The phenotypic and genetic assessment of antithrombin deficiency.

INTRODUCTION: Antithrombin (AT) deficiency is associated with an increased risk of deep vein thrombosis and pulmonary embolism which are major causes of morbidity and death. The incidence of deficiency in healthy populations has been reported to vary from 1/600 to 1/5000, with the variation being due to the different populations studied and detection methods used. When reduced activity levels are identified it is important to measure the AT antigen levels to differentiate type I from type II disorders, as type II defects have varying thrombotic risk.

METHODS: Functional AT assays detect the ability of AT to inactivate thrombin or factor Xa, and AT antigen assays detect the quantity of AT in plasma. In functional assays, reducing the incubation time of sample with enzyme/heparin reagent may increase sensitivity to type II defects. An excess of antigen over activity level suggests the presence of functionally defective AT, which can be characterized further by assaying AT in the absence of heparin, electrophoresis to investigate the ability of heparin to bind to AT, and gene sequencing.

RESULTS: Many patients with AT deficiency have a type II defect and these defects may not be detected by all routine diagnostic assays. Assays using human thrombin may lack specificity and assays that use factor Xa may fail to detect the common variant, AT Cambridge II, which can be detected by assays using bovine thrombin, especially if activity is compared to antigen by ratio. Factor Xa based assays may be particularly sensitive to certain heparin binding defects, and sensitivity of assays to both heparin binding and reactive site defects can be improved by shortening the incubation time with enzyme.

CONCLUSION: uAT activity assays are essential for the detection of AT deficiency because type II defects are relatively common in patients with heritable deficiency. No one functional assay can be assumed to detect all forms of AT deficiency, and assays can sometimes be improved by reducing reaction time of AT with thrombin or factor Xa.