Which fluids are considered isotonic with respect to body fluids?

• A. Normal saline (0.9% NaCl) and Lactated Ringer's solution
• B. Dextrose 5% in water
• C. Half-normal saline (0.45% NaCl)
• D. Hypotonic saline (0.45% NaCl with dextrose)

Answer: (A)

Fluids that are isotonic with body fluids have an osmolality similar to plasma, so they expand the extracellular fluid without causing water to shift into or out of cells. Normal saline (0.9% NaCl) has an osmolality around 308 mOsm/L, and Lactated Ringer's solution is in a similar range; both stay mainly in the intravascular space and are routinely used for fluid resuscitation because they match the body's fluid balance.

Dextrose 5% in water initially appears similar in osmolality, but once in the body, the glucose is rapidly taken up by cells, leaving free water that lowers the effective osmolality and can cause water to move into cells—so it doesn't act as isotonic in the long term. Half-normal saline (0.45% NaCl) is clearly hypotonic, with lower osmolality, leading to a shift of water into cells. Hypotonic saline with dextrose follows the same logic after the dextrose is metabolized, becoming hypotonic as free water remains.

Therefore, the fluids that are truly isotonic with body fluids are normal saline and Lactated Ringer's solution.