Composition of Normal Saline (NS) 0.9%
Normal saline 0.9% contains 0.9% sodium chloride (NaCl), which equals 154 mmol/L of sodium and 154 mmol/L of chloride, with an osmolarity of 308 mOsmol/L. 1
Chemical Composition
- Sodium concentration: 154 mmol/L (or mEq/L) 1
- Chloride concentration: 154 mmol/L (or mEq/L) 1
- Total NaCl concentration: 0.9% weight/volume 1
- Osmolarity: 308 mOsmol/L, making it slightly hyperosmolar rather than truly isotonic 1
Important Clinical Context
Despite being called "normal" or "physiological" saline, 0.9% saline is not truly physiological - it contains significantly higher chloride concentrations than human plasma and has no convincing scientific basis for its routine use, with its widespread adoption appearing to be based on historical misconception rather than physiological rationale. 2
Key Physiological Discrepancies
- The sodium and chloride concentrations in 0.9% saline (154 mmol/L each) are supraphysiologic compared to normal plasma values 1
- The term "normal saline" is a misnomer - it is neither normal nor physiological in composition 2
- The reflection coefficient of the endothelial membrane for sodium is only 0.1, meaning fluid shifts occur primarily from intracellular rather than interstitial spaces 1
Clinical Implications of Composition
Adverse Effects from High Chloride Content
- Hyperchloremic metabolic acidosis is a well-documented complication of large-volume normal saline administration 1, 3
- In kidney transplant recipients, normal saline was associated with significantly more hyperkalemia (19% vs 0%) and metabolic acidosis requiring treatment (31% vs 0%) compared to balanced crystalloid solutions 3
- The high chloride load can prolong insulin infusion duration and ICU length of stay in diabetic ketoacidosis patients 4
When Balanced Crystalloids Are Preferred
- Current guidelines recommend balanced crystalloid solutions over 0.9% saline for large-volume resuscitation to avoid hyperchloremic acidosis 1, 5
- Balanced solutions should be preferred in liver surgery, septic shock, and critically ill patients 1, 6
Comparison with Other Saline Concentrations
- Half-normal saline (0.45% NaCl): 77 mmol/L sodium, used in hypernatremia or hyperglycemic crises with elevated corrected sodium 6
- Quarter-normal saline (0.225% NaCl): Approximately 38.5 mmol/L sodium, used for severe hypernatremia but associated with risk of hemolysis 7
- Hypertonic saline (3%): 513 mmol/L sodium, 1026 mOsmol/L osmolarity 1
- Hypertonic saline (7.5%): 1283 mmol/L sodium, 2566 mOsmol/L osmolarity 1
Common Clinical Pitfalls
- Do not assume 0.9% saline is "physiological" - it causes hyperchloremia with large volumes 1, 2
- Avoid in crush syndrome - use normal saline instead of potassium-containing balanced solutions, but be aware of chloride load 5
- Monitor for acidosis when using large volumes, particularly in renal transplant, DKA, or critically ill patients 4, 3