive detection of the "chemical" heparin in plasma, independent of any anticoagulant activity. The quantification range is about 0-5μg/mL for both unfractionated heparin (corresponding to 0-1IU/mL) and the low molecular weight heparin enoxaparin. The Heparin Red assay is of particular value for the quantification of non-anticoagulant heparins, as exemplified by the low molecular weight heparin derivative tafoxiparin and a N-desulfated-N-reacetylated heparin. Heparin octa- and decasaccharides are also detected. Graphical abstract Heparin quantification in plasma by mixing the sample with the Heparin Red reagent and fluorescence readout.

Laboratory monitoring of rivaroxaban and assessment of its bleeding risk

Background: The aims of this study were to investigate the effects of rivaroxaban on routine coagulation assays using our local, widely available, reagents and to study the relationship between sensitive coagulation assays and bleeding risk caused by rivaroxaban.Methods: Prothrombin time (PT), activated partial thromboplastin time (APTT) and anti-factor Xa (FXa) chromogenic assays (Biophen DiXaI) and inhibition of FXa activity were performed in normal pooled plasma (NPP) spiked with rivaroxaban and plasma samples from patients treated with rivaroxaban.Results: In vitro, the linear correlation coefficient of measured concentrations of rivaroxaban, by Biophen DiXaI, and spiked concentrations of rivaroxaban was 0.99. PT and APTT showed good linear correlation with rivaroxaban concentrations, while other assays showed poor correlation. In vivo, PT showed a moderate linear correlation with rivaroxaban concentrations while APTT had a weak correlation with rivaroxaban concentrations. In vitro and in vivo, the rivaroxaban concentrations, measured by Biophen DiXaI, always showed good correlation with the inhibition of FXa activity, and PT values showed moderate correlation with the inhibition of FXa activity.Conclusions: Biophen DiXaI can be considered as a quantitative method to monitor the anticoagulation activity of rivaroxaban, and could be used to evaluate bleeding risk caused by rivaroxaban. The PT reagent (Thrombosis S) could be considered as a rough method to monitor the anticoagulation activity of rivaroxaban and evaluate bleeding risk caused by rivaroxaban.

Laboratory Monitoring of Low-Molecular-Weight Heparin and Fondaparinux

Compared with older agents, low-molecular-weight heparins (LMWH) and fondaparinux offer improved bioavailability and more predictable, dose-independent clearance. While routine monitoring of coagulation parameters is not usually necessary with these agents, certain populations (including pregnant patients, children, obese patients, and patients with renal impairment) may benefit from the monitoring of anti-factor Xa activity, thromboelastography, or other coagulation assays to help guide therapy. The chromogenic anti-factor Xa assay is currently the gold standard for monitoring LMWH and fondaparinux therapy. Thromboelastography has been used to monitor LMWH therapy in special situations but is not needed for routine use.

Comparison of Low-Molecular-Weight Heparins Prepared From Bovine Lung Heparin and Porcine Intestine Heparin

Currently porcine intestine is the only approved source for producing pharmaceutical heparin in most countries. Enoxaparin, prepared by benzylation and alkaline depolymerization from porcine intestine heparin, is prevalent in the anticoagulant drug market. It is predicted that porcine intestine heparin–derived enoxaparin (PIE) will encounter shortage, and expanding its production from heparins obtained from other animal tissues may, therefore, be inevitable. Bovine lung heparin is a potential alternative source for producing enoxaparin. Critical processing parameters for producing bovine lung heparin–derived enoxaparin (BLE) are discussed. Three batches of BLEs were prepared and their detailed structures were compared with PIEs using modern analytical techniques, including disaccharide composition, intact chain mapping by liquid chromatography-mass spectrometry and 2-dimensional nuclear magnetic resonance spectroscopy. The results suggested that the differences between PIEs and BLEs mainly result from N-acetylation differences derived from the parent heparins. In addition, bioactivities of BLEs were about 70% of PIEs based on anti-factor IIa and Xa chromogenic assays. We conclude that BLE has the potential to be developed as an analogue of PIE, although some challenges still remain.

The Influence of Assay Selection on Prothrombin Time Measured in Patients Treated With Rivaroxaban for Nonvalvular Atrial Fibrillation.

