The Second International Standard for unfractionated heparin sodium was released in 1958
The average molecular weight for heparin is 15,000 daltons but there is a range in the size of the mucopolysaccharide chains.
1 USP is defined as the concentration of heparin that will inhibit 1.0 mL of citrated sheep plasms from clotting for 1 hour after the addition of 0.2 mL of calcium chloride solution in a 1:100 dilution.
A USP unit is not the same as an international unit (IU) since different methods are used to determine activity.
1 mg of the Second International Standard = 130 IU
1 IU = 0.00769 mg
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