Several blood tests check kidney function, including serum creatinine, glomerular filtration rate and blood urea nitrogen, or BUN, explains the National Kidney Foundation. These tests measure how well the kidneys remove waste products and extra fluids from the blood.
Normal wear and tear of the body's muscles produces creatinine, and the breakdown of protein in foods produces urea nitrogen, according to the National Kidney Foundation. Elevated BUN and creatinine levels usually indicate kidney problems.
Glomerular filtration rate, or GFR, measures how well the kidneys are filtering the blood, explains MedlinePlus. Doctors calculate GFR using a formula that accounts for age, gender, body size, ethnicity and creatinine level.