Is there a difference? Osmolarity vs. Osmolality..

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

The difference between the calculated osmolarity and measured osmolality is called the “osmolar gap” and is often used as part of differential diagnostic evaluation of patients. If the concentration of solutes in the given fluid is very low, then osmolarity and osmolality are considered to be equivalent.

When all body fluids have an osmolarity near 300 mOsm/L, the osmotic pressure of the two fluid compartments are equal = no net water movement occurs. This is called iso-osmotic or isotonic.

  • IV fluid solutions  270-300 mOsm/L = Isotonic (eg 0.9% Sodium Chloride)
  • IV fluid solutions  >300 mOsm/L = Hypertonic (eg D2.5% in Water)
  • IV fluid solutions <270 mOsm/L = Hypotonic (eg D5% 0.9%Sodium Chloride)

Hypotonic solutions will move water into the cell, causing the cell to swell and potential burst. By lowering the serum osmolarity, the body fluids shift out of the  blood vessels into the interstitial tissue and cells. Hypotonic solutions hydrate the cells and can deplete the circulatory system.

Hypertonic solutions conversely cause the water from within a cell to move to the ECF compartment, causing the cell to shrink. These solutions are used to replace electrolytes. Hypertonic dextrose solutions when used alone, shifts ECF from interstitial to plasma.

As infusion nurses, we know anything with an osmolarity of greater than 600, is not recommended for peripheral IV infusion.

So now you know….is it osmolarity or osmolality???

Erstad BL. Osmolarity and Osmolality: narrowing the terminology gap. Pharmacotherapy 2003;23(9):1085-1086