Role of sponsors: The sponsors played no role in the development of these guidelines. Sponsoring organizations cannot recommend panelists or topics, nor are they allowed prepublication access to the manuscripts and recommendations. Guideline panel members, including the chair, and members of the Health & Science Policy Committee are blinded to the funding sources. Further details on the Conflict of Interest Policy are available online at http://chestnet.org. Endorsements: This guideline is endorsed by the American Association for Clinical Chemistry, the American College of Clin- ical Pharmacy, the American Society of Health-System Pharma- cists, the American Society of Hematology, and the International Society of Thrombosis and Hematosis. References 1. McLean J. The thromboplastic action of cephalin. Am J Physiol. 1916;41:250-257. 2. Brinkhous K, Smith H, Warner E, et al. The inhibition of blood clotting: an unidentified substance which acts in conjunction with heparin to prevent the conversion of pro- thrombin into thrombin. Am J Physiol. 1939;125:683-687. 3. Abildgaard U. Highly purified antithrombin 3 with heparin cofactor activity prepared by disc electrophoresis. Scand J Clin Lab Invest. 1968;21(1):89-91. 4. Lindahl U, B ckstr m G, H k M, Thunberg L, Fransson LA, Linker A. Structure of the antithrombin-binding site in hep- arin. Proc Natl Acad Sci U S A. 1979;76(7):3198-3202. 5. Merlini PA, Bauer KA, Oltrona L, et al. Persistent activation of coagulation mechanism in unstable angina and myocar- dial infarction. Circulation. 1994;90(1):61-68. 6. Rosenberg RD, Lam L. Correlation between structure and function of heparin. Proc Natl Acad Sci U S A. 1979;76(3): 1218-1222. 7. Hirsh J, Bauer KA, Donati MB, et al. Parenteral anticoagu- lants: American College of Chest Physicians evidence-based clinical practice guidelines (8th edition). Chest. 2008;133(6 suppl):141S-159S. 8. Andersson LO, Barrowcliffe TW, Holmer E, Johnson EA, Sims GE. 