The ex vivo correlate of the antithrombotic action of heparin

Heparins: A Shift of Paradigm

Heparins inhibit the thrombin forming capacity of plasma, i. e., the endogenous thrombin potential (ETP), by their anti-thrombin (aIIa) activity, the anti-factor Xa (aXa) activity is of minimal importance. This holds for both unfractionated heparin (UFH) and low molecular weight heparin (LMWH) at aXa/aIIa ratios < 25. Clinical experience and epidemiological evidence show a direct relationship between the ETP and the risk of thrombosis and bleeding. Consequently, the therapeutic potency of a heparin is determined by its aIIa activity, i.e., the concentration of a domain in which 12 sugar flank the high affinity antithrombin-binding pentasaccharide (HA5) at one side. The response of individual plasmas to a fixed dose of any heparin is highly variable. This suggests that individualization of heparin dosage, on basis of the ETP, might reduce bleeding or re-thrombosis. There exist simple laboratory methods for both the ETP and the concentration of the active domain. These methods can be used both for unequivocally characterization of a heparin preparation and for controlling heparin therapy and allow arbitrary units relative to a standard to be abandoned. These tests are as robust as any hematological routine test but not yet routinely available, which severely encumbers progress in the field.

Elements from in vitro studies that help understand the action of heparins

In determining heparin one has the choice to test a specific activity, such as the decay constant of thrombin or factor Xa on a global test such as the aPTT. The best test would be a global test that directly reflects the only important global effect, to wit antithrombotic efficiency. Such a test does not yet exist. We propose that the thrombin potential, i.e. the time concentration integral of thrombin activity appearing in plasma after triggering is a plausible candidate for such a test.