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Background: Patients with hereditary antithrombin deficiency (HAD) have an increased risk of venous thromboembolism (VTE). The American Thrombosis and Hemostasis Network (ATHN) 12: HAD Pilot Project established a registry to collect data on patients with HAD., Objectives: To inform current practice and serve as a platform to design a multicenter global registry for patients with HAD., Methods: The HAD registry was designed in 2020 to identify 100 patients with HAD receiving care at ATHN-affiliated centers. Demographics, type of HAD, thrombotic events, risk factors, anticoagulants, and antithrombin (AT) concentrate administration were recorded., Results: Ninety-four patients were included; 65% were females; 51% had type 1 HAD. Mean age at diagnosis was 26 years (SD, 18 years); 61% had VTE: 55% deep vein thrombosis and 27% pulmonary embolisms. Eight patients had arterial thrombosis. Recurrent thrombosis occurred in 58.6% of patients (44.8%) despite anticoagulation. The main risk factor for thrombosis in females was estrogen. Direct oral anticoagulants were prescribed in 30%, heparin in 34%, and warfarin in 32%. There were 139 pregnancies. Low-molecular-weight heparin was administered in 33% and AT concentrate in 19% and 11% prior to and after delivery, respectively. Twelve patients developed thrombosis in pregnancy. Seventy-nine patients underwent 239 surgeries or procedures, mainly gastrointestinal and vascular. Overall, 35% of participants received AT concentrate without adverse events., Conclusion: In ATHN 12, VTE was the predominant manifestation, frequently recurrent. There was a trend toward using direct oral anticoagulants. Low-molecular-weight heparin was administered in one-third of pregnancies and AT concentrate in one-fifth without adverse events. These data should encourage prospective studies to optimize the management of these patients., Competing Interests: Declaration of competing interests M.T.D.: participation on Pharmacosmos Advisory Board and member of American Society of Hematology Panel on Practice Guidelines for Anticoagulation in COVID-19. E.S.: no conflict of interest. J.C.R.: has received honorarium from F. Hoffman-La Roche AG, Genentech, Novo Nordisk, Octapharma, Pfizer, Sanofi, and Takeda. M.D.T.: received consulting fees from Pfizer, Sanofi, Sobi-Swedish Orphan, Biovitrum, Takeda Amgen, Octapharma Inc, Genentech, BioMarin, UCB Biosciences Dova Pharmaceuticals, and Novo Nordisk and payment/honoraria for lectures, presentations, speakers’ bureaus, manuscript writing, or educational event from Sobi-Swedish Orphan Biovitrum, Genentech, Amgen, Novartis, BioMarin, and Sanofi. J.S.: no conflict of interest. J.C.: no conflict of interest. M.R.: consulting for Catalyst Biosciences, CSL Behring, Genentech, Hema Biologics, Kedrion, Novo Nordisk, Pfizer, Sanofi, Takeda, and uniQure and leadership or fiduciary role in Foundation for Women and Girls with Blood Disorders; Partners in Bleeding Disorders, Thrombosis and Hemostasis Societies of North America., (Copyright © 2024 International Society on Thrombosis and Haemostasis. Published by Elsevier Inc. All rights reserved.)

Background: Although P5 (preventive, personalized, predictive, participatory, psychocognitive) medicine and patient-focused healthcare are gaining ground in various healthcare areas, the diagnosis of antithrombin deficiency (ATD) is still based on crude diagnostic tests, clustering patients into clinically heterogeneous subgroups whereby relevant thrombophilia phenotypes may go unnoticed. Clinical pathways and the majority of evidence are based on these tests; therefore, generic treatment is still the norm., Objectives: To unravel the heterogeneity of ATD, a mass spectrometry (liquid chromatography coupled to multiple-reaction-monitoring mass spectrometry [LC-MRM-MS])-based test for antithrombin was developed allowing molecular characterization of the antithrombin proteoforms in patient plasma. This study provides the first insight into the tests' clinical performance., Methods: Plasma from 91 unrelated ATD patients and 41 patients with a congenital disorder of glycosylation affecting antithrombin glycosylation were characterized functionally, genetically, and analyzed by LC-MRM-MS. An established data analysis strategy was applied for quantitation and molecular characterization of antithrombin proteoforms., Results: The test recognized patients with a quantitative defect, discriminated between type I and type II ATD, and identified variant proteoforms. Overall, the diagnostic sensitivity for ATD was 100% for LC-MRM-MS compared with 81.1% by the functional test. Type II ATD, a subtype prone to misdiagnosis, revealed an even larger difference of 100% identification by LC-MRM-MS vs 56.8% by functional test., Conclusion: The qualitative and quantitative mass spectrometry-based AT-test can serve as a platform for investigating the molecular basis of the clinical heterogeneity of ATD. This "precision diagnostics" approach for ATD can lower diagnostic uncertainty and modernize the ATD diagnostic and clinical pathways., Competing Interests: Declaration of competing interests There are no competing interests to disclose., (Copyright © 2024 The Author(s). Published by Elsevier Inc. All rights reserved.)