What is the normal serum osmolality range?

What is the normal serum osmolality range?

• A. 300-320 mOsm/kg
• B. 250-270 mOsm/kg
• C. 320-340 mOsm/kg
• D. 275-295 mOsm/kg

Answer: (D)

Serum osmolality reflects how concentrated the solutes are in the blood, and water follows sodium, which is the main extracellular solute. Because of this, the body keeps serum osmolality in a narrow window: about 275 to 295 mOsm/kg. Glucose and urea add smaller contributions to the total osmolar load, so the balance is mostly driven by sodium and water status. In practice, you can estimate osmolality with 2 times the sodium level plus glucose divided by 18 plus BUN divided by 2.8, all with values in mg/dL for glucose and BUN. Values above the normal range indicate hyperosmolality (often from dehydration or hyperglycemia), while values below indicate hypoosmolality (from excess free water or conditions like SIADH). The normal range, 275–295 mOsm/kg, is the benchmark.

Serum osmolality reflects how concentrated the solutes are in the blood, and water follows sodium, which is the main extracellular solute. Because of this, the body keeps serum osmolality in a narrow window: about 275 to 295 mOsm/kg. Glucose and urea add smaller contributions to the total osmolar load, so the balance is mostly driven by sodium and water status.

In practice, you can estimate osmolality with 2 times the sodium level plus glucose divided by 18 plus BUN divided by 2.8, all with values in mg/dL for glucose and BUN. Values above the normal range indicate hyperosmolality (often from dehydration or hyperglycemia), while values below indicate hypoosmolality (from excess free water or conditions like SIADH). The normal range, 275–295 mOsm/kg, is the benchmark.