Renal function tests are useful for estimating kidney function before medical procedures and in diseases affecting the kidneys like hypertension and diabetes. The kidney filters blood to form urine, circulating the total blood volume through the kidneys 300 times per day to produce about 1 liter of urine. Basic renal function tests measure blood urea nitrogen (BUN) and serum creatinine levels, which rise with kidney dysfunction. Estimated glomerular filtration rate (eGFR) is now commonly used instead of creatinine clearance to estimate kidney function based on serum creatinine levels along with age, sex, and other factors. A kidney biopsy examines tissue samples under light microscopy, direct immunofluorescence, and electron microscopy to identify signs of kidney disease.