The anticoagulant rivaroxaban is not routinely monitored due to its predictable anticoagulant action. However, drug levels are clinically useful in patients requiring surgery as well as in assessing compliance. We have modified a commercial heparin assay based on inhibition of activated factor X (Xa) that was previously modified for the assay of fondaparinux (Am J Clin Pathol 2009, 132:608-612) by replacing the fondaparinux calibrators with commercial rivaroxaban calibration plasmas. Technoview rivaroxaban calibrator plasmas were purchased from Dia Pharma (West Chester, Ohio). These calibrators were substituted for the fondaparinux calibrators. The assay was run on a Siemens BCS XP, as previously described. The assay was linear in the range of 0 to 650 ng/ml. The limit of detection was 14 ng/ml, and carryover at 285 ng/ml was < 1%. The intra-assay precision at 100 ng/ml was standard deviation of 1.04 ng/ml, coefficient of variation of 1.03%. The inter-assay precision at 105 ng/ml was standard deviation 3.5 ng/ml, coefficient of variation of 3.3%. The assay is unaffected by warfarin, but can be affected by other Xa inhibitors such as heparin, low molecular weight heparin, fondaparinux, and apixaban.
No relevant conflicts of interest to declare.
