To the Editor.— The MEDICAL NEWS article "Automated Test Measures Functional Antithrombin, Coagulation Enzymes" (report of Jayed Fareed, PhD, and Harry L. Mesmore, MD) confirms my observations 1 and those of others 2,3 that there is poor correlation between immunological (total) and functional antithrombin III levels in disease states. Fareed and Mesmore point out that at Loyola University Stritch School of Medicine antithrombin III assay by chromogenic substrates (still an investigational procedure) is being used to identify three groups of patients: hereditary thrombophiliacs, patients with hypercoagulable states, and patients who may be resistant to heparin therapy. Clearly, a fourth area where this test may be indicated is in women ingesting oral contraceptives; 13 of 14 international studies have provided data showing that women with an increased predisposition to thromboembolic phenomena while ingesting oral contraceptives have decreased antithrombin III levels after ingesting the agents. 4 One caution is that chromogenic substrates