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.