Test ID: UOSMS    
Osmolality, Serum

Evaluating acutely ill or comatose patients

Osmolality is a measure of the number of dissolved solute particles in solution. It is determined by the number and not by the nature of the particles in solution.

Dissolved solutes change the physical properties of solutions, increasing the osmotic pressure and boiling point and decreasing the vapor pressure and freezing point.

Serum osmolality increases with dehydration and decreases with overhydration. The patient receiving intravenous fluids should have a normal osmolality. If the osmolality rises, the fluids contain relatively more electrolytes than water. If the osmolality falls, relatively more water than electrolytes is being administered.

Normally, the ratio of serum sodium, in mEq/L, to serum osmolality, in mOsm/kg, is between 0.43 and 0.5. The ratio may be distorted in drug intoxication.

Generally, the same conditions that decrease or increase the serum sodium concentration affect the osmolality.

A comparison of measured and calculated serum osmolality produces a delta-osmolality. If this is  above 40 mOsm/kg H2O in a critically ill patient, the prognosis is poor.

An easy formula to calculate osmolality is: 

275-295 mOsm/kg

An increased gap between measured and calculated osmolality may indicate ingestion of poison, ethylene glycol, methanol, or isopropanol.

No significant cautionary statements

Murphy JE, Henry JB: Evaluation of renal function, and water, and electrolyte, and acid base balance. In Todd-Sanford-Davidsohn Clinical Diagnosis and Management by Laboratory Methods. 19th edition. Edited by JB Henry. Philadelphia, PA, WB Saunders Company, 2006