Which IV solution is isotonic?

• A. 0.45% saline
• B. D5W
• C. 0.9% normal saline
• D. 3% saline

Answer: (C)

IV fluid osmolality determines where fluid shifts occur in the body. Isotonic solutions have an osmolality close to plasma, so they stay mainly in the intravascular space and expand circulating volume without causing cells to swell or shrink. The solution that matches plasma osmolality and is commonly used to rapidly restore circulating volume is normal saline. It effectively increases fluid volume in the bloodstream, which is ideal for dehydration or hypovolemia.

The other options behave differently: a hypotonic solution has fewer solutes than plasma and can cause water to move into cells, risking cellular edema; a hypertonic solution has more solutes and pulls water out of cells; and a dextrose-containing solution becomes free water after the dextrose is metabolized, so it behaves as hypotonic in the body.