Unfractionated versus low-molecular-weight heparin in the treatment of venous thromboembolism.

Low-molecular-weight heparin (LMWH) fractions are prepared from standard unfractionated heparin (UFH) and are thus similar to UFH in many aspects. The main advantages of this new class of antithrombotic agents as compared with UFH are: (1) an improved bioavailability and a prolonged half-life, which alleviate cumbersome laboratory monitoring and may permit one single daily subcutaneous injection; and (2) an improved efficacy-to-safety ratio, with less bleeding despite similar or improved efficacy. For these reasons, LMWH is progressively replacing UFH for preventing postoperative thromboembolism and for treating established deep vein thrombosis and pulmonary embolism. However, the effects of the new compounds need to be evaluated carefully in some other indications (arterial thrombosis, unstable angina, or myocardial infarction-the latter also in conjunction with thrombolytic treatment) before they can generally replace UFH in pharmacotherapy.