In the derivation cohort, we will determined the conversion factor linking apixaban, rivaroxaban, fondaparinux, or danaparoid measured level, on the one hand, and heparin anti-Xa activity, on the other hand.

In the validation cohort, for each tested anticoagulant, we will used the conversion factor determined in the derivation cohort to infer the estimated level of anticoagulant from heparin anti-Xa activity:

estimated anticoagulant level = conversion factor for this anticoagulant × heparin anti-Xa activity

The agreement between measured and estimated levels of each factor-Xa inhibitor will be assessed.

Estimated anticoagulant level (conversion factor x anti-Xa activity)

An estimated anticoagulant level between 80 and 120% of the value obtained by the reference method ("specific assay") will be deemed clinically acceptable, i.e. the patient's management would have been the same.