A test for the level of creatinine in the blood evaluates kidney function, according to the National Kidney foundation. Doctors use this test, along with information such as age, race and gender, to calculate glomerular filtration rate (GFR), which determines kidney function.
When kidneys no longer filter the blood effectively, a build up of waste and excess fluid forms in the body. Blood Urea Nitrogen (BUN) and creatinine are examples of waste that build up in the blood. When checking kidney function, doctors test for the abnormal presence of these wastes, according to the National Kidney Foundation. Doctors also test protein amounts in urine when evaluating kidney health.