Summary
We describe a new method for determining the biological activity of heparin in plasma
with use of thrombin and the substrate Tos-Gly-Pro-Arg-pNA. The procedure is based
on the photometric determination of the inactivation of thrombin after incubation
with plasma in the presence of endogenous antithrombin III (At III). The method allows
the specific determination of heparin concentrations from 0.02 USP to 0.8 USP/ml of
plasma in the presence of normal At III levels. It has been carried out manually by
use of an Eppendorf spectrum line photometer or automatically by use of a Vitatron
Akes analyzer. For evaluation, the results were compared with two standard samples
which contained heparin in the low and high therapeutic range, respectively.
