Anti‐factor IIa (FIIa) heparin assay for patients on direct factor Xa (FXa) inhibitors

Direct factor Xa (FXa) inhibitors are increasingly prescribed for outpatients, and those transitioning to unfractionated heparin (UFH) for hospital admission are monitored via an anti‐FXa assay. Because of assay interference, UFH results would often be critically elevated, confounding dosing. An anti‐factor IIa (FIIa) UFH assay was evaluated for clinical use. The BIOPHEN ANTI‐IIa (Aniara Diagnostica) assay and anti‐FXa INNOVANCE Heparin assay (Siemens Healthcare Diagnostics Products GmbH) were compared on the Siemens BCS XP system. Samples included UFH controls and calibrators and specimens from patients transitioning from apixaban or rivaroxaban to UFH. Method comparison, linearity, recovery, precision, and interference by direct FXa inhibitors were evaluated. The effect of the BIOPHEN ANTI‐IIa assay on the rate of critically high UFH results was retrospectively reviewed 4 months after implementation. Accuracy studies using 0.24 and 0.50 IU/mL UFH yielded means and standard deviations of 0.26 ± 0.01 and 0.58 ± 0.01 IU/mL, respectively. Within‐run and between‐run coefficients of variation were 4.6% and 15.5% for the low control, and 1.8% and 10.6% for the high control. The method comparison slope was 0.9965 ( r 2  = 0.9468). The linear range was 0.1 to 1.3 IU/mL. The assay measured UFH in the presence of 192 ng/mL apixaban or 158 ng/mL rivaroxaban. Introduction of the assay for clinical use reduced the monthly percentage of critically high results from 9.4% to 3.8% for admitted heparinized patients who recently discontinued apixaban or rivaroxaban. The BIOPHEN ANTI‐IIa assay is suitable for patients transitioning off apixaban or rivaroxaban.