Saline (also known as saline solution) is a mixture of sodium chloride (salt) and water. It has several uses in medicine including cleaning wounds, removal and storage of contact lenses, and help with dry eyes. By injection into a vein, it is used to treat hypovolemia such as that from gastroenteritis and diabetic ketoacidosis. Large amounts may result in fluid overload, swelling, acidosis, and high blood sodium. In those with long-standing low blood sodium, excessive use may result in osmotic demyelination syndrome. Saline is in the crystalloid family of medications. It is most commonly used as a sterile 9 g of salt per litre (0.9%) solution, known as normal saline. Higher and lower concentrations may also occasionally be used. Saline is acidic, with a pH of 5.5 (due mainly to dissolved carbon dioxide). The medical use of saline began around 1831. It is on the World Health Organization's List of Essential Medicines. In 2023, sodium salts were the 227th most commonly prescribed medication in the United States, with more than 1 million prescriptions.
Normal concentration
Normal saline (NSS, NS or N/S) is the commonly used phrase for a solution of 0.90% w/v of NaCl, 308 mOsm/L or 9.0 g/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 physiologically normal solution). Although neither of those names is technically accurate because normal saline is not exactly like blood serum, they convey the practical effect usually seen: good fluid balance with minimal hypotonicity or hypertonicity. NS is used frequently in intravenous therapy for people who cannot take fluids orally and have developed or are in danger of developing dehydration. NS is also used for aseptic purpose. NS was typically the first fluid used when one's hypovolemia was severe enough to threaten the adequacy of blood circulation, and has long been believed to be the safest fluid to give quickly in large volumes; however, it is now known that rapid and/or high amount of NS infusion will cause metabolic acidosis, which is why other means of therapy like Ringer's lactate solution or Low Titer O Whole Blood (LTOWB) infusion have been proven to be better in the treatment of hypovolemia, especially in traumatically induced hypovolemic shock. The solution is 9 grams of sodium chloride (NaCl) dissolved in water, to a total volume of 1000 mL. The mass of 1 mL of normal saline is 1.0046 grams at 22 °C. The molar mass of sodium chloride is approximately 58.4 g/mol. A concentration of 9 g/L therefore gives 154 mmol/L. Since NaCl dissociates into two ions (sodium and chloride), 1 molar NaCl is 2 osmolar. Thus, normal saline contains 154 mEq/L of Na+ and the same amount of Cl−. This gives an osmolarity of 154 + 154 = 308, which is higher (i.e. more solute per litre) than that of blood (approximately 285). However, if the osmotic coefficient (a correction for non-ideal solutions) is taken into account, then the saline solution is much closer to isotonic. The osmotic coefficient of NaCl is about 0.93, which yields an osmolarity of 0.154 × 1000 × 2 × 0.93 = 286.44. Therefore, the osmolarity of normal saline is a close approximation to that of blood.
Usage For medical purposes, saline is often used to flush wounds and skin abrasions. However, research indicates that it is no more effective than potable tap water. Normal saline will not burn or sting when applied. Saline is also used in IV therapy, intravenously supplying extra water to rehydrate people or supplying the daily water and salt needs ("maintenance" needs) of a person who is unable to take them by mouth. Because infusing a solution of low osmolality can cause problems such as hemolysis, intravenous solutions with reduced saline concentrations (less than 0.9%) typically have dextrose (glucose) added to maintain a safe osmolality while providing less sodium chloride. The amount of normal saline infused depends largely on the needs of the person (e.g., ongoing diarrhea or heart failure). IVs and catheters (e.g. a nephrostomy) may need to be flushed with saline to prevent clogging. Saline is also often used for nasal washes to relieve some of the symptoms of rhinitis and the common cold. The solution exerts a softening and loosening influence on the mucus to make it easier to wash out and clear the nasal passages for both babies and adults. In very rare instances, fatal infection by the amoeba Naegleria fowleri can occur if it enters the body through the nose; therefore tap water must not be used for nasal irrigation. Water is only appropriate for this purpose if it is sterile, distilled, boiled, filtered, or disinfected. Sterile isotonic saline is also used to fill breast implants for use in breast augmentation surgery, to correct congenital abnormalities such as tuberous breast deformity, and to correct breast asymmetry. Saline breast implants are also used in reconstructive surgery post-mastectomy.
Eyes Eye drops are saline-containing drops used on the eye. Depending on the condition being treated, they may contain steroids, antihistamines, sympathomimetics, beta receptor blockers, parasympathomimetics, parasympatholytics, prostaglandins, non-steroidal anti-inflammatory drugs (NSAIDs), antibiotics or topical anesthetics. Eye drops sometimes do not have medications in them and are only lubricating and tear-replacing solutions.
Nose
There is tentative evidence that saline nasal irrigation may help with long term cases of rhinosinusitis. Evidence for use in cases of rhinosinusitis of short duration is unclear.
Scleral tattooing Saline is used in scleral tattooing, coloring the white part of the human eye.
Tattoo lightening
Saline is used to lighten tattoos (including microblading tattoos) through the process of osmosis.
Other concentrations Concentrations lower and higher than normal also exist. High concentrations are used rarely in medicine but frequently in molecular biology.
Hypertonic saline
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