ACTICHROME® AT-III, activity a

INTENDED USE
Actichrome AT III is intended for the quantitative determination of antithrombin III in human plasma by chromogenic assay.

PRINCIPLE
Antithrombin III is an inhibitor of plasma serine proteases. An important function of antithrombin III is the inhibition of thrombin activity. Normally the rate of thrombin inhibition by antithrombin III is slow (progressive antithrombin activity). However, the rate of inhibition can be enhanced several thousandfold in the presence of heparin (heparin cofactor activity).

In the present two-stage method (Odegard, et al.), thrombin is added to a plasma dilution containing antithrombin III in the presence of excess heparin. After an initial incubation (stage 1) residual thrombin is determined with a thrombin-specific chromogenic substrate (stage 2). The residual thrombin activity is inversely proportional to the antithrombin III concentration.

Tolefsen and Blank have reported another rapid heparin-dependent thrombin inhibitor, Heparin Cofactor II, in human plasma. This protein can interfere with antithrombin III determinations especially at high (2 USP units/mL) heparin concentrations. In order to confer specificity to antithrombin III the present assay system uses a lower (1.0 USP units/mL) final ~heparin concentration where heparin-enhanced inactivation of thrombin by heparin cofactor II is negligible. In addition, human heparin cofactor II reacts more readily with human thrombin than with bovine thrombin (Friberger et al.).
Thus, further specificity for antithrombin III is imparted in the present assay system by the u se of bovine thrombin.