What is normal saline, and is it ‘normal’?
‘Saline’ (also known as saline solution) is a mixture of sodium chloride (salt, NaCl) and water. The medical use of saline began around 1831.
Saline is in the crystalloid family of fluids. It is most commonly used as a sterile solution of 9g of salt per litre (0.9%). It is then known as ‘normal saline’. Higher and lower concentrations may also occasionally be used.
As an intravenous fluid, it is used to treat dehydration. Being hyperosmolar to the blood (see below) it will increase the sodium and chloride in the blood.
Large amounts may result in fluid overload, swelling, hypertension, acidosis, and high blood sodium and chloride.
Also, more worryingly, in those with long-standing hyponatraemia (low blood sodium; especially chronic), excessive use of normal saline may result in brain damage caused by central pontine myelinolysis (osmotic demyelination syndrome).
It has a number of other uses in medicine, including cleaning wounds, removal and storage of contact lenses, and help with dry eyes.
But is normal saline really ‘normal’?
No. It is quite abnormal actually.
Normal saline is:
- Hyperosmolar – 308 mOsm/L (normal is 285-295 mOsm/L). Less commonly, this solution is referred to as ‘physiological saline’ or ‘isotonic saline’ (because it is approximately isotonic to blood serum, which makes it a moderately physiological solution). Although neither of those names is technically accurate because normal saline is not exactly like blood serum
- Hypertonic – to blood (because of above)
- Hypernatraemic – as it contains NaCl
- Hyperchloraemic. Ditto
- Hypertensive – it is used to get BP up
- Acidic – with a pH of 5.5 (due mainly to dissolved carbon dioxide). It causes (or worsens) metabolic acidosis.
Alternatives to normal saline
- Other crystalloids:
- Isotonic fluids – e.g. Hartmann’s solution (popular with anaesthetists)
- 0.45% (half normal) saline (hypotonic) – not often used
- 5% dextrose (hypotonic) – this is effectively IV water, and so can cause hyponatraemia. For this reason, many with give it as:
- Alternating IV normal saline with 5% dextrose – common usage
- Colloids.
Conclusion
‘Normal’ saline is a misnomer. It is far from normal. It is a hyperosmolar/hypernotic, hypernatraemic, hyperchloraemic, hypertensive solution that causes (or worsens) metabolic acidosis. Thus it should only be used by health professionals who know what is in it, and its effects when given IV as a treatment.
Note. Osmolality is the concentration of particles dissolved in a body fluid. It is based on the number of particles in a specific amount of fluid.
Other resources
0.9% saline is neither normal nor physiological (Li, 2016)
Acid-base balance (Hopkins, 2022)
