The osmolality test is an indirect measurement of the number of particles (solutes) in a fluid (solvent). It is used to evaluate the water balance in the body and is based on the principle that changes in the number of solutes result in changes in the physical properties of the fluid they are in. Water balance in the body is a dynamic process that is regulated by increasing or decreasing the amount of water coming into the body in response to thirst and by controlling the amount of water excreted in the urine. Serum osmolality primarily measures sodium, as well as blood urea nitrogen and blood glucose levels. HIGH serum osmolality may cause hyperosmolarcoma due to excess glucose or alcohol, or due to over-treatment with hyperosmolar solutions. It is also associated with dehydration, diabetes insipidus, hyperglycemia, hypernatremia, kidney damage, and mannitol therapy. Serum osmolality may be DECREASED with excess hydration, hyponatremia, or inappropriate ADH secretion.