Introduction: Heparins exert their anticoagulant effect through activation of antithrombin. Whether antithrombin deficiency leads to clinically relevantly reduced anti-Xa activity of heparins is unknown. We investigated the relation between antithrombin deficiency and anti-Xa activity measurements of plasma samples spiked with unfractionated heparin (UFH) or low-molecular-weight heparin (LMWH).
Materials and methods: Plasma samples from 34 antithrombin-deficient subjects and 17 family controls were spiked with UFH and LMWH (nadroparin) aimed to correspond with an anti-Xa activity of 0.8 IU/mL. Antithrombin, β-antithrombin and anti-Xa activities were measured.
Results: Mean anti-Xa activity with LWMH was 0.55 IU/mL (0.30–0.74) (recovery 69%, 38–93%) in antithrombin-deficient subjects and 0.82 (0.71–0.89) IU/mL in controls (recovery 103%, 89–111%). Expected anti-Xa measurements after LMWH spiking were found in 17/17 non-deficient subjects and in 8/34 antithrombin-deficient subjects. Anti-Xa measurements in the expected range (0.6–1.0 IU/mL) after UFH spiking were found in17/17 non-deficient subjects and in 1/22 antithrombin-deficient subjects. Antithrombin activity correlated with anti-Xa activity of UFH (R= 0.77) and LMWH (R= 0.66). Mixing studies of pooled normal plasma and antithrombin-deficient plasma showed that anti-Xa recovery was linearly reduced with antithrombin activity de-creasing below 100%.
Conclusions: Reduced antithrombin activity causes significantly reduced anti-Xa levels. Standard LWMH- or UFH-doses are likely to lead to under treatment in antithrombin-deficient individuals.

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Journal Thrombosis Research: vascular obstruction, hemorrhage and hemostasis
Croles, F.N, Lukens, M.V, Mulder, R., de Maat, M.P.M, Mulder, A.B, & Meijer, K. (2019). Monitoring of heparins in antithrombin-deficient patients. Thrombosis Research: vascular obstruction, hemorrhage and hemostasis, 175, 8–12. doi:10.1016/j.thromres.2019.01.007