Factor Xa, which has a molecular weight of 44 KDa, is the activated form of Factor X (MW: 59 KDa).
The International Units of Factor X correspond to the amount of Factor X contained in 1 ml of normal plasma. This is about 8 mg/l or 0.13 ?mol/l.
Since there is no WHO standard for FXa, one would assume that if all the Factor X in normal plasma was converted to the activated form, the Factor Xa concentration would be approximately 5.7 mg/l.
The activity of human Factor Xa as calculated from the kinetic tables is 1.5 nkat/?g with the substrate S-2222, and 4.4 ?kat/?g with the substrate S-2765.
The activity of 1 ?g of Factor Xa as determined by Frieberger (1) is 1.9 nkatS-2222.
Thus, 1 plasma equivalent unit of Factor X would correspond to 15.2 nkatS-2222.

1. Friberger P et al.
   Synthetic peptide substrate assays and fibrinolysis and their application on automates. In: Seminars in Thrombosis and Haemostasis, Vol. 9, 281-300 (1983).