['The results of studies among patients with antithrombin deficiency have suggested that the use of warfarin will increase the level of antithrombin.', 'To reevaluate the effect of warfarin on antithrombin levels using an automated amidolytic method in current use.', 'Antithrombin levels were measured in patients who were receiving warfarin for atrial fibrillation and were compared with antithrombin levels in preoperative patients who had not received warfarin.', 'Patients receiving warfarin had a mean antithrombin level of 100.40% (range, 81%-153%). Patients not receiving warfarin had a mean antithrombin level of 99.97% (range, 79%-120%). The Student t test was not significant for a difference between the mean antithrombin levels of the 2 populations.'] Are antithrombin levels unaffected by warfarin use?