True or False: D5W is isotonic in the container but becomes hypotonic after metabolism.

• A. True
• B. False
• C. Always isotonic during infusion
• D. Never isotonic

Answer: (A)

Dextrose 5% in water looks isotonic when prepared and infused, because the dextrose contributes osmotic particles similar to plasma at that moment. But once the dextrose is metabolized by cells, it is converted to CO2 and water, leaving behind mostly water. That reduces the osmolar load of the solution, making it hypotonic relative to plasma. So the fluid is effectively isotonic in the container but becomes hypotonic after metabolism. This is why D5W isn’t used as a reliable isotonic maintenance fluid, even though its bag content appears isotonic.