Prothrombin time (PT) can provide a qualitative assessment of the relative intensity of anticoagulation by rivaroxaban. More than ten types of assay are available for the measurement of PT in clinical settings, but it is not yet fully understood whether their interactions with rivaroxaban are uniform or inconsistent. We examined 139 blood samples from patients taking rivaroxaban. We measured PT using five different commercially available assays. We also evaluated the estimated rivaroxaban concentration using a chromogenic anti-factor Xa assay. The median estimated concentration of rivaroxaban was 192 ng/ml (interquartile range 85-284 ng/ml). The correlation coefficient (r) between PT and the estimated concentrations of rivaroxaban was as follows: Thromborel S, r = 0.768; Thrombocheck PT, r = 0.861; Coagpia PT-N, r = 0.909; Neoplastin Plus, r = 0.882; and Triniclot PT Excel S, r = 0.870. The gradients of the regression plots differed more than fourfold, and the standard deviation of the regression line ranged from 1.001 to 2.980, which tended to be higher for the assays with the higher regression slope gradients. The estimated concentration of rivaroxaban varied greatly depending on the assay, so the PT measured in patients taking rivaroxaban should be interpreted with caution.

Evaluation of a Heparin-Calibrated Antifactor Xa Assay for Measuring the Anticoagulant Effect of Oral Direct Xa Inhibitors

The introduction of oral direct anti-Xa anticoagulants apixaban and rivaroxaban has significantly impacted the treatment and prevention of thromboembolic disease. Clinical scenarios exist in which a quantitative assessment for degree of anticoagulation due to these agents would aid management. The purpose of this work was to evaluate the chromogenic antifactor Xa assay calibrated with heparin standards at our institution for assessment of intensity of anticoagulation with rivaroxaban or apixaban in addition to its current use for unfractionated heparin or low-molecular-weight heparin. We also aimed to propose expected steady state peak and trough antifactor Xa activities for these agents based upon dosing regimens approved for nonvalvular atrial fibrillation. Antifactor Xa activity correlated very strongly with apixaban and rivaroxaban concentration in both spiked samples and treated patient plasma samples (r (2) = .99, P < .001). This correlation was observed over a broad range (20-500 ng/mL) of drug concentrations, as sample dilution with pooled normal plasma significantly extended the range of quantitative assessment. Based on drug concentrations previously published in pharmacokinetic studies, the expected steady state peak and trough antifactor Xa activity ranges for apixaban are 1.80 to 2.20 IU/mL and 0.70 to 1.10 IU/mL, respectively. For rivaroxaban, these ranges are 3.80 to 6.20 IU/mL and 0.60 to 1.00 IU/mL, respectively. In conclusion, our findings demonstrate that heparin-calibrated antifactor Xa activity correlates strongly with apixaban and rivaroxaban concentration. The dilution of samples allowed for this correlation to be extended over the majority of on-therapy drug concentrations.

Oral Factor Xa Inhibitors and Clinical Laboratory Monitoring

Oral anticoagulation therapy is currently undergoing great changes with the development and use of several new medications. The newer drugs have a more specific mechanism of action for inhibiting coagulation and do not require continuous monitoring like their predecessor. Their efficacy and safety has been proven, but problems arise in the clinical laboratory with developing accurate procedures to measure the therapy when needed. Methods being used are the prothrombin time and anti-factor Xa chromogenic assay which require calibration with the drug being measured. The prothrombinase-induced clotting test shows promise as a measure for Xa inhibitors. The article reviews the oral Factor Xa inhibitor9s clinical usefulness, effects on current laboratory coagulation tests and methods for measuring their anticoagulant activity. <b>ABBREVIATIONS:</b>CYP – cytochrome; DOAC – direct oral anticoagulant; DVT – deep venous thrombosis; INR – international normalized ratio; PE – pulmonary embolism; PiCT – prothrombinase-induced clotting time; PT – prothrombin time; APTT – activated partial thromboplastin time.

Is anti-factor Xa chromogenic assay for Rivaroxaban appropriate in clinical practice? Advantages and comparative drawbacks

IntroductionNo routine monitoring is required with factor Xa inhibitor rivaroxaban. Yet, its titration must be adapted, and its misuse may lead to increased risk of bleeding; therefore, therapeutic rivaroxaban monitoring might help in specific situations. Material and methodsAs asked by clinicians of our medical center, we measured rivaroxaban plasma concentrations in real conditions of use and checked their corresponding prescriptions. Measurement of 112 samples from 94 consecutive patients was performed with a Biophen LRT® anti-Xa chromogenic assay and compared blindly to the HPLC-MSMS “gold standard” method.Rivaroxaban was effectively given to 80 out of 94 patients but a mere 57% through an adequate prescription (within the scope of indications/titration). All chromogenic measurements were over the pre-specified 30ng/ml LOQ, whereas only 98 /114 samples had quantifiable rivaroxaban with HPLC-MSMS (LOQ 1ng/ml). Correlation between the two methods and linear regression were highly significant (p<0.0001). However, chromogenic values (mean 141.6ng/ml[96.6]) overestimated HPLC-MSMS values (119.7ng/ml[79.5]) by 22ng/ml according to Bland-Altman analysis (p<0.001). After re-assessing the chromogenic LOQ at 52ng/ml, 83 quantifiable samples had a mean concentration of 176.9ng/ml as compared to 158.5ng/ml with HPLC-MSMS, with no false positive anymore. ConclusionsIn our medical center, rivaroxaban concentrations could be assessed by a rapid chromogenic method. Its pre-specified LOQ proved too high after being checked “on site” against HPLC-MSMS. Prescriptions for rivaroxaban were not optimal. An overestimated LOQ may impair observance monitoring or predispose patients to either risky thrombolysis or otherwise adjournable surgery in clinical practice.

