Antithrombin III: Comparison of Functional and Immunologic Assays

A comparison of functional and immunologic assays of antithrombin III was carried out in several groups of patients. The control group consisted of 120 male and female patients on medical and obstetrics–gynecology wards, ages 14 to 70 years. They were not receiving estrogen therapy or anticoagulant drugs and had no known disturbance of coagulation or thrombotic episode. The experimental groups consisted of 42 female patients, ages 14 to 32 years, who were receiving oral contraceptive therapy; ten postpartum patients; one patient who had disseminated intravascular coagulation; and seven patients with thromboembolic diseases. Additional coagulation studies were performed on all patients throughout the study. With the exception of those for women taking contraceptives, results of the functional and immunologic assays were generally consistent. The contraceptive group showed a difference between the two assays. The functional assay was low in approximately 55% of the cases, whereas the corresponding immunologic test disclosed normal concentrations of antithrombin III in all but 12% of those tested. The discrepancy between the two tests found in individuals taking contraceptive medication implies the presence of a substance that interferes with the functional assay but not the immunologic assay. Thus, even though the amount of protein, as detected immunologically, appears to be normal in patients taking contraceptives, there is, nonetheless, evidence of dysfunction in some of the patients, possibly related to blocking factors.