Multicenter Evaluation of Different Anti-Xa Assays and Diluted Russell’s Viper Venom Time in Ex Vivo Plasma Samples from Patients Treated with Rivaroxaban or Apixaban

<b>Background/Objectives</b>: Different anti-Xa assays are routinely used to evaluate the plasma concentrations of direct factor X inhibitors (DXIs) rivaroxaban and apixaban, despite a lack of data on assay equivalence. Information on assay performance is particularly important at clinical decision cut-offs, such as 50 ng/mL or 30 ng/mL. The aim of this study was to evaluate the equivalence of different anti-Xa assays with the reference LC-MS/MS method for measuring rivaroxaban and apixaban concentrations in a multicenter study. In addition, the usefulness of the dRVVT as a simple coagulation test for emergency situations was evaluated. <b>Methods</b>: We included 122 patients with atrial fibrillation. Trough and peak blood samples were collected from 60 patients treated with rivaroxaban and 62 patients treated with apixaban. Rivaroxaban and apixaban plasma levels were measured by LC-MS/MS. Different anti-Xa assays were used in three laboratories to evaluate equivalence. <b>Results</b>: The concentrations in the analyzed samples ranged from 2 to 781 ng/mL for rivaroxaban and 9 to 568 ng/mL for apixaban. Only one of the anti-Xa assays gave equivalent results to LC-MS/MS for rivaroxaban, and none for apixaban. All anti-Xa assays significantly underestimated apixaban concentration, with a proportional bias between 10% and 20%. A high correlation was found between DXI concentration and dRVVT clotting time, but dRVVT was not consistently prolonged at clinically relevant DXI concentrations in plasma. <b>Conclusions</b>: Only one of the anti-Xa assays showed equivalence with LC-MS/MS for rivaroxaban, and none for apixaban. Several anti-Xa assays provided reliable results for rivaroxaban in the range of clinically relevant cut-off values, but none for apixaban, which could expose patients to a higher risk of bleeding and urgently needs further clinical research. The dRVVT test was not sensitive enough for reliable detection of clinically relevant DXI plasma concentrations and therefore cannot replace the anti-Xa assay in emergency situations.