Cystatin C is a cysteine proteinase inhibitor, produced by nucleated cells and destroyed at a constant rate. It is found in a variety of body fluids such as blood, spinal fluid, and breast milk, and is filtered out of the blood by the glomeruli. This test is used to evaluate kidney function. It is not affected by muscle mass, gender, age, or race unlike creatinine, and therefore may be used as an alternative to creatinine and creatinine clearance to screen and monitor kidney dysfunction. An INCREASED serum cystatin C corresponds to a decreased glomerular filtration rate (GFR) and hence to kidney dysfunction.