Test Catalog

Test ID: UOSMS    
Osmolality, Serum

Useful For Suggests clinical disorders or settings where the test may be helpful

Evaluating acutely ill or comatose patients

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

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: 

Osmolality (mOsm/kg H2O)=2 Na+






Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

275-295 mOsm/kg

Interpretation Provides information to assist in interpretation of the test results

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

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

No significant cautionary statements

Clinical Reference Recommendations for in-depth reading of a clinical nature

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