These terms have always been confusing and perhaps like many other nurses, sometimes, I can’t figure out which one to use. In infusion therapy, particularly with Fluids and Electrolytes and IV solutions, these two terms certainly have important roles and understanding each term will be helpful to us.
Osmolarity and Osmolality are both units of measurements. What these two terms measure and calculate is the osmotic activity. Remember, osmotic activity is when one solution , usually lower in conductivity or mineral content, passes through a semi-permeable barrier for the purpose of diluting the concentration of the solution on the other side, usually containing a higher conductivity or mineral content.
The measurements are expressed as:
Osmoles and milliosmoles – to describe the solute particles contained in a solution
Osmolarity is the number of milliosmoles/liter (mOsm/L) of solution. It is the concentration of an osmotic solution. This is the common bedside calculation used in clinical settings for osmotic activity.
Example: Plasma and other body fluid osmolarity: 270 – 300 mOsm/L
Osmolality is the number of milliosmoles/ kg (mOsm/kg ) of solvent. It is the concentration of the particles that is dissolved in a fluid. This is the clinical laboratory measurement using an osmometer.
Example: Urine osmolality: extreme range of 50 – 1400 mOsm/kg water, but average is about 500 – 800 mOsm. After an overnight fast, the urine osmolality should be at least 3 times the serum osmolality.
Example: Serum osmolality: 282 – 295 mOsm/kg water