Rapid point-of-care detection and classification of direct-acting oral anticoagulants with the TEG 6s: Implications for trauma and acute care surgery.

BACKGROUND: The trauma patient on direct oral anticoagulant (DOAC) therapy preinjury presents a challenge in trauma and acute care surgery. Our understanding of these patients is extrapolated from vitamin K antagonists. However, DOACs have different mechanisms of action, effects on laboratory coagulation assays, and reversal strategies. Rapid identification of DOACs in the blood will allow timely reversal of factor Xa inhibitors and direct thrombin inhibitors when necessary. The present study evaluated viscoelastic testing to detect and classify DOACs in patient blood samples. METHODS: This observational, prospective, open-label, multicenter study used point-of-care viscoelastic testing to analyze blood samples taken from patients with and without DOAC treatment, and healthy volunteers. Antifactor Xa and direct thrombin inhibition (DTI) assays were used to establish reference ranges for viscoelastic testing parameters on the TEG 6s system. These ranges were applied to produce a DOAC identification algorithm for patient blood samples. Internal consistency of the measurements, as well as algorithm sensitivity and specificity, was evaluated. RESULTS: Using the TEG 6s system, the R parameter reference range was 0.6 minutes to 1.5 minutes for the Antifactor Xa assay and 1.6 minutes to 2.5 minutes for the DTI assay. Our identification algorithm using these ranges for 2.5 minutes or less has sensitives of 98.3% and 100% for factor Xa inhibitor and direct thrombin inhibitor detection, respectively. Specificity was 100%. Both classes of DOAC were detectable, even when samples were collected during the "trough" between doses of medication. CONCLUSION: Point-of-care viscoelastic testing with TEG 6s can detect and classify DOACs with high sensitivity and specificity. This tool can be used to better determine the need for reversal in trauma and acute care surgery patients and guide optimal surgical timing in the acute setting. LEVEL OF EVIDENCE: Prognostic and epidemiological study, level II.

Determination of non-Vitamin K oral anticoagulant (NOAC) effects using a new-generation thrombelastography TEG 6s system.

Non vitamin K oral anticoagulants (NOACs) do not require regular monitoring but information about their pharmacodynamic effect may be importantin situations like trauma, stroke oremergent surgery. Currently, no standardized point-of-care test is available to evaluate the anticoagulant effects of NOACs. We evaluated the anticoagulant effect of NOACs with the next generation point-of-care TEG assay (TEG® 6S) based on a fully-automated thrombelastography system. We used two TEG® 6S assays, the DTI assay and Anti-Factor Xa (AFXa) assay, to detect anticoagulant effects and classify NOACs. Blood from healthy volunteers (n = 26) was used to obtain a baseline reference range. Data derived from patients on factor Xa inhibitors (FXi) (rivaroxaban and apixaban) (n = 39), and direct thrombin inhibitors (DTIs) (dabigatran) (n = 25) were compared against the reference range for detection of drug effect and drug classification. TEG®6s R-time highly correlated to each NOAC. Presence of NOACs caused elongation of R-time on the AFXa assay compared to the reference range (4.3 ± 1.7 vs. 1.3 ± 0.3 min. for FXi, p < 0.001 and 3.5 ± 1.2 vs. 1.3 ± 0.3 min. for DTI, p < 0.001). R-time on the DTI assay was elongated only in presence of a DTI (3.4 ± 1.0 vs. 1.5 ± 0.2 min, p < 0.001). The cutoff for detection of a DTI effect was an R time of 1.9 min and for anti-Xa effect was 1.95 min. For detection of NOAC therapy, there was ≥92% sensitivity and ≥95% specificity. The automated TEG®6s NOAC assay may be an effective tool to identify an anticoagulant effect from NOAC therapy and facilitate care of patients with bleeding or at risk of bleeding in the event of needing emergency surgery.

A computational analysis of an in vitro vessel wall injury model.

