Hypersensitivity reactions, including anaphylaxis. Risk of transmitting infectious agents. Regularly perform coagulation tests (eg, aPTT & anti-factor Xa activity). Monitor patients for the occurrence of bleeding or thrombosis. Measure functional levels of antithrombin in plasma by amidolytic assays using chromogenic substrates or by clotting assays. Pregnancy & lactation. Childn. Elderly.