Cystatin C is a low-molecular-weight protein produced by all nucleated cells at a constant rate and freely filtered by the kidneys. This test measures cystatin C levels in the blood to evaluate kidney function and estimate glomerular filtration rate (GFR). Unlike creatinine, cystatin C is not affected by muscle mass, age, gender, or diet, making it a more reliable marker of renal function. Elevated cystatin C levels indicate decreased kidney function and reduced GFR. This test is particularly valuable for detecting early kidney disease, monitoring renal function in patients with muscle wasting or malnutrition, and providing more accurate GFR estimates than creatinine-based calculations. Cystatin C can detect kidney dysfunction earlier than creatinine, as levels rise when GFR falls below 50%, while creatinine may remain normal until GFR is significantly reduced.