PB1084 - Comparability Of Anti-Xa Assessments Using Alternate Calibration Curves In Patients Receiving Rivaroxaban And Apixaban Therapy

Background: Oral anti-factor Xa (anti-Xa) inhibitors are widely used. Routine coagulation tests cannot exclude clinically significant drug activity, therefore an anti-Xa assay using a drug-specific calibrator is required. In the emergency setting, determining the specific drug can be challenging, however, most centres can perform a standard anti-Xa assay with a low molecular weight heparin (LMWH) calibrator. While clinical data are limited, a rivaroxaban or apixaban level below 50ng/mL appears safe for most procedures. Therefore establishing an anti-Xa level with a LMWH calibrator that corresponds to a rivaroxaban or apixaban level below 50ng/mL would be helpful.

Aims: 1. To measure the correlation between rivaroxaban and apixaban levels using drug-specific and LMWH calibrators.
2. To establish the anti-Xa level with a LMWH calibrator that correlates with trough levels of apixaban and rivaroxaban ( < 50ng/mL).

Methods: 60 frozen samples from patients on rivaroxaban or apixaban (30 per drug) were selected and run in duplicate. Each 30 samples comprised: 10 samples with anti-Xa levels 20-100ng/mL, 10 samples 100-200ng/mL, 10 samples 200-300ng/mL, and 2 data points at 0. They were analysed with a drug-specific, and LMWH calibrator, using a one stage chromogenic assay (ACL TOP analyser, IL Liquid Anti-Xa kit reagent). The mean anti-Xa activity was determined for each sample, and a line of best fit established for each drug both separately and combined.

Results: We confirmed a strong, statistically significant correlation with apixaban and rivaroxaban drug levels using both calibrators (table 1), and established a combined correlation curve (figure 1). From this data, an anti-Xa level of 0.35 U/mL corresponds to a drug level of 50ng/mL.

Conclusion(s): There is a strong correlation between anti-Xa levels for rivaroxaban or apixaban using a LMWH calibrator. This can establish an anti-Xa cut-off to exclude significant levels of either drug.