Glomerular filtration is the process by which the kidneys filter the blood, removing excess wastes and fluids. GFR is a calculation that determines how well the blood is filtered by the kidneys, which is one way to measure remaining kidney function. GFR is usually estimated (estimated GFR or eGFR) using a mathematical formula that compares a person's size, age, sex, and race to serum creatinine levels. A GFR under 60 ml/min/1.73 m² may indicate kidney disease; the lower the GFR number, the worse the kidney function.