Low-molecular-weight heparins (LMWHs), like unfractionated heparin (UFH), exert their action primarily by accelerating the interaction between antithrombin (AT) and thrombin. At the levels of aXa activity that are attained in human pharmacology, it does not cause significant (>15%) inhibition of the clotting system. The essential differences between LMWHs and UFH are: (a) LMWHs attain higher plasma concentrations after subcutaneous injection (high bioavailability), and (b) in contrast to LMWHs, UFH contains very large heparin molecules with a putative hemorrhagic action. The reputedly higher aXa activity of LMWH can be shown to be largely due to the absence of Ca2+ using the current laboratory methods to estimate this activity. Via this artifact the apparently high aXa activity of LMWHs is correlated but not related to their favorable pharmacokinetic properties. Consequently dosage guidelines for the use of different LMWHs cannot be based upon their aXa activity. Until better laboratory methods are available, clinical results are the only reliable guideline to heparin dosage.

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