DOAC plasma levels measured by chromogenic anti‐Xa assays and HPLC‐UV in apixaban‐ and rivaroxaban‐treated patients from the START‐Register

Introduction To measure direct factor Xa inhibitor (apixaban, edoxaban, rivaroxaban) concentrations, dedicated chromogenic anti‐Xa assays are recommended as suitable methods to provide rapid drug quantification. Moreover, the high‐performance liquid chromatography with ultraviolet detection (HPLC‐UV) is reported as a reliable quantitative technique. We investigated seven anti‐Xa assays and an HPLC‐UV method for measurement of apixaban and rivaroxaban levels in patients enrolled in the START‐Register. Methods A total of 127 apixaban and 124 rivaroxaban samples were tested by HPLC‐UV and the following anti‐Xa assays: Biophen DiXaI and Heparin LRT (Hyphen BioMed), Berichrom and Innovance Heparin (Siemens), STA‐Liquid Anti‐Xa (Stago Diagnostics), Technochrom anti‐Xa (Technoclone), and HemosIL Liquid Anti‐Xa (Werfen). Each method was performed in one of the participating laboratories: Bologna, Cremona, Florence, and Padua. Results Our data confirmed the overestimation of apixaban and rivaroxaban levels by the antithrombin‐supplemented anti‐Xa method (Berichrom). Performances and reproducibility of the six anti‐Xa assays not supplemented with antithrombin and the HPLC‐UV method were good, with limits of quantification from 8‐39 ng/mL (apixaban) and 15‐33 ng/mL (rivaroxaban). The six chromogenic methods showed good concordances with the quantitative HPLC‐UV [bias: −26.9‐22.3 ng/mL (apixaban), −11.3‐18.7 ng/mL (rivaroxaban)]. Higher bias and wider range between limits of agreement were observed at higher concentrations [200 ng/mL: bias −42.2‐36.8 ng/mL (apixaban) and −20.1‐68.9 ng/mL (rivaroxaban)]. Conclusion Overall, the anti‐Xa assays not supplemented with antithrombin and the HPLC‐UV method proved to be suitable for apixaban and rivaroxaban quantification.