New oral anticoagulants are directed towards a single target, essentially factor 
Xa (FXa) or factor IIa. They do not require routine coagulation monitoring. 
However, in special clinical settings (emergency surgery, bleeding, thrombosis, 
control of the patient's compliance, suspected overdose, potential drug 
interference, and so on), measurement of plasma levels is needed. Several 
available anti-FXa assays are used for monitoring anticoagulant activity of 
heparins and fondaparinux. They must be modified and standardized for the 
measurement of direct FXa inhibitors (rivaroxaban, apixaban, edoxaban, 
betrixaban and others). The use of calibrators (lyophilized plasma with a known 
concentration of drug) allows an expression of the results in ng per ml of 
plasma. Two categories of assays - endogenous and exogenous assays are 
available. Endogenous assays are useful in pharmaceutical research, while 
exogenous assays are used in clinical laboratories. The preferred anti-FXa assay 
is a specific method in contrast to prothrombin time and activated partial 
thromboplastin time, but it is not available everywhere at any time. A specific 
measurement of direct FXa inhibitors is feasible with the use of a new test 
developed by the authors' group. The physicians must be aware of the possibility 
to measure the plasma concentration of FXa inhibitors in patients at high risk 
of bleeding and in several other special clinical situations.