LMWHs can be administered once daily or bid by subcutaneous injection, without coagulation monitoring. Based on their greater convenience, LMWHs have replaced UFH for many clinical indications. Fonda- parinux, a synthetic pentasaccharide, catalyzes the inhibition of factor Xa, but not thrombin, in an antithrombin-dependent fashion. Fondaparinux binds only to antithrombin. Therefore, fonda- parinux-associated HIT or osteoporosis is unlikely to occur. Fondaparinux exhibits complete bio- availability when administered subcutaneously, has a longer half-life than LMWHs, and is given once daily by subcutaneous injection in xed doses, without coagulation monitoring. Three addi- tional parenteral direct thrombin inhibitors and danaparoid are approved as alternatives to hep- arin in patients with HIT. CHEST 2012; 141(2)(Suppl):e24S e43S Abbreviations: aPTT 5 activated partial thromboplastin time; AT 5 antithrombin; CrCl 5 creatinine clearance; HCII 5 heparin cofactor II; HIT 5 heparin-induced thrombocytopenia; INR 5 international normalized ratio; Parenteral Anticoagulants Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines David A. Garcia, MD; Trevor P. Baglin, MBChB, PhD; Jeffrey I. Weitz, MD, FCCP; and Meyer Michel Samama, MD 2012 American College of Chest Physicians by Kimberly Henricks on February 10, 2012 chestjournal.chestpubs.org Downloaded from www.chestpubs.org CHEST / 141 / 2 / FEBRUARY, 2012 SUPPLEMENT e25S At even higher concentrations, low-af nity heparin impairs factor Xa generation through AT- and HCII- independent mechanisms13 (Table 2). The heparin/AT complex inactivates thrombin (factor IIa) and factors Xa, IXa, XIa, and XIIa.5 Hep- arin catalyzes AT-mediated thrombin inhibition in a nonspeci c charge-dependent fashion to form a ter- nary heparin/AT/thrombin complex. In contrast, to catalyze factor Xa inhibition by AT, heparin needs only to bind to AT.14 In both cases binding occurs at the unique pentasaccharide sequence found within some heparin molecules. 