Summary
We have compared the non-specific binding of unfractionated heparin (UFH) with that
of low molecular weight heparin (LMWFl) to plasma proteins both ex vivo and in vitro.
Non specific binding to plas ma proteins was assessed by comparing the heparin levels
measured as anti-factor Xa activity before and after the addition of low affinity
heparin, which is essentially devoid of anti-factor Xa activity, in order to displace
heparin bound to plasma proteins. For the ex-vivo studies, we compared the recovery
of UFH and a LMWH (ardeparin) from the plasma of patients participating in a randomized
trial of post operative venous thrombosis prophylaxis. For the in-vitro studies, we
compared the recovery of UFH and 4 different LMWHs when added to the plasma from healthy
volunteers and from patients with suspected venous thromboembolic disease. The results
indicate that the recovery of LMWH is much less affected by nonspecific binding to
plasma proteins both ex-vivo and in-vitro. In addition, there are differences between
the LMWHs with respect to their plasma protein-binding.