Prospective cohort study on the use of low molecular weight heparin calibrated anti-Xa assay for measurement of direct oral Xa inhibitors in ex vivo patient samples.

Drug-specific anti-Xa chromogenic assays are recommended for measurement of direct anti-Xa inhibitor levels but are not routinely available in many institutions. We performed a prospective study to determine: (1) the relationship between low molecular weight heparin (LMWH) calibrated anti-Xa measurements and apixaban or rivaroxaban levels measured using drug-specific anti-Xa assays and, (2) if a LMWH calibrated anti-Xa assay can be used to detect clinically significant apixaban or rivaroxaban levels. Haematology outpatients on rivaroxaban or apixaban for at least 72 h were recruited for this study. Anti-Xa LMWH assay was performed using the Innovance Heparin Anti-Xa kit/calibrator. Drug-specific levels were determined using STA-Liquid anti-Xa kit/STA-Apixaban or STA-Rivaroxaban calibrators. Serial dilutions with pooled normal plasma were performed for specimens with anti-Xa LMWH activity greater than 1.50 ng/mL to obtain anti-Xa levels within the reportable range (0.10-1.50 ng/mL) and multiplied by the dilution factor to determine actual anti-Xa level. Seventy-five (39 rivaroxaban, 36 apixaban) specimens from 67 patients (mean age 60.3 years; 53.3% males) were available for analysis. Rivaroxaban levels ranged from <25 to 500 ng/mL while apixaban levels ranged from <20 to 236.1 ng/mL. For both rivaroxaban and apixaban, there was linear and good correlation (R2  = 0.96) between direct oral anticoagulants and anti-Xa LMWH levels. Using the correlation equation from our data, a rivaroxaban concentration of 50 ng/mL [International Society on Thrombosis and Haemostasis (ISTH) threshold for consideration of antidotes in bleeding patients] and 30 ng/mL (ISTH threshold for consideration of reversal agents prior to interventions), corresponds to anti-Xa LMWH levels of 0.50 and 0.35 IU/mL, respectively. For apixaban the corresponding anti-Xa LMWH levels were 0.35 and 0.20 IU/mL, respectively. In conclusion, LWWH calibrated anti-Xa assay can be used in emergency situations to screen for clinically significant apixaban or rivaroxaban levels when drug-specific calibrators are not available.