Which IV fluid is isotonic relative to plasma?

• A. 0.9% NSS
• B. 0.33% NaCl
• C. Lactated Ringer's (LR)
• D. 5% H2O

Answer: (A)

Isotonic fluids have an osmolality similar to blood plasma, so they stay mainly in the extracellular fluid and expand circulating volume without causing water to move into or out of cells. The solution with 0.9% sodium chloride matches plasma osmolality and is commonly used to expand extracellular fluid, making it isotonic. The other options don’t maintain that balance: the hypotonic saline (low salt) draws water into cells; pure water has no solutes and causes cells to swell; a dextrose solution is initially isotonic but, once the glucose is metabolized, it becomes hypotonic and can shift water into cells.