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Point-Of-Care Testing for Anticoagulation Monitoring In Point-of-Care Testing for Anticoagulation PATIENT SAFETY Monitoring in Neuroendovascular Procedures H.M. Hussein SUMMARY: POC testing is defined as diagnostic testing at or near the site of patient care. Rapid A.L. Georgiadis measurement of the intensity of anticoagulation and, more recently, platelet inhibition allows dose titration of adjuvant medications such a heparin and antiplatelet agents during neuroendovascular A.I. Qureshi procedures. However, knowledge among practicing physicians regarding the pathophysiologic basis of these measurements and variations in knowledge about the differences among devices is often limited. This review discusses the role of anticoagulation in endovascular procedures and the currently available POC tests for anticoagulation monitoring. ABBREVIATIONS: ACT ϭ activated clotting time; Anti-Xa ϭ quantitative chromogenic heparin assay; aPTT ϭ activated partial thromboplastin time; AT ϭ antithrombin; CI ϭ confidence interval; GP IIb/IIIa ϭ glycoprotein IIb/IIIa; Hemonox-CT ϭ Hemonox clotting time; HIT ϭ heparin-induced thrombocytopenia; HMT ϭ Heparin Management Test; IV ϭ intravenous; LMWH ϭ low-molecular- weight heparin; MI ϭ myocardial infarction; OR ϭ odds ratio; PCI ϭ percutaneous coronary intervention; POC ϭ point of care; UFH ϭ unfractionated heparin hromboembolism ensues from activation of platelets and Brief Overview of Anticoagulant Medications Tthe coagulation cascade and is a major source of compli- Anticoagulants reduce the activity of proteases in the coagula- cations during endovascular procedures. Multiple studies tion cascade. On the basis of their mechanism of action (Fig 1), have demonstrated accelerated platelet activation during cor- anticoagulants can be divided into 4 major groups: vitamin K onary and cerebral angioplasty.1,2 The coagulation cascade antagonists (warfarin), heparins (UFH and LMWH), direct (Fig 1) consists of a sequential conversion of a series of proen- thrombin inhibitors, and direct factor Xa inhibitors. This re- zymes, or inactive precursor proteins (zymogens) to active view will focus on the anticoagulants relevant to endovascular enzymes, resulting in the formation of thrombus. It can be procedures: UFH, LMWH, and parenteral direct thrombin PATIENT SAFETY divided into 3 pathways: the extrinsic pathway (tissue factor inhibitors. and factor VIIa), which is the primary activator of the cascade; the intrinsic pathway (factors XIIa, XIa, IXa, and VIIIa), which amplifies the cascade; and the common pathway (factor Xa, UFH factor Va, and thrombin), which generates thrombin and UFH activates AT, which neutralizes multiple enzymes, espe- fibrin. cially factors IIa (thrombin) and Xa. Individual patients re- Vessel injury and the introduction of foreign bodies set off spond differently to the same dose of heparin, largely due to the coagulation cascade.3 Catheters and wires used in endo- differences in their levels of acute-phase-reactant proteins, vascular procedures are thrombogenic, independent of inti- which can bind and neutralize heparin. Other factors that can mal injury.3 Traversing atherosclerotic lesions with guidewires interfere with heparin efficacy include AT deficiency, in- and catheters can dislodge plaque or thrombus. Inflation of creased heparin clearance, and elevation in factor VIII and/or balloons can produce intimal cracks, flaps, or dissections, all of fibrinogen.6 Monitoring of the anticoagulant effects of UFH which act as thrombogenic surfaces.3 In vivo studies have by means of the aPTT or ACT is, therefore, essential.7 demonstrated that platelets rapidly accumulate on the stent surface after placement, especially if the underlying artery is LMWH injured.4 The role of fibrin and platelets in stent-related LMWHs are produced by depolymerization of heparin.6 thrombosis has been confirmed in pathologic studies.5 Anticoagulants and antiplatelet agents are, therefore, es- LMWHs exert their anticoagulant properties by activating AT. sential for reducing the rate of thromboembolic complica- However, they have a much smaller effect on factor IIa and are Ϫ 7 tions. On the other hand, their use may increase the risk of thus relatively factor Xa specific. LMWHs cause less platelet bleeding complications. In this review, we will focus on activation and exhibit less binding to cells and plasma proteins 8 anticoagulants and available POC devices that measure compared with UFH. Therefore, LMWHs have more predict- anticoagulation. able pharmacokinetic and pharmacodynamic properties and a lower risk of side effects.6 Recently, the safety and efficacy of IV LMWH anticoagulation for patients undergoing PCI has been From Zeenat Qureshi Stroke Research Center, University of Minnesota, Minneapolis, demonstrated.9 LMWH