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HelpTest Code UOSMS Osmolality, Serum
Reporting Name
Osmolality, SUseful For
Evaluating acutely ill or comatose patients
Performing Laboratory
Mayo Clinic Laboratories in Rochester
Specimen Type
SerumSpecimen Required
Supplies: Sarstedt Aliquot Tube, 5 mL (T914)
Collection Container/Tube:
Preferred: Red top
Acceptable: Serum gel
Submission Container/Tube: Plastic vial
Specimen Volume: 2 mL Serum
Collection Instructions: Centrifuge and aliquot serum into a plastic vial.
Specimen Minimum Volume
Serum: 0.25 mL
Specimen Stability Information
| Specimen Type | Temperature | Time |
|---|---|---|
| Serum | Refrigerated (preferred) | 7 days |
| Frozen | 7 days | |
| Ambient | 24 hours |
Reference Values
275-295 mOsm/kg
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 |
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/20) + (Blood Urea Nitrogen/3)
Reject Due To
| Gross hemolysis | OK |
| Gross lipemia | OK |
Method Name
Freezing Point Depression