Analysis of Anticoagulation Activities of Heparins - Is usp Heparin Assay Reliable?

The in vitro “over-all” activity of heparins was analyzed by the technique of rigidity measurements with plasmas upon recalcification. This technique allows the measurement of the entire clotting process of a heparinized plasma, and is thus more sensitive and reliable than clotting time determination. Contrary to its action to the clotting of whole blood, heparin reduces the final rigidity of a plasma clot only when the degree of heparinization exceeds certain limit. A log-log plot of clotting half-time vs. amount of heparin may be used to compare the anticoagulation activity of various heparins in human plasma and in sheep plasma. Results show that heparin with molecular weight around 20,000 possesses highest specific activity, the activity drops sharply when molecular weight increases or decreases. The anticoagulation activity of heparin in human plasma, expressed by the increase in clotting half-time is up to 100 times more effective than that in sheep plasma but responds less sensitively to the change in heparin concentration. Using the same heparin standard, the specific activity of certain heparin fractions assayed in human plasma differs from that assayed in sheep plasma. The discrepancy increases with the decrease in heparin molecular weight. The discrepancy was also observed with some heparins of different tissues and sources. The USP heparin assay, which uses sheep plasma as the assay medium, therefore does not necessarily reflect the true activity in human blood clotting system.