Bridging laboratory assays, genetics, and clinical phenotypes in antithrombin deficiency: Rethinking the diagnostic...

Diagnosis of antithrombin deficiency (ATD), the most severe inherited thrombophilia, commonly relies on functional assays despite their uncertain sensitivity. Moreover, routine characterization of ATD remains uncommon due to limited supporting clinical data.

This study aimed to evaluate the diagnostic sensitivity of commercial antithrombin (AT) activity assays and assess clinical differences among ATD types to refine the current diagnostic approach.

Eighty-eight patients with decreased AT activity and 124 consecutive patients with unprovoked venous thromboembolism were included. AT activity was measured using six assays. Genetic analysis of SERPINC1 was performed by Sanger sequencing and multiplex ligation-dependent probe amplification; additionally, long-read whole-genome sequencing was conducted.

Fifteen SERPINC1 variants were detected, including two novel ones. AT Padua I (p.Arg79His) was the most prevalent (49%). Sensitivity varied across AT activity assays, particularly for types IIRS and IIHBS, with variant-specific discrepancies. AT Dublin (p.Val30Glu), causing transient deficiency, was undetected by all assays. Two assays demonstrated very high sensitivity (93%; p < 0.001), while two others showed poor sensitivity (46%). Regardless of measured AT activity, characterization of ATD type enabled better risk stratification. Type I was associated with early-onset and recurrent venous thromboembolism, and type IIHBS was distinctly linked to arterial thrombosis.

Assay sensitivity varies considerably, and only a few proved suitable as first-line tests. Genetic testing for common variants undetectable by even the most sensitive assays, along with ATD characterization, should be integrated into diagnostic algorithms. Our results also suggest that young patients with arterial thrombosis should be screened for ATD.