Composition of Normal Saline 0.9%
Normal Saline (NS) 0.9% is composed of 154 mmol/L of sodium (Na+) and 154 mmol/L of chloride (Cl-), with an osmolarity of 308 mOsm/L and a pH range of 5-6.5. 1
Detailed Electrolyte Composition
- Sodium concentration: 154 mmol/L 1
- Chloride concentration: 154 mmol/L 1
- No other electrolytes: Unlike balanced crystalloids, 0.9% NaCl contains no potassium, calcium, magnesium, bicarbonate, or buffer substances 1
- Osmolarity: 308 mOsm/L, making it isotonic (within the 280-310 mOsm/L range) 1
- pH: 5-6.5, which is acidic compared to physiologic pH of 7.4 1
Comparison to Plasma
When compared to normal plasma composition, 0.9% saline is notably different:
- Sodium: NS contains 154 mmol/L versus plasma's 142 mmol/L (approximately 8% higher) 1
- Chloride: NS contains 154 mmol/L versus plasma's 103 mmol/L (approximately 50% higher) 1
- Lacks physiologic electrolytes: Plasma contains potassium (4 mmol/L), calcium (2.4 mmol/L), magnesium (1 mmol/L), and bicarbonate (27 mmol/L), none of which are present in NS 1
Clinical Implications of Composition
The supraphysiologic chloride content has important clinical consequences:
- Hyperchloremic metabolic acidosis: Large volumes of 0.9% saline can cause this condition due to the excessive chloride load 1
- Renal effects: The high chloride concentration may cause renal vasoconstriction and increase the risk of acute kidney injury 1
- Not truly "normal" or "physiological": Despite its name, the composition bears little resemblance to plasma and has minimal scientific basis for being called "normal" 2
Alternative Formulations
For context, balanced crystalloids have compositions closer to plasma:
- Lactated Ringer's: Contains Na+ 130 mmol/L, Cl- 108 mmol/L, K+ 4 mmol/L, Ca2+ 0.9 mmol/L, Mg2+ 3 mmol/L, and lactate 27.6 mmol/L 1
- Plasma-Lyte: Contains Na+ 140 mmol/L, Cl- 98 mmol/L, K+ 5 mmol/L, Mg2+ 1 mmol/L, acetate 27 mmol/L 1
Important Caveats
- Sodium overload risk: When used to dilute drugs or keep catheters open, 0.9% saline is a major source of iatrogenic hypernatremia in intensive care settings 3
- Historical misconception: The widespread use of 0.9% saline appears to be based on historical fallacy rather than physiologic rationale 2
- Hemolysis concerns: Normal saline causes substantially more in vitro hemolysis than balanced solutions during red cell washing and storage 4