Using New Oral Anticoagulants in Patients Undergoing Major Orthopedic Surgery

Four new oral anticoagulants (NOAC), apixaban, rivaroxaban, edoxaban, and dabigatran, are now available in the USA; however, only apixaban and rivaroxaban are FDA approved for the prevention of venous thromboembolism following orthopedic surgery. Apixaban, rivaroxaban, and edoxaban's anticoagulant activity can be measured using a chromogenic anti-factor Xa assay but there is no widely available means of measuring dabigatran blood levels. None of the NOAC has an antidote. Dabigatran is 80% renally excreted, and patients with atrial fibrillation taking dabigatran for stroke prevention should stop the drug 4-5 days prior to major orthopedic surgery. Apixaban, rivaroxaban, and edoxaban should be held for 48 h preoperatively in this setting.

Anticoagulation intensity of rivaroxaban for stroke patients at a special low dosage in Japan.

ObjectivesIn Japan, low-dose rivaroxaban [15 mg QD/10 mg QD for creatinine clearance of 30–49 mL/min] was approved for clinical use in NVAF patients partly because of its unique pharmacokinetics in Japanese subjects. The aim of the study was to determine the anticoagulation intensity of rivaroxaban and its determinant factors in Japanese stroke patients.MethodsConsecutive stroke patients with NVAF admitted between July 2012 and December 2013 were studied. Prothrombin time (PT), activated partial thromboplastin time (aPTT), and estimated plasma concentration of rivaroxaban (Criv) based on an anti-factor Xa chromogenic assay were measured just before and 4 and 9 h after administration at the steady state level of rivaroxaban. Determinant factors for Criv were explored using a linear mixed-model approach.ResultsOf 110 patients (37 women, 75±9 years old), 59 took 15 mg QD of rivaroxaban and 51 took 10 mg QD. Criv at 4 h was 186 ng/mL for patients taking 15 mg QD and 147 ng/mL for those taking 10 mg QD. Both PT and aPTT were positively correlated with Criv. Criv was 72% lower at 4 h in 15 patients receiving crushed tablets than in the other patients, and tablet crushing was significantly associated with lower Criv (adjusted estimate −0.43, 95% CI −0.60 to −0.26) after multivariate-adjustment.ConclusionThe anticoagulation effects of rivaroxaban in the acute stroke setting for Japanese NVAF patients were relatively low as compared with those in the ROCKET-AF and J-ROCKET AF trials. Tablet crushing, common in dysphagic patients, decreased Criv.

Rivaroxaban and Hemostasis in Emergency Care

Rivaroxaban is an oral, direct Factor Xa inhibitor, approved for the prevention and treatment of several thromboembolic disorders. Rivaroxaban does not require routine coagulation monitoring and has a short half-life. However, confirmation of rivaroxaban levels may be required in circumstances such as life-threatening bleeding or perioperative management. Here, we explore the management strategies in patients receiving rivaroxaban who have a bleeding emergency or require emergency surgery. Rivaroxaban plasma concentrations can be assessed quantitatively using anti-Factor Xa chromogenic assays, or qualitatively using prothrombin time assays (using rivaroxaban-sensitive reagents). In patients receiving long-term rivaroxaban therapy who require elective surgery, discontinuation of rivaroxaban 20–30 hours beforehand is normally sufficient to minimize bleeding risk. For emergency surgery, we advise against prophylactic use of hemostatic blood products, even with high rivaroxaban concentrations. Temporary rivaroxaban discontinuation is recommended if minor bleeding occurs; for severe bleeding, rivaroxaban withdrawal may be necessary, along with compression or appropriate surgical treatment. Supportive measures such as blood product administration might be beneficial. Life-threatening bleeding demands comprehensive hemostasis management, including potential use of agents such as prothrombin complex concentrate. Patients taking rivaroxaban who require emergency care for bleeding or surgery can be managed using established protocols and individualized assessment.

Progressive chromogenic anti-factor Xa assay and its use in the classification of antithrombin deficiencies.

