Heparin must be administered subcutaneously or intravenously, because the drug does not readily cross membranes. The LMWHs are administered subcutaneously. Heparin is often initiated as an intravenous bolus to achieve immediate anticoagulation. This is followed by lower doses or continuous infusion of heparin, titrating the dose so that the activated partial thromboplastin time (aPTT) is 1.5- to 2.5-fold that of the normal control.   The half-life of heparin is approximately 1.5 hours, whereas the half-life of the LMWHs is longer than that of heparin, ranging from 3 to 12 hours. Key Concept:  The aPTT is the standard test used to monitor the extent of anticoagulation with heparin.