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.
