Cystatin C is a low molecular weight protein (13.3 kD) continuously produced by all nucleated cells. Its production is not affected by gender, muscle mass or the patient's diet. The low molecular weight and the net positive charge of cystatin C allow it to be freely filtered by the renal glomerulus. It is mostly reabsorbed but it is catabolized by cells of the renal tubule, without secretion or reabsorption of the intact form. Serum cystatin C levels therefore depend only on the glomerular filtration rate; as with creatinine, the serum level increases in case of glomerular renal insufficiency and returns to normal when kidney function improves.