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HelpTest ID UOSMS Osmolality, Serum
Useful For
Evaluating acutely ill or comatose patients
Method Name
Freezing Point Depression
Reporting Name
Osmolality, SSpecimen Type
SerumContainer/Tube:
Preferred: Red top
Acceptable: Serum gel
Specimen Volume: 2 mL
Specimen Minimum Volume
0.25 mL
Specimen Stability Information
| Specimen Type | Temperature | Time |
|---|---|---|
| Serum | Refrigerated (preferred) | 7 days |
| Frozen | 7 days | |
| Ambient | 24 hours |
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.
The osmolality of serum 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 >40 mOsm/kg a 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
Cautions
No significant cautionary statements
Day(s) Performed
Monday through Sunday; Continuously
Report Available
1 dayPerforming Laboratory
Mayo Medical Laboratories in Rochester
Test Classification
This test has been cleared or approved by the U.S. 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