Abstract

The antithrombin III (AT III) concentration in plasma was measured in 63 patients on oral anticoagulant treatment (mean age 57.7 years), 26 healthy laboratory controls (mean age 28 years), and 21 patients attending the hypertensive clinic who had never been on oral anticoagulants (mean age 50 years). Three methods were used to measure AT III: a coagulation assay, a chromogenic substrate assay, and an immunological assay. In patients on oral anticoagulants, the mean values for AT III in the three assays were: 124%, 107%, and 96% respectively. The mean AT III concentration in laboratory staff was 103.4%, 94%, and 104.1% for the three assays; patients attending the hypertensive clinic had AT III concentrations indistinguishable from those in patients on oral anticoagulants: 117.9%, 110.5%, and 93.9%. The difference between both patient groups and laboratory staff was statistically highly significant, but no difference was demonstrated between patients on anticoagulant treatment and those not receiving it. Our results show that the increase in the functional AT III concentration (measured by coagulation and chromogenic assays) observed in patients on oral anticoagulants is probably due to the effects of age and underlying disease rather than to the anticoagulant treatment itself.