Glomerular filtration rate (GFR) is the best blood test that measures the level of kidney function and also determines the stage of kidney disease. The kidneys have small filters called glomeruli which help in removing the waste from the blood. The GFR is the rate at which the blood is filtered each minute. The test measures how much the blood passes each minute through the glomeruli. The GFR can be estimated with greater accuracy from the results of another blood creatinine test, or other factors like age, gender or body weight. The test helps the doctor to plan for further treatment regarding kidney. Low GFR will result in the improper working of the kidney.