The principle of the double antibody sandwich ELISA is represented in Figure 1. In this assay the Fibrinogen present in samples reacts with the anti-Fibrinogen antibodies which have been adsorbed to the surface of polystyrene microtitre wells. After the removal of unbound proteins by washing, anti-FIB antibodies conjugated with horseradish peroxidase (HRP), are added. These enzyme-labeled antibodies form complexes with the previously bound FIB. Following another washing step, the enzyme bound to