Serum Osmolality

Blood Osmolality Test, Osmolality Test

Test Overview

A serum osmolality test measures the amount of chemicals dissolved in the liquid part (serum) of the blood. Chemicals that affect serum osmolality include sodium, chloride, bicarbonate, proteins, and sugar (glucose). A serum osmolality test is done on a blood sample taken from a vein.

Serum osmolality is controlled partly by a hormone called antidiuretic hormone (ADH). Water constantly leaves your body as you breathe, sweat, and urinate. If you do not drink enough water, the concentration of chemicals in your blood (serum osmolality) increases. When serum osmolality increases, ADH is released. This increases the amount of water in the blood and helps restore serum osmolality to normal levels.

When you drink too much water, serum osmolality decreases. When serum osmolality decreases, ADH is suppressed. This increases the amount of water in your urine and prevents too much water from building up in your body (overhydration).

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Author:	Maria G. Essig, MS, ELS	Last Updated: November 27, 2007
Medical Review:	Caroline S. Rhoads, MD - Internal Medicine David C.W. Lau, MD, PhD, FRCPC - Endocrinology & Metabolism
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