This kit is used for quantitative determination of cystatin C (Cys-C) in human serum, plasma or whole blood in vitro. Cystatin C (CysC) is a kind of basic non glycosylated protein with 13kda molecular weight produced by most nucleated cells in a constant way, which does not depend on any foreign factors, such as gender, age and diet. CysC is almost completely filtered from renal cell membrane in normal renal tissue and reabsorbed and degraded through the nearest tubular cells. Therefore, the concentration of CysC in serum is almost entirely determined by glomerular filtration rate (GFR), which is an ideal homologous marker reflecting the change of glomerular filtration rate.