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Mayo Test ID UOSMS Osmolality, Serum


Specimen Required


Collection Container/Tube:

Preferred: Red top

Acceptable: Serum gel

Submission Container/Tube: Plastic vial

Specimen Volume: 2 mL

Collection Instructions: Centrifuge and aliquot serum into a plastic vial.


Useful For

Evaluating acutely ill or comatose patients

Method Name

Freezing Point Depression

Reporting Name

Osmolality, S

Specimen Type

Serum

Specimen Minimum Volume

0.25 mL

Specimen Stability Information

Specimen Type Temperature Time Special Container
Serum Refrigerated (preferred) 7 days
  Frozen  7 days
  Ambient  24 hours

Reject Due To

Gross hemolysis OK
Gross lipemia OK

Clinical Information

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+

Glucose

+

BUN

20

3

Reference Values

275-295 mOsm/kg

Interpretation

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

Cautions

No significant cautionary statements

Clinical Reference

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

2. Delaney MP, Lamb EJ: Kidney disease. In: Rifai N, Horvath AR, Wittwer CT, eds: Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. Elsevier; 2018:1306

Method Description

The depression of the freezing point of serum or other fluid is used to measure osmolality in most osmometers. The extent of lowering below 0° C (the freezing point of water) is a function of the concentration of substances dissolved in the serum. By definition, 1 milliosmole per kilogram lowers the freezing point 0.001858° C.(Schindler EI, Brown SM, Scott MG: Electrolytes and blood gases. In: Rifai N, Horvath AR, Wittwer CT, eds: Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. Elsevier; 2018:610-612)

Specimen Retention Time

7 Days

Performing Laboratory

Mayo Clinic Laboratories in Rochester

Test Classification

This test has been cleared, approved, or is exempt by the US Food and Drug Administration and is used per manufacturer's instructions. Performance characteristics were verified by Mayo Clinic in a manner consistent with CLIA requirements.

CPT Code Information

83930

LOINC Code Information

Test ID Test Order Name Order LOINC Value
UOSMS Osmolality, S 2692-2

 

Result ID Test Result Name Result LOINC Value
UOSMS Osmolality, S 2692-2

Day(s) Performed

Monday through Sunday

Report Available

1 day