At the end of the 70s heparin derivatives were produced by depolymerization, that were as heterogeneous as the original heparin, but had lesser bandwidth of molecular size. With a molecular weight between 4000 to 9000 daltons they were in a range where they predominantly act against factor Xa and thus, were very effective in prophylaxis of venous thromboembolism. The portion with the higher molecular weight (>= 13000 daltons), which acts against thrombin (factor IIa) and which is imperative for treatment of thrombotic diseases, was missing. In unfractionated heparin (UFH), only about 20% has a molecular size of < 10,000 daltons and only 2% has a size between 4000 and 6000 daltons. Besides there specifity the newer fractioned heparins, or low molecular weight heparins (LMWH), also have the advantage of a longer and more predictable plasma half-life (3 to 4 hours). This is due to their reduced affinity to endothelial cells or plasma proteins, which is mainly a function of the length of the heparin molecule. LMWHs are primarily cleared by the kidneys, and their biological half-life is prolonged in patients with renal failure. The LMWHs differ to some extent in their pharmacocinetic and anticoagulant properties due to the different kinds of chemical or enzymatic depolymerization.
Laboratory monitoring of LMWH-therapy is usually not necessary. In certain clinical situations, such as renal failure, a variety of laboratory assays, that measure factor Xa activity are available and thus, allow for determination of the anticoagulant effect. The chromogenic anti-Xa assay is the widest available and is currently recommended by the College of American Pathologists.
