FXIa functional activity is determined in a chromogenic method. FXIa in the sample activates human FIX to FIXa in the presence of calcium ions. Generated FIXa activates human FX in the presence of human FVIII, calcium ions and phospholipid. The amount of activated FX is determined from the hydrolysis of a chromogenic FXa substrate and is related to the FXIa activity of the sample. The concentration of functionally active FXIa is assigned vs. a FXIa Calibrator and expressed in International Units.
The activation of Factor IX by Factor XIa is performed in two steps:
Initial activation of FIX in the absence of phospholipids.
Continued activation of FIX in the presence of FX and phospholipids, therewith allowing concomitant activation of FX.