A serum osmolality test measures the amount of chemicals dissolved in the fluid portion of blood (serum). Chemicals that affect serum osmolality include sodium, chloride, bicarbonate, proteins, sugar (glucose), and urea. A serum osmolality test is done to evaluate electrolyte and water balance.
Serum osmolality is controlled partly by a hormone called antidiuretic hormone (ADH), or vasopressin. ADH is produced by a part of the brain called the hypothalamus and is released by the pituitary gland into the blood.
Water constantly leaves your body when you breathe, sweat, and urinate. If you do not drink enough water, the concentration of chemicals in your blood (serum osmolality) increases. In response to an increase in serum osmolality, ADH is released, which reduces the amount of water excreted by the kidneys. This helps restore serum osmolality to normal levels.
When you drink too much water, serum osmolality decreases. In response to a decrease in serum osmolality, ADH is suppressed and the kidneys release more water into urine. This prevents excessive amounts of water from building up in your body (overhydration).
A urine osmolality test is used to measure the ability of the kidneys to concentrate urine. This is a good indicator of the proper functioning of the kidneys. It can also be used to identify electrolyte and water imbalance.
Often, the serum and urine osmolality tests are run together. When done together, the two tests are most informative about the body’s ability to properly maintain water and electrolyte balance, and may also indicate dehydration or over-hydration.
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