Kidney function is most commonly determined by measuring the level of creatinine in the blood. as kidney function decreases the level of creatinine rises. the creatinine test is a simple blood draw and the result is used to determine glomerular filtration rate (gfr). you may be required to have your urine tested; this helps your doctor understand if there is any leakage of protein or blood from the kidneys. nephrologists also can use an ultrasound to look at the size and shape of the kidneys.