PB0584 - Using heparin-calibrated assays to assess anti-factor Xa activity of factor Xa inhibitors (FXaI): a literature correlation analysis

Background: Measurement of drug activity in patients on direct oral anticoagulant (DOAC) therapy with factor Xa inhibitors (FXaI; rivaroxaban, apixaban, edoxaban) may guide the management of acute complications such as major bleeding. As specific calibrated FXaI assays are not readily available, studies have evaluated correlation between heparin-calibrated and specific FXaI assays, suggesting that heparin-calibrated assays may be suitable to assess FXaI plasma activity. LEX-210 (NCT04867837, Octapharma) will evaluate the hemostatic efficacy of a four-factor PCC (Octaplex®) in patients with acute major bleeding on FXaI and one of the main inclusion criteria is that patients must have a baseline anti-factor Xa (anti-Xa) activity equivalent to ≥100 ng/mL of FXaI.

Aims: To enable investigators to convert heparin-calibrated anti-factor Xa assay results in international units (IU)/mL to equivalent FXaI concentration in ng/mL, based on correlation analysis of published data, to recruit patients.

Methods: Literature was searched (PubMed, June 2021) for original data correlating heparin-calibrated assays and FXaI-specific assays only in real-life patients. Fitted regression equations were extracted/calculated from included papers. Where more than one report was available for a device/reagent/FXaI combination, data were meta-analysed.

Results: 8/57 screened articles contained relevant data. Correlation curves against FXaI-calibrated anti-Xa assays were obtained for heparin-calibrated assays, for different combinations of device/reagent/calibrator. The correlation curves and corresponding conversion table for a commonly used device (STA-R coagulation analyzer) and FXaI (apixaban) are shown (Figure 1/Table 1). Data were also available for rivaroxaban and edoxaban. Enrolment in LEX-210 is ongoing, with investigators consulting conversion tables where only heparin-calibrated assays are available.

Conclusion(s): Conversion tables based on correlation data enable clinicians to estimate FXaI plasma activity using anti-Xa assays calibrated for heparin, if FXaI-calibrated assays are unavailable.