Saline (medicine)

In medicine, saline is a general term referring to a sterile solution of sodium chloride (table salt) in water, frequently used for intravenous infusion, rinsing contact lenses, and nasal irrigation. Saline solutions are available in various formulations for different purposes. Salines are also used in cell biology, molecular biology and biochemistry experiments. Normal saline is a solution of 0.9% w/v of NaCl (this nomenclature is confusing - "normal sodium chloride" to a chemist means a concentration of 5.85% w/v, also expressed as 1M NaCl_(aq) ). It contains 154 mEq/L of Na⁺ and Cl⁻. It has a slightly higher degree of osmolality (i.e. more solute per liter) compared to blood (hence, though it is referred to as being isotonic with blood in clinical contexts, this is a technical inaccuracy), about 300 mOsm/L. Normal saline (NS) is therefore used frequently in intravenous drips (IVs) for patients who cannot take fluids orally and have developed severe dehydration. Normal saline is typically the first fluid used when dehydration is severe enough to threaten the adequacy of blood circulation and is the safest fluid to give quickly in large volumes. Physiological saline is 9g NaCl dissolved in 1 liter water. The molecular weight of sodium chloride is approximately 58 g/mole, so 58g NaCl is 1 mole. Since saline contains 9 gram NaCl, the concentration is 9g/L divided by 58g/mole = 0.154 mole/L. Since NaCl dissociates into two ions - sodium and chloride - 1 molar NaCl is 2 osmolar.

Additional recommended knowledge

Daily Visual Balance Check

What is the Correct Way to Check Repeatability in Balances?

Essential Laboratory Skills Guide

Other concentrations of saline are frequently used for other medical purposes, such as supplying extra water to a dehydrated patient or supplying the daily water and salt needs ("maintenance" needs) of a patient who is unable to take them by mouth. Because infusing a solution of low osmolality can cause problems, intravenous solutions with reduced saline concentrations typically have dextrose (glucose) added to maintain a safe osmolality while providing less sodium chloride. As the molecular weight (MW) of dextrose is greater, this has the same osmolality as normal saline but contributes less sodium to the circulation. Because dextrose monohydrate (MW 198 in contrast to MW 180 for glucose) is the commercial form of dextrose used in these preparations, 5% dextrose actually contains only 4.5 g/dL of glucose.

Concentrations commonly used include

1. Half-normal saline (0.45% NaCl), often with "D5" (5% dextrose), contains 77 mEq/L of Na and Cl and 4.5 g/L glucose.
2. Quarter-normal saline (0.22% NaCl) has 39 mEq/L of Na and Cl and always contains 5% dextrose for osmolality reasons.
3. Dextrose (glucose) 4% in 0.18% saline is used sometimes for maintenance replacement.

The amount of normal saline infused depends largely on the needs of the patient (e.g. ongoing diarrhea or heart failure) but is typically between 1.5 and 3 litres a day for an adult.

Common types of salines in medicine include:

• Lactated Ringer's solution
• Acetated Ringer's solution

And in cell biology, the above, as well as the following are used:

• Phosphate buffered saline (PBS) (recipes from Dulbecco = D-PBS, Galfre, Kuchler, Ausubel etc.)
• TRIS-buffered saline (TBS) (recipes from Goldsmith, Ausubel etc.)
• Hank's balanced salt solution (HBSS)
• Earle's balanced salt solution (EBSS)
• Standard saline citrate (SSC)
• HEPES-buffered saline (HBS) (recipes from Dittmar, Liu, Ausubel etc.)

See also

• Intravenous therapy
• Oral rehydration therapy