Which IV fluid is isotonic with respect to osmolality?

• A. Dextrose 5% in water
• B. Normal saline
• C. Half-normal saline
• D. Hypertonic saline

Answer: (B)

Isotonic fluids have an osmolality similar to that of plasma, roughly 275–295 mOsm/kg, so they expand the intravascular space without causing water to move into or out of cells.

Normal saline (0.9% NaCl) has an osmolality around 308 mOsm/L, very close to plasma, so it remains in the vascular compartment and is considered isotonic.

Dextrose 5% in water seems isotonic in the bag, but after infusion the glucose is rapidly taken up by cells, leaving free water behind and making the solution hypotonic to plasma. Half-normal saline is about 154 mOsm/L, clearly hypotonic, and hypertonic saline has a higher osmolality than plasma and draws water out of cells.