How to Calculate Serum Osmolarity

Written by gabriel dockery | 13/05/2017
How to Calculate Serum Osmolarity
Calculation of serum osmolarity requires a blood test. (blood donation image by robert mobley from

Serum osmolarity is a measure of the concentration of materials in the blood against the overall mass of the blood being measured. Substances such as sodium, chloride, potassium, other ions, and urea are found in varying quantities in the blood. The kidneys filter the blood and keep the levels of these substances within a small variance. Changes in this variance of osmolarity signify one of many problems from kidney failure to diabetes. The normal range of serum osmolarity is between 285-295 mOsm/L (milliosmoles per litre).

Multiply the amount of sodium within the specimen by two.

Divide the amount of glucose by 18.

Divide the amount of blood urea nitrogen by 2.8.

Add all three products together. This is the serum osmolarity of the specific blood sample per litre.

Multiply the resulting sum by 0.995. This converts the serum osmolality to serum osmolarity. Osmolality is a measure of serum osmotic concentration in terms of mass, while osmolarity is a measure of serum osmotic concentration per volume. This final calculation multiplies the osmolality by the mass density of blood times the result of the density of blood plasma (1.025 g/ml) minus the anhydrous solute concentration (0.03g/ml) to produce osmolarity.

Things you need

  • Blood panel showing the relative levels of sodium, glucose, blood urea nitrogen

Show MoreHide

By using the site, you consent to the use of cookies. For more information, please see our Cookie policy.