MLT GlobeOsmolality (serum)
Electrolytes · also called Serum osmolality, Osm
What it measures
Serum osmolality measures the total concentration of dissolved particles. It is used to investigate sodium disorders and to detect unmeasured solutes such as toxic alcohols through the osmolal gap.
Specimen and preparation
- Specimen: Serum (urine osmolality on a paired random urine)
- Tube: Gold top
- Preparation: Separate promptly; tightly capped to avoid evaporation.
Reference range
| Group | Range |
|---|---|
| Adult serum | 275–295 mOsm/kg |
| Random urine | 50–1200 mOsm/kg |
Reference ranges differ between laboratories, methods and populations. Always use the range printed on your own lab report.
Causes of high values
- Dehydration and hypernatraemia
- Hyperglycaemia (HHS)
- Uraemia
- Ethanol, methanol, ethylene glycol or mannitol
Causes of low values
- Hyponatraemia with SIADH
- Excess water intake
- Overhydration
Method
Freezing point depression osmometry
Good to know
Calculated osmolality = 2×Na + glucose/18 + BUN/2.8 (mg/dL units). An osmolal gap over 10 mOsm/kg suggests an unmeasured substance.
Related tests
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