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HelpTest Code OSMO Osmolality, Serum/Plasma
Specimen and Container/Tube
Specimen Type: Plasma
Preferred Specimen: Li-Heparin Mint Green Top
Other Acceptable specimens: Serum/Gold Top, Serum/Red Top
Specimen Handling and Transport Instructions
UCMC Onsite Instructions:
Collect specimen per standard collection procedure and send to laboratory immediately.
UCMC Offsite and UC MedLab Outreach Instructions:
- Samples should be centrifuged within 2 hours of collection.
- Centrifuge specimens at >2500 x g for 10 minutes at room temperature.
- Aliquot sample into plastic transport tube containing a minimum of 0.2 mL of specimen.
- Samples must be sent room temperature at 15-25oC.
Rejection Criteria
- Common specimen rejected situations include: incorrect specimen type, insufficient volume, missing or incomplete specimen identifiers, incorrect specimen transport, or specimens outside stability limits.
- Specimens that are clotted or grossly hemolyzed.
- Specimen collection tubes that are less than 50% full
- Specimens that were transported unspun/whole blood and draw time is greater than 6 hours.
Reference Values
|
Test |
Age |
Sex |
Reference Range |
|
Osmolality |
All |
All |
289 - 308 mOsm/kg |
Critical Values
Not applicable
CPT Codes
83930
LOINC Codes
2692-2
Volume
Preferred Volume: 0.5 mL
Minimum Volume: 0.1 mL
Specimen Stability
|
Storage Temperature |
Stability |
|
Room temperature |
8 hours |
|
Refrigerated |
7 days |
|
Frozen |
N/A |
Collection Instructions
Collect specimen per standard laboratory collection procedures.
Test Components
Osmolality
Clinical Indications
- 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.
- An increased gap between measured and calculated osmolality may indicate ingestion of poison, ethylene glycol, methanol, or isopropanol.
Methodology
Freezing point depression (Advanced Instruments Micro-Osmometer Model 3320)
Additional Information
- 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)
Turnaround Time
Turnaround times are relative to the time the specimen is received in the test laboratory.
STAT Turnaround Time: 1 hour
Routine Turnaround Time: 4 hours
Testing Schedule: 24/7
Synonyms/Keyword Search
Osmolality, Osmo
Last Review Date
4/7/2025