Effect of oral anticoagulant treatment on plasma and serum antithrombin III: a study on 172 patients at different levels of anticoagulation.

Antithrombin III (AT III) functional levels are much lower in serum than in plasma; during oral anticoagulation this difference is reduced. Plasma and serum of 172 patients taking vitamin K antagonists were tested for AT III antigen and both AT III heparin cofactor and anti-Xa heparin cofactor. Crossed immunoelectrophoresis of AT III on heparin-agarose was also carried out in plasma and serum. The patients were divided into four groups: (1) international normalized ratio (INR) 9.3-4.1, n = 25; (2) INR 4.0-2.5, n = 73; (3) INR 2.4-2.0, n = 40, and (4) INR 1.9-1.5, n = 34. 66 healthy subjects were used as controls. Plasma levels of AT III antigen, AT III heparin cofactor, and anti-Xa heparin cofactor were the same in all groups. In all groups all serum AT III parameters were higher than in controls; crossed immunoelectrophoresis of AT III on heparin-agarose indicated that this finding was due to a lower formation of complexed AT III in serum. AT III heparin cofactor serum values were the same whatever the INR over a large range (9.3-1.5); the highest anti-Xa heparin cofactor serum levels were noted in the groups treated more intensely (groups 1 and 2).