Plasma osmolarity reflects the body’s water–solute balance.

It is determined mainly by sodium, chloride, glucose, and urea concentrations.

Clinical uses include:

Diagnosing dehydration or overhydration.

Evaluating electrolyte imbalances (especially sodium disorders).

Detecting metabolic conditions such as diabetes mellitus or renal dysfunction.

Assessing patients with confusion, seizures, or altered mental status.

The test is performed using a blood sample.

Analyte: Plasma osmolarity (solute concentration).

Normal Range: ~275 – 295 mOsm/kg H₂O (varies slightly by lab).

Abnormal Findings:

High osmolarity: Suggests dehydration, hypernatremia, hyperglycemia, or renal dysfunction.

Low osmolarity: Suggests overhydration, hyponatremia, or SIADH (Syndrome of Inappropriate Antidiuretic Hormone).

Clinical Use:

Provides a direct measure of hydration and electrolyte status.

Helps differentiate causes of abnormal sodium levels.

Guides treatment in metabolic, renal, and critical care settings.