Implantation of vascular grafts or stents causes significant injury to the vessel wall. Activated coagulation factors, such as FXa are generated at the injury site. The size of the injury and the flow conditions influence the transport of these activated factors. A simulation model has been developed to evaluate surface bound coagulation inhibitors on medical devices. Tissue factor pathway inhibitor (TFPI) isa potent inhibitor of FXa in vivo. This physiologically relevant in vitro model studies the mechanism by which immobilized rTFPI effectively inhibits TF initiated thrombosis.Computational fluid dynamics was used to develop the model and validated by experiments performed in a parallel plate flow chamber. The lower plate was divided into two zones. The first zone represents fibroblasts that catalyze FXto FXa by TF-FVIIa. The second represents passively absorbed surface bound rTFPI. The efficacy of rTFPI in inhibiting FXa from the injury site was studied under both venous and moderate arterial shear rates. This model extends a previous model to include a more physiologic model of vessel wall injury in which FXa generation by TF:VIIa occurs at the wall upstream of the TFPI coated surface. The previous model estimated a uniform inlet FXa concentration of 20 nM (20% conversion of the approximate physiologic concentration of 100 nM) to the parallel plate chamber.

Evaluating surface bound rTFPI through an in vitro model of vessel wall injury.

INTRODUCTION: Injury to the surrounding vessel wall is one of the major reasons for failure of implantable medical devices. The surgical procedure itself or the altered flow conditions after implantation can cause damage to the vessel wall. This damage exposes tissue factor (TF), the initiator of the extrinsic pathway of coagulation. One approach to combat thrombosis is to use an anticoagulant on the surface of the device. The primary aim of this study is to develop a simplified physiologically relevant in vitro model of vessel wall injury to study the mechanisms by which immobilized recombinant tissue factor pathway inhibitor (rTFPI) effectively inhibits TF initiated thrombosis. MATERIALS AND METHODS: A two well chamber slide was used for the study. Fibroblasts were cultured on the upstream portion of the slide. Fibroblast cells stimulated with TNF-α acted as a source of surface TF. The downstream portion of the slide was coated with rTFPI. A mixture of FX, FVIIa and calcium was perfused over the slides to generate FXa. Effluent collected at the outlet was used to analyze the inhibition of this surface generated FXa by the rTFPI present downstream. RESULTS AND CONCLUSIONS: Different shear rates and rTFPI densities were used to study this effect. In most cases rTFPI inhibited FXa generated upstream as a function of the wall shear rate and rTFPI dosage (surface density). This study shows the effectiveness of the surface bound inhibitor when FXa is generated from an upstream injury site and the bulk of FXa is near the wall.

Factor Xa inhibition by immobilized recombinant tissue factor pathway inhibitor.

The recombinant form of the endogenous anticoagulant, tissue factor pathway inhibitor (rTFPI), is a potent inhibitor of Factor Xa (FXa) and the tissue factor-factor VIIa (TF:VIIa) complex. Surface-immobilized rTFPI reduces the thrombogenicity and intimal hyperplasia associated with synthetic vascular grafts in animal models and specifically reduces fibrin deposition on collagen-impregnated Dacron grafts from native blood in an in vitro flow model. The FXa inhibitory capacity of rTFPI in the bulk phase has been demonstrated in static systems and immobilized rTFPI reduces fibrin deposition in whole blood in vitro and animal studies; however, the specific mode of this anticoagulation has not been studied. Therefore, a comparison was made between the FXa binding capacity of two forms of immobilized rTFPI, i.e., passively adsorbed and covalently bound. The rTFPI-coated surfaces were evaluated using a parallel-plate flow reactor and comparing the amount of FXa exiting the flow chamber after exposure to an rTFPI-coated versus an uncoated plate. The results demonstrate that adsorbed rTFPI exhibits increased binding capacity (1.5-3.6 times) the expected stoichiometry via interactions with the C-terminus, whereas covalently-bound rTFPI interacts with FXa in a 1:1 stoichiometry. Thus, the results imply that specific FXa inhibition is a key component of the anticoagulant effect of rTFPI-coated surfaces and that passive adsorption of rTFPI to glass surfaces produces a more effective coating than covalent binding of rTFPI.