Osmotic strength, osmolality, and osmolarity.

The confusion that often exists between osmolality and osmolarity is discussed, and the clinical importance of careful distinction between these terms is emphasized. The clinical effects of abnormal osmotic strength of parenteral solutions, methods of determining osmolality and osmolarity, and the practical applications of these concepts are reviewed. Although osmolality can be determined experimentally (whereas osmolarity cannot) it is difficult or impossible to use osmolality in calculations associated with the extemporaneous preparation and administration of fluids such as i.v. admixtures. Hence, there is a need in many cases for product labels to state both osmolality and osmolarity.