# Assays for Monitoring Apixaban and Rivaroxaban in Emergency Settings, State-of-the-Art Routine Analysis, and Volumetric Absorptive Microsamples Deliver Discordant Results

## TL;DR

This study compares different methods for measuring anticoagulant drug levels in blood, finding significant differences in results between techniques.

## Contribution

The study highlights critical discrepancies between anti-Xa assays and LC-MS/MS for DOAC monitoring in clinical and home-collected samples.

## Key findings

- Anti-Xa chromogenic assays and LC-MS/MS methods produced discordant plasma concentrations for apixaban and rivaroxaban.
- VAMS samples showed lack of agreement with plasma concentrations, emphasizing the need for careful method selection.
- Hematocrit measurement is essential when using VAMS to ensure accurate drug concentration results.

## Abstract

Our aim was to compare the performance of complementary clinical laboratory approaches to monitoring exposure to apixaban and rivaroxaban, the most prescribed direct-acting oral anticoagulants (DOAC’s): an automated commercial anti-Xa chromogenic assay suitable for emergency and pre-surgery testing and a laboratory-developed liquid chromatography-tandem mass spectrometry (LC-MS/MS) method employed for non-emergency analysis in plasma and in dried blood volumetric absorptive microsamples (VAMS) collectible by the patients in their homes. The full validation of the LC-MS/MS method was performed. Cross-validation of the methodologies was accomplished by processing 60 specimens collected for whole blood count and DOAC monitoring in a central clinical laboratory. For VAMS samples, dried plasma and whole blood calibrators were found to be suitable, and a cycle run for seven days could be implemented for rational and economic sample processing. The anti-Xa chromogrenic assay and the LC-MS/MS method delivered discordant plasma analyte concentrations. Moreover, the lack of agreement between plasma and VAMS concentrations was observed. Clinical laboratories must be aware of the differences between the performance of apixaban and rivaroxaban LC-MS/MS and anti-Xa assays. Hematocrit must always be measured along with VAMS samples to obtain accurate results.