Why Normal Saline Is Called "Normal"
Normal saline (0.9% NaCl) is called "normal" due to historical misconception rather than physiological accuracy, as it is neither truly isotonic to human plasma nor physiologically normal in composition. 1
Historical Origin of the Term "Normal"
The term "normal" in normal saline has a complex historical background:
- The use of saline solutions originated during the cholera pandemic of 1831, though the solutions used then bore little resemblance to today's 0.9% saline 2
- The term "normal" likely stems from laboratory chemistry terminology, where a "normal" solution contains one gram-equivalent weight of solute per liter 1
- The designation appears to have been based on Hamburger's in vitro studies of red cell lysis rather than actual physiological compatibility 2
Composition and Physiological Considerations
Normal saline has several key characteristics that make it non-physiological:
- Contains equal concentrations of sodium (154 mmol/L) and chloride (154 mmol/L) 1
- Has an osmolarity of 308 mOsmol/L, making it slightly hyperosmolar compared to plasma 1
- Despite being called "normal," it is actually slightly hyperosmolar, not iso-osmotic 1
Comparison with Human Plasma:
| Solution | Osmolarity (mOsmol/L) | Sodium (mmol/L) | Chloride (mmol/L) |
|---|---|---|---|
| 0.9% Normal Saline | 308 | 154 | 154 |
| Human Plasma | ~290 | 135-145 | 95-105 |
Clinical Implications
The non-physiological nature of normal saline has important clinical consequences:
- Large volume infusion can cause hyperchloremic metabolic acidosis 3, 4
- May negatively impact vascular and renal function compared to balanced crystalloids 3
- Guidelines specifically mandate its use for certain procedures despite these concerns:
Modern Perspective
Recent research has increasingly questioned the continued widespread use of normal saline:
- Balanced crystalloid solutions (like Lactated Ringer's or Plasma-Lyte A) may be safer alternatives for many applications 3
- Studies show that balanced fluids may lead to faster resolution of conditions like diabetic ketoacidosis compared to normal saline 5
- Despite its limitations, normal saline remains widely used due to historical precedent, availability, and established protocols 6
Key Takeaways
- The term "normal" in normal saline is a historical misnomer that persists despite being physiologically inaccurate
- Normal saline is neither truly isotonic to human plasma nor balanced in its electrolyte composition
- While still widely used, growing evidence suggests that balanced crystalloid solutions may be preferable for many clinical applications
- The continued use of normal saline should be based on specific clinical indications rather than historical convention