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HelpTest ID: UOSMS Osmolality, Serum
Reporting Name
Osmolality, SUseful For
Evaluating acutely ill or comatose patients
Specimen Type
SerumSpecimen 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.
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 |
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 |
+ |
BUN |
|
20 |
3 |
Interpretation
An increased gap between measured and calculated osmolality may indicate ingestion of poison, ethylene glycol, methanol, or isopropanol.
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 Name
Freezing Point Depression
Forms
If not ordering electronically, complete, print, and send a Renal Diagnostics Test Request (T830) with the specimen.
Day(s) Performed
Monday through Sunday
Report Available
1 daymcl-kidney