Human pharmacokinetics of low molecular weight heparins.

Pharmacokinetic parameters of low molecular weight heparins (LMWHs) are commonly determined by quantifying the biological activity. Such pharmacodynamic determinations should be extrapolated with care to provide pharmacokinetic information. Direct comparisons should only be drawn between LMWHs given at the same dose, determined by the same test. This is especially important since different tests estimate potency, bioavailability, and duration differently. There is a strong correlation between the ratio of anti-Xa:anti-IIa activities determined in vitro and mean residence time in vivo. Each LMWH has a characteristic pharmacokinetic and pharmacodynamic profile and cannot be considered bioequivalent either at doses recommended for prophylaxis or at anti-Xa adjusted doses. These conclusions emphasize that LMWHs are not interchangeable drugs and recommendations for drug dosing should be made based on pharmacological data available for each LMWH.