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Paediatric Laboratory Medicine

Osmolarity, Blood

Clinical Significance

Clinical uses of serum osmolality include screening for toxin ingestion (alcohols, glycols), monitoring concentration of osmotically active agents (eg., mannitol), and evaluation of hyponatremia (exclusion of pseudohyponatremia, presence of other osmotic agents such as glycine and glucose). Clinical uses of urine osmolality include evaluation of increased urine output (as in primary polydipsia, diabetes insipidus, diabetes mellitus), evaluation of decreased urine output (dehydration, acute tubular injury), and evaluation of renal acidification defects. Stool osmolality may be used to separate secretory diarrhea (due to agents damaging the mucosa) from osmotic diarrhea (due to the presence of unabsorbed solutes, laxative abuse, malabsorption).

Test Name

Osmolarity, Blood

Test Code



Biochemistry - Biochemistry


Freezing Point Depression

Turn Around Time


Specimen Type

Serum or Sodium/Lithium Heparin plasma/ Urine/ Pharmacy Fluids

Minimum Specimen Requirements

60 uL



Approval is not required

CPT Codes


Shipping and Contact Information

The Hospital for Sick Children
Rapid Response Laboratory
170 Elizabeth Street, Room 3642
Toronto, ON
M5G 2G3
Phone: 416-813-7200
Phone: 1-855-381-3212

Reference Range

<2d : 275-300 mmol/kg H2O
2-7d : 276-305 mmol/kg H2O
8-28d : 274-305 mmol/kg H2O
≥29d : 282-300 mmol/kg H2O