Serum Osmolality

A summary measure of blood concentration — reflects hydration status.

What it is

Osmolality measures the concentration of solutes in blood, primarily sodium, glucose, and urea. It is tightly regulated by the kidneys and antidiuretic hormone (ADH). High osmolality = concentrated blood = dehydration or excess solutes. Low osmolality = dilute blood.

Why it matters

Serum osmolality helps diagnose hyponatraemia causes (is it dilutional or depletional?) and SIADH (syndrome of inappropriate ADH) — a paraneoplastic phenomenon in some cancers. It is rarely tested routinely but valuable in specific clinical contexts.

How to test

Blood test. Can also be calculated: 2×Na + Glucose + Urea (all in mmol/L).

This information is for educational purposes only. Always discuss testing and interpretation of results with your care team.

Optimal range

280–295 mOsm/kg.

How often

When sodium is abnormal, or investigating fluid balance issues.