Heparin has long been used as an antithrombotic to treat and prevent thromboembolic events, as well as for systemic anti-coagulation during cardiopulmonary bypass and dialysis. Heparin continues to have distinct advantages when intense anti-coagulation is needed, but newer low molecular weight derivatives of heparin (LMWH) have advantages in the management of thromboembolism. One advantage of LMWH is that they often do not require laboratory monitoring; when monitoring is indicated it can be done with an anti-factor Xa activity assay. Monitoring is recommended for heparin because the patient response to a given dose is highly variable. The aPTT and anti-Xa assays are used for monitoring. However, the therapeutic ranges have not been rigorously validated, and current monitoring is not highly predictive of patient outcome.