Antithrombin (AT) is a slow-acting progressive inhibitor of activated clotting factors, particularly thrombin and activated factor X (FXa). However, the presence of heparin or heparan sulfate accelerates its effect by several magnitudes. AT deficiency, a severe thrombophilia, is classified as type I (quantitative) and type II (qualitative) deficiency. In the latter case mutations may influence the reactive site, the heparin binding-site (HBS) and exert pleiotropic effect. Heterozygous type II-HBS deficiency is a less severe thrombophilia than other heterozygous subtypes. However, as opposed to other subtypes, it also exists in homozygous form which represents a very high risk of venous thromboembolism. A modified anti-FXa chromogenic AT assay was developed which determines both the progressive (p) and the heparin cofactor (hc) activities, in parallel. The method was evaluated and reference intervals were established. The usefulness of the assay in detecting type II-HBS AT deficiency was tested on 78 AT deficient patients including 51 type II-HBS heterozygotes and 18 homozygotes. Both p-anti-FXa and hc-anti-FXa assays showed excellent reproducibility and were not influenced by high concentrations of triglyceride, bilirubin and hemoglobin. Reference intervals for p-anti-FXa and hc-anti-FXa AT activities were 84%-117% and 81%-117%, respectively. Type II-HBS deficient patients demonstrated low (heterozygotes) or very low (homozygotes) hc-anti-FXa activity with normal or slightly decreased p-anti-FXa activity. The p/hc ratio clearly distinguished wild type controls, type II-HBS heterozygotes and homozygotes. Concomitant determination of p-anti-FXa and hc-anti-FXa activities provides a reliable, clinically important diagnosis of type II-HBS AT deficiency and distinguishes between homozygotes and heterozygotes.

Clinical Judgment When Using Coagulation Tests during Direct Oral Anticoagulant Treatment: A Concise Review

Because their anticoagulant effect is dose predictable, steady, and little influenced by diet and drugs, laboratory monitoring was deemed unnecessary in trials on venous and arterial thromboprophylaxis with the direct oral anticoagulant drugs (DOACs) rivaroxaban, apixaban, edoxaban, or dabigatran. However, there are special clinical settings in which measurement of their anticoagulant effect may be clinically desired. Because of lack of standardization between reagents employed and their global nature, the activated partial thromboplastin time (APTT) and the prothrombin time (PT) are not generally suitable for accurately assessing the anticoagulant effect of DOACs. The modified (diluted) PT reliably quantifies the anticoagulant effect of rivaroxaban or of apixaban. However, a higher intraindividual variability from one PT reagent to another is found when the data are compared with those obtained with standard PT. The HEMOCLOT (HYPHEN BioMed, France) assay is a modified (diluted) thrombin time (TT) provided with dabigatran calibrators. However, it is performed only in few specialized centers. Anti-factor Xa (anti-FXa) chromogenic assays employ routine automated coagulometers or manual spectrometers, and kits for anti-FIIa assays that measure dabigatran levels and kits for anti-Xa with rivaroxaban calibrators are commercially available. However, no correlation has been identified between any of these tests and clinical outcomes in patients taking any of the DOACs. Thus, currently, there are evidence gaps regarding the role of laboratory testing for surveillance and management of adverse events associated with DOACs. In view of this, in addition to standardization, validation of such assays is mandatory before they can be used to make clinical decisions.

Rivaroxaban and other novel oral anticoagulants: pharmacokinetics in healthy subjects, specific patient populations and relevance of coagulation monitoring

Unlike traditional anticoagulants, the more recently developed agents rivaroxaban, dabigatran and apixaban target specific factors in the coagulation cascade to attenuate thrombosis. Rivaroxaban and apixaban directly inhibit Factor Xa, whereas dabigatran directly inhibits thrombin. All three drugs exhibit predictable pharmacokinetic and pharmacodynamic characteristics that allow for fixed oral doses in a variety of settings. The population pharmacokinetics of rivaroxaban, and also dabigatran, have been evaluated in a series of models using patient data from phase II and III clinical studies. These models point towards a consistent pharmacokinetic and pharmacodynamic profile, even when extreme demographic factors are taken into account, meaning that doses rarely need to be adjusted. The exception is in certain patients with renal impairment, for whom pharmacokinetic modelling provided the rationale for reduced doses as part of some regimens. Although not routinely required, the ability to measure plasma concentrations of these agents could be advantageous in emergency situations, such as overdose. Specific pharmacokinetic and pharmacodynamic characteristics must be taken into account when selecting an appropriate assay for monitoring. The anti-Factor Xa chromogenic assays now available are likely to provide the most appropriate means of determining plasma concentrations of rivaroxaban and apixaban, and specific assays for dabigatran are in development.