Test Catalog

Interpretive Handbook

Test 9257 :
Osmolality, Body Fluid

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

Osmolality is an index of the solute concentration. It is determined by the number and not by the nature of the particles in solution. Dissolved solutes change the physical properties of solutions; they increase the osmotic pressure and decrease freezing point.


Body fluids have the same osmolality as a corresponding serum specimen taken at the same time. "True body fluids" include: ascitic, cerebrospinal, hydrocele, edema, pericardial, pleural, spermatocele and synovial fluids. Secretions not in equilibrium with the extra-cellular fluids of the body include gastric juice, saliva and sweat.


Serum osmolality is normally between 275 to 295 mOsm/kg; it increases with dehydration and decreases with overhydration. Urine osmolality reflects the ability of the kidney to maintain tonicity and water balance. The normal kidney can concentrate urine to 800 to 1,400 mOsm/kg, and with excess fluid intake, a minimal osmolality of 40 to 80 mOsm/kg can be obtained.

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

Determining the source and type of fluid

Interpretation Provides information to assist in interpretation of the test results

No normals are available.

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

Avoid viscous fluids

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.

No established reference values

Clinical References Provides recommendations for further in-depth reading of a clinical nature

Clinical Diagnosis and Management by Laboratory Methods. 20th edition. Edited by JB Henry. Philadelphia, WB Saunders Company, 2001

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