Which IV solution is hypotonic?

• A. 0.45% saline
• B. 0.9% normal saline
• C. Lactated Ringer's
• D. D5W

Answer: (A)

Hypotonic solutions have a lower osmolality than extracellular fluid, so they pull water from the bloodstream into cells. The 0.45% saline option has a much lower osmolarity than plasma—roughly 150 mOsm/L compared with plasma’s about 275–295 mOsm/L. Because of this, it is classified as hypotonic and causes water to shift into cells, increasing intracellular fluid. That’s the hallmark of a hypotonic IV fluid.

The other fluids listed are isotonic with plasma, meaning they primarily stay in the extracellular space (the vascular and interstitial compartments) and don’t drive water into cells as readily. D5W is a special case: while it’s labeled as isotonic in the bag, once the glucose is metabolized, it effectively becomes free water and can act hypotonically. In many contexts, the clear, classic example of a hypotonic solution among these is the half-normal saline.