Diagnostic Blood Test: Cystatin C with eGFR Most cells in the body naturally make a protein called cystatin C, which is then removed from the blood by the kidneys. High concentrations of cystatin C are often a signal of kidney dysfunction. Cystatin C levels can be used to calculate the estimated glomerular filtration rate (eGFR), which describes how well the kidneys are able to filter waste.Cystatin C is a more accurate marker of kidney function than Creatinine as it is less influenced by diet, supplements and exercise.Note: Fasting is recommended for 12 hours prior to blood draw.