Which solution may be referred to both isotonic and hypotonic depending on the context?

The choice of 5% Dextrose in Water (D5W) as the solution that can be referred to as both isotonic and hypotonic is based on its behavior in the body once administered. Initially, D5W is considered isotonic when it is in the bag, with an osmolarity similar to that of blood plasma.

However, once D5W is infused and metabolized, the glucose is rapidly utilized by the cells for energy, leaving behind free water. This unbound water concentration causes a decrease in the overall osmolarity of the remaining solution in the bloodstream, making it hypotonic compared to plasma.

Thus, in a clinical context, D5W is recognized as isotonic upon administration, but it acts hypotonic during its physiological effects because it dilutes plasma osmolarity after glucose is metabolized. This dual characterization is crucial for understanding fluid and electrolyte management in various medical settings, particularly in patients requiring careful monitoring of fluid balance.