When renal failure is advanced (glomerular filtration rate [GFR] ≤ 15 mL / min/1.73 m 2), the ability to effectively dilute or concentrate urine is lost; thus, urine osmolality is usually fixed at about 300 to 320 mOsm / kg, close to that of plasma (275 to 295 mOsm / kg), and urinary volume does not respond readily to variations in water intake.