A Method of Antithrombin Estimation using Plasma Defibrinated with Ancrod

S ummary . A method for the measurement of progressive antithrombin activity is described. Higher levels of antithrombin activity are found in plasma defibrinated with ancrod than in plasma defibrinated by heating to 56°C or by thrombin. As ancrod itself has no antithrombin activity, it is recommended as the best agent for defibrinating plasma prior to antithrombin estimation. The relationship between plasma and serum antithrombin concentration was studied in patients during oral contraceptive therapy. There was a fall in both plasma and serum antithrombin during this therapy but the fall in serum antithrombin was significantly greater than that seen in plasma; this reflects the fact that the serum antithrombin concentration is dependent on the degree of thrombin generation which has taken place during clotting.