Anticoagulant and antithrombotic effects of heparin and low molecular weight heparin fragments in rabbits

Heparin and heparin fragments of different molecular weight and with different anti-factor X_a/APTT activity ratios were studied with respect to their ability to inhibit thrombus formation in an animal model. It is concluded that: a) Neither anti-factor X_a nor the APTT activity alone is a good reflector of the antithrombotic activity. b) Anti-factor X_a active fragments must have a minimum molecular weight in order to elicit good antithrombotic activity. c) High affinity for antithrombin III is important for good antithrombotic activity. d) A heparin fragment of molecular weight 4 000 has the same antithrombotic activity as heparin but less effect on the clotting time.