Urine concentration test

The kidney has a strong ability to regulate water. It can retain water and concentrate 180L / d of glomerular filtrate to only 0.4L / d and hypertonic urine with osmotic pressure up to 1400mOsm / kg of water. The kidneys can also remove excess water from the body, such as excluding 10 to 20 L / d of hypotonic urine with an infiltration volume of 40 mOsm / kg. The ability to concentrate and dilute urine is mainly related to the function of distally curved tubules and collecting tubes. Currently, clinical laboratories still measure the specific gravity of urine as an indicator of urine concentration or dilution. Common methods are urine hydrometer method and urine refractive index measurement.