For Laboratory Use Only For the determination of Heparin in plasma. Antithrombin in conjunction with heparine like substances is a potent inhibitor of thrombin, factor Xa, factor Xla and factor lXa. Antithrombin is prepared from human plasma and purified by affinity chromatography with an Heparine Sepharose gel according to the protocol of Miller-Andersson et al. (Thromb. Res. 5, 439-452, 1974).