Development of a novel, rapid assay for detection of heparin-binding defect antithrombin deficiencies: the...

Qualitative and quantitative antithrombin deficiency predisposes to thrombosis, although patients with heparin-binding dysfunction have a lower incidence than other sub-types. Assays discriminating between qualitative sub-types are not widely available.

Extended heparin incubation in antithrombin activity assays can overestimate levels in patients with HBDs. Plasmas from genetically proven HBD patients were assayed for antithrombin activity by factor Xa-inhibition and thrombin-inhibition at varying incubation times. Optimal pairings were assessed for generating a quantifiable discrepancy in HBDs by deriving a ratio between results from short and prolonged heparin-incubation assays respectively, the Heparin-antithrombin binding (HAB) ratio. Fourteen patients with hereditary antithrombin deficiency, including five with HBDs, were analysed.

The FXa-inhibition assay with 30s and 300s incubations clearly identified a heterozygous p.Pro73Leu and homozygous p.Leu131Phe, giving HAB ratios of 0.24 and 0.67 respectively (reference range 0.90 - 1.01). However, three plasmas containing mutations with markedly reduced or absent heparin affinity (p.Lys146Glu, p.Gln150Pro, p.Arg79Cys) gave normal results. Nine antithrombin deficient plasmas were tested with the thrombin-inhibition assay and all generated reduced HAB ratios whilst two normal donors did not. The three available HBD plasmas generated lower values than non-HBD plasmas. The mildly reduced HAB ratios in non-HBD deficiencies may have been due to heparin cofactor II reacting with bovine thrombin during extended incubation.

HAB ratio from FXa-inhibition assays distinguishes some but not all HBD from non-HBD antithrombins, and thrombin-inhibition assays may be diagnostically applicable with sub-type specific cut-offs.