Serum Osmolality

Excerpt

Osmolality is a colligative property of solutions that depends on the number of dissolved particles in the solution. The term osmolality expresses concentrations relative to the mass of the solvent, whereas the term osmolarity expresses concentrations per volume of solution. Osmolality is thermodynamically accurate because solution concentrations expressed on a weight basis are temperature-independent, whereas those based on volume vary with temperature. Although the term osmolarity is used in the medical literature, osmolality is usually measured.

Osmolality is routinely measured in clinical laboratories for the differential diagnosis of disorders related to hydrolytic balance regulation, renal function, and small-molecule poisonings. Serum and urine osmolality tests are usually measured and compared to determine the diagnosis of any disease that influences osmolality. Serum osmolality is affected by the concentration of blood chemicals like chloride, sodium, proteins, bicarbonate, and glucose. The blood urea nitrogen (BUN) is important for calculating serum osmolality. Specific therapies and toxins that affect an individual’s fluid balance should also be evaluated with serum osmolality. Several formulas are used for serum osmolality, but the Smithline-Gardner formula is considered the most accurate.

The normal serum osmolality ranges from 275 to 295 mOsm/kg. Water normally flows from low to high osmolality. When water moves between plasma and intracellular compartments, the direction depends on both compartments' osmolalities. For example, if a cell is in a relatively hyperosmolar solution, fluid will move toward the highly concentrated compartment to reach homeostasis. As a result, the cell will shrink.