Cystatin C is a small protein produced by the cells in the body that is filtered out of the blood by the kidneys and excreted in the urine. The Cystatin C test is a blood test that measures the level of Cystatin C in the blood, which can help estimate the glomerular filtration rate (GFR) of the kidneys. The GFR is a measure of how well the kidneys are functioning, and a decrease in GFR can indicate kidney damage or disease. The Cystatin C test may be used to evaluate kidney function, particularly in patients with chronic kidney disease (CKD) or those at risk for CKD. It may also be used to monitor the effectiveness of treatment for CKD or other kidney-related conditions. Unlike other markers of kidney function, Cystatin C is not affected by muscle mass or diet, making it a potentially more accurate measure of GFR.