*2.1. Functional Biomarker—Serum Cystatin C*

Cystatin C is a non-glucosylated 13 kD basic protein which belongs to the cysteine protein inhibitors family and is produced at a constant rate by all nucleated cells. Cystatin C is an inhibitor of lysosomal proteinases and one of the most important extracellular inhibitors of cysteine proteases [5,9]. Cystatin C is freely filtered in glomeruli and then reabsorbed and metabolized in the proximal tubule. Studies on diabetes, protein-induced glomerular hyperfiltration and extreme exercise demonstrated that acute changes in serum (s)Cyst-C provide a better approximation of GFR than serum creatinine (sCr). sCyst-C is affected by sex and race and to a small degree, by inflammation [20]. In clinical studies on acute kidney injury, an increase in serum and urine cystatin C levels is observed earlier than an increase in creatinine [5,9,20,21].
