All cells with a nucleus in the body produce the relatively small protein known as cystatin C which is found in many bodily fluids including blood. It is continuously created removed from the blood by the kidneys and decomposed. The glomeruli groups of microscopic blood capillaries in the kidneys that permit water dissolved substances and wastes to flow past their walls while retaining blood cells and bigger proteins filter out cystatin C from the blood. In order to assess kidney function this test analyses the amount of cystatin C in the blood.