To the Editor.—
In a letter to the editor, Roger L. Bick, MD (239:296, 1978), commented on the clinical determination of antithrombin III, pointing out the usefulness of chromogenic substrate and that an increased predisposition to thromboembolic phenomena is associated with a decreased antithrombin III level. In reply, Harry L. Messmore, MD, and Jawed Fareed, PhD (240:345, 1978), questioned the use of autolytic forms of thrombin and the chromogenic substrate in the antithrombin III assay.The methods employed for antithrombin III assay can be roughly classified into two categories: immunological and functional. There are two types of functional assays available: the rate determination procedure and the end-point analysis. The rate of the inhibition of thrombin by antithrombin III depends on (1) the amount of antithrombin III present, (2) the amount of heparin or heparin-like substance present in the sample, and (3) the quantity of thrombin employed. It was first pointed