activity can be measured by chromo- Minnesota. genic anti-Xa. More recently, POC tests such as Hemonox Please address correspondence to Haitham M. Hussein MD, Department of Neurology, Zeenat Qureshi Stroke Research Center, University of Minnesota, 420 Delaware St. SE, clotting time (measured by the Hemochron Jr. Hemonox de- MMC 295, Minneapolis, MN 55455; e-mail: [email protected] vice; International Technidyne, Edison, New Jersey)10 and the Enox test (measured by the Cascade device; Helena Laborato- Indicates open access to non-subscribers at www.ajnr.org ries, Beaumont, Texas)11 have been developed to monitor the http://dx.doi.org/10.3174/ajnr.A2621 anticoagulant effect of LMWH. Currently available LMWHs AJNR Am J Neuroradiol 33:1211–20 ͉ August 2012 ͉ www.ajnr.org 1211 Fig 1. Simplified diagram depicting the coagulation cascade and targets of action of different anticoagulants. The coagulation cascade consists of the intrinsic, extrinsic, and common pathways. Note the positive feedback effect of thrombin on further activation of the intrinsic and common pathways. All anticoagulants target factors of the intrinsic and common pathway except warfarin, which decreases the rate of hepatic synthesis of factor VII. in the United States include enoxaparin, tinzaparin, and Methods of Measurement of Anticoagulation Intensity dalteparin. Anti-Xa assay Fondaparinux The anti-Xa measures heparin concentration indirectly by us- Fondaparinux is a synthetic pentasaccharide structurally re- ing either the thrombin or factor Xa enzyme as the assay re- 22,23 lated to the AT-binding site of heparin. It potentiates the anti- agents. For LMWHs, factor Xa assay is used. No POC test factor Xa activity of AT.7 Fondaparinux is not currently used is available. in endovascular procedures because it may be associated with aPTT a slightly increased risk of coronary guide-catheter thrombus The aPTT test is used to monitor anticoagulation with hepa- formation.9 rin. It measures the time in seconds from the activation of factor XII to the formation of fibrin clot, thus assessing the Direct Thrombin Inhibitors integrity of the intrinsic and common pathways of coagula- Hirudin was the first direct thrombin inhibitor to be used tion. However, at heparin concentrations of Ͼ1 U/mL, aPTT clinically. It directly and irreversibly binds with thrombin. is infinitely prolonged, and it can, therefore, not be used to Lepirudin is a recombinant form of hirudin.12 Lepirudin has 2 monitor anticoagulation during cardiac surgery or endovas- important side effects: 1) the formation of antibodies against cular procedures. hirudin, which occurs in up to 40% of patients and can pro- long the plasma half-life of lepirudin, resulting in drug accu- ACT mulation; and 2) anaphylaxis, which can occur if patients with The ACT is a whole-blood clotting time test (Table 1).24 It is antibodies are re-exposed to the drug. In the United States, routinely used to monitor anticoagulation during cardiopul- lepirudin is licensed for the treatment of thrombosis compli- monary bypass surgery and endovascular procedures. Clot- cating HIT. ting is initiated in a blood sample by an activator of the intrin- Bivalirudin is a synthetic hirudin analog with a half-life of sic pathway (most commonly Celite [Celite Corporation, 25 minutes. The binding of bivalirudin to the active site of Santa Barbara, California], glass particles, or kaolin), and the 13,14 thrombin is transient. Several trials have suggested its su- time until clot formation is measured in seconds.25 ACT val- periority over UFH (alone or in combination with GP IIb/IIIa ues are device-specific. All available devices have acceptable inhibitors) during PCI in reducing the risk of bleeding com- reproducibility, but reference and therapeutic ACT ranges 13,15-19 plications. Bivalirudin is licensed as an alternative to vary considerably.21,26,27 Therefore, every ACT test should be heparin in patients with HIT who require PCI.6 regarded as unique. Argatroban is a competitive thrombin inhibitor. Its plasma ACT is determined predominantly by the anti-IIa activity half-life is 45 minutes.20 Argatroban is licensed for treatment of the anticoagulant. Thus, ACT is higher with heparin or and prevention of HIT-associated thrombosis and for antico- bivalirudin than with LMWH for the same intensity of antico- agulation during PCI when heparin is contraindicated because agulation.28 However, ACT can be used to monitor LMWH of a recent history of HIT.6 Activity can be monitored by ACT anticoagulation with appropriate adjustments.29,30 or aPTT.21 In a study comparing aPTT with ACT, a total of 175 paired 1212 Hussein ͉ AJNR 33 ͉ August 2012 ͉ www.ajnr.org Table 1: Overview of some important facts on anticoagulation and There is no criterion standard with which ACT can be
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