Osmolality of Normal Saline
Normal Saline (0.9% NaCl) has an osmolarity of 308 mOsm/L, making it an isotonic solution that matches human plasma osmolality. 1
Composition and Osmolarity
Normal Saline contains:
This osmolarity precisely matches the normal human plasma osmolality range of 275-295 mOsm/kg (with an upper reference of 308 mOsm/L). 1, 2
Clinical Significance of Isotonicity
Normal Saline is classified as an isotonic solution because it does not result in osmotic shifts across cell membranes. 1 When infused intravascularly:
- Cells maintain their normal size without expansion or contraction 1
- No net vector of force acts on cells relative to the semipermeable membrane 1
- Plasma osmolality is maintained during infusion 3
Comparison to Other IV Fluids
The American Academy of Pediatrics guideline provides clear distinctions: 1
- Hypotonic solutions (e.g., 0.45% NaCl): 154 mOsm/L - causes cellular expansion
- Normal Saline (0.9% NaCl): 308 mOsm/L - isotonic, no cellular shifts
- Lactated Ringer's: 273 mOsm/L - slightly hypotonic, tends to decrease plasma osmolality 3
Evidence from Clinical Studies
Research confirms that Normal Saline maintains plasma osmolality during infusion. In a randomized trial comparing volume loading solutions, Normal Saline maintained baseline osmolality (299 ± 5 mOsm/kg at baseline vs. 298 ± 5 mOsm/kg after 2L infusion), whereas Lactated Ringer's decreased osmolality significantly. 3 This makes Normal Saline preferable in patients with blood-brain barrier damage or conditions requiring osmolality maintenance. 3
Important Clinical Context
The tonicity of Normal Saline (~300 mOsm/kg H₂O) is clinically relevant in specific conditions: 4