We read with great interest the study by Mansour et al., which provided a longitudinal examination of plasma antithrombin levels during extracorporeal membrane oxygenation (ECMO) and investigated their correlation with heparin responsiveness.1  The study offers important clinical insights for intensivists managing ECMO; however, it merits further discussion on certain clinical and technical points.

The authors’ decision to utilize Liquid anti-Xa and Stachrom ATIII assays for quantifying antithrombin and anti-Xa levels was justified, given the established anti-Xa and anti-IIa activities of heparin. Nonetheless, interpreting these results must be done with caution. Specifically, the Liquid anti-Xa assay employed does not incorporate dextran sulfate, which is typically included in commercial kits to reduce nonspecific binding of heparin to plasma proteins such as histidine-rich glycoproteins and platelet factor 4. The absence of dextran sulfate leads to biases of as much as 0.24 units/ml between anti-Xa assays.2  An underestimation of heparin’s activity...

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