Defination of serum osmolality test
- Definition of Osmolality:
- A more exact measurement of urine concentration than specific gravity (SG), Depends on the number of solute particles in a unit of solution.
- Clinical Importance:
- Provides more information on renal function when both serum and urine osmolality are tested together. The normal urine-to-serum osmolality ratio is 3:1.
- A high ratio indicates concentrated urine and a low ratio suggests poor kidney concentrating ability
Specimen :
Urine, Spot for 24 hours without preservatives. Samples are stable for 7 days at 15-250°C, 7 days at 2-8°C; andstable for long time at 00°C.
Serum Osmolality Test Calculator
About the Author
Clinical Laboratory Scientist, Lab Manager, Clinical Microbiologist
Related Caculator
Normal serum osmolality Adults: 280–303 mOsm/kg H₂O 2O (280–303 mmol/kg H 2O)
Newborns: as low as 266 mOsm/kg H 2O (266 mmol/kg H 2O)
Serum Osmolality Calculator
Formula: Osm = (Na × 2) + (Glucose ÷ 18) + (BUN ÷ 2.8)
Result
Calculated Serum Osmolality:
-
Interpretation
-
Note: This calculator provides a calculated serum osmolality. Clinical labs may report a measured osmolality; differences (osmolal gap) can occur, especially with substances like ethanol, methanol, ethylene glycol, or mannitol.
Reference Values
Urine (mOsm/kg H2O):
- 24-hour specimen: 300–900 mOsm/kg H2O
- Random specimen: 50–1200 mOsm/kg H2O
Serum (mOsm/kg H2O):
- Adults: 280–303 mOsm/kg H2O (≈ 280–303 mmol/kg H2O)
- Newborns: as low as 266 mOsm/kg H2O (≈ 266 mmol/kg H2O)
Interfering factors
- Decreases in osmolal gap are associated with altitude, diurnal variation with water retention at night, and
some drugs. - Some drugs also cause increases in osmolal gap.
- Hypertriglyceridemia and hyperproteinemia cause an elevated osmolal gap.
- Radiographic contrast medium within 3 days
- Intravenous sodium administration
- Intravenous dextrose and water administration
