Cystatin C with Glomerular Filtration Rate, Estimated (eGFR) - This test measures the level of Cystatin C, a protein found in your body fluids, to assess kidney function. Unlike other markers, the level of Cystatin C's in the blood does not change due to infections, diet, or medication, making it a reliable indicator of how well your kidneys are filtering. The test also estimates your Glomerular Filtration Rate (eGFR), which indicates the rate at which your kidneys are filtering the blood. It's especially useful for people with unusual muscle mass or rapidly changing kidney function.