Development and validation of a novel rivaroxaban calibrator: A routine anti-Xa assay solution with high agreement with LC-MS/MS

OBJECTIVES: Therapeutic drug monitoring of rivaroxaban is essential for select patients, but anti-Xa assays can show discrepancies with the Liquid chromatography-tandem mass spectrometry (LC-MS/MS) reference method due to calibrator differences. This study aimed to develop and validate a novel rivaroxaban calibrator to achieve high interchangeability between the CX-9010 anti-Xa assay and LC-MS/MS. METHODS: Samples were measured concurrently by the CX-9010 anti-Xa assay and LC-MS/MS. Agreement was assessed by Pearson correlation, Passing-Bablok (PB) regression, and Bland-Altman (BA) analysis stratified by concentration: absolute differences for samples ≤50 ng/mL (n = 108) and relative differences for samples >50 ng/mL (n = 92). RESULTS: Pearson correlation coefficient was 0.998. PB regression yielded a slope of 0.988 (95% CI: 0.978-0.998). For samples ≤50 ng/mL, absolute BA showed a mean bias of -0.73 ng/mL and 95% limits of agreement (LoA) of -8.74 to +7.28 ng/mL. For samples >50 ng/mL, relative BA showed a mean bias of -0.44% and LoA of -10.1% to +9.3%. Intra-vial CVs ranged from 0.85% to 2.52%, and inter-vial CVs from 0.95% to 2.21% across all six calibrator levels. Working calibrator stability over 34 h showed a maximum deviation of +4.91% relative to baseline, with all time-point deviations remaining within ±5%. CONCLUSIONS: The CX-9010 assay demonstrated strong agreement with LC-MS/MS in this single-center cohort. The calibrator quality met all predefined targets, supporting its use for routine clinical monitoring; multicenter validation is warranted before broader claims of interchangeability can be made.