Measurements of urea nitrogen are used in the diagnosis and treatment of certain renal and metabolic disorders. Urea nitrogen makes up approximately 75% of the total nonprotein nitrogen (NPN) fraction of the blood. It is synthesized in the liver from ammonia produced as a result of deamination of proteins. Filtration of urea from the blood into the urine by the renal glomeruli is the chief means of eliminating surplus nitrogen from the body. Blood Urea Nitrogen (BUN) levels are a measure of kidney function and also of prerenal and postrenal conditions. Prerenal causes of elevated BUN include cardiac decompensation, water depletion or increased protein catabolism. Among the renal causes of increased levels are acute glomerulonephritis, chronic nephritis, polycystic kidney, nephrosclerosis, and tubular necrosis. Both urea and creatinine are cleared by the renal glomeruli, however, urea is subsequently partially reabsorbed by the renal tubules, while creatinine is not. Consequently, serum urea nitrogen and serum creatinine determinations are frequently performed together in the differential diagnosis of kidney function.