The anticoagulant activities of different heparin preparations have been compared using two clotting (APTT and CaTT) and two amidolytic assays. The heparin samples analyzed included two heparin standards, heparin isolated from human mastocytoma tissue, four commercial heparin preparations and two heparin preparations ("high affinity heparin" and "low affinity heparin") separated by affinity chromatography on immobilized antithrombin III. In each assay system, specific activities were expressed relative to the specific activity of the 3rd International standard (assigned to 100). The specific activities obtained ranged from 3 to 198. In all assay systems "high affinity heparin" had the highest and "low affinity heparin" the lowest specific activity. For each individual heparin preparation the variation in specific activity recorded with the various assay methods was relatively slight. However, the specific activities of the commercial heparins determined in the present study differed considerably from those reported by the manufacturers using pharmacopoeial methods. Exchanging purified antithrombin III with plasma in the amidolytic assay systems caused relatively small changes in specific activities, but suggested that plasma antiheparin activity was more effective in the heparin-antithrombin III-thrombin than heparin-antithrombin III-Xa reaction. In view of the reproducibility and simplicity of amidolytic methods, it is suggested that they are adapted for heparin standardization.
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