The principle of the double antibody sandwich ELISA is represented in Figure 1. In this assay the Cystatin C present in samples reacts with the anti-CYSTATIN C antibodies which have been adsorbed to the surface of polystyrene microtitre wells. After the removal of unbound proteins by washing, antiCYSTATIN C antibodies conjugated with horseradish peroxidase (HRP), are added. These enzymelabeled antibodies form complexes with the previously bound CYSTATIN C. Following another washing step, the enzyme bound to the immunosorbent is assayed by the addition of a chromogenic substrate, 3,3