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What is Free Water Deficit?

By Solomon Branch
Updated May 17, 2024
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If there is not enough free water in the body, water that is not bound to any organelles or macromolecules, it can cause an excess of sodium in the blood, a condition known as hypernatremia. The free water deficit is the amount of water that the body is lacking that has brought about the hypernatremia. Hypernatremia can cause a variety of symptoms, and the severity of symptoms primarily depends on the underlying cause.

Hypernatremia is an excess of sodium in the blood, but is more typically due to a lack of free water than excess consumption of sodium. Normally, if there is even a small change in the amount of sodium in the blood, the thirst response will kick in and cause someone to consume water. Most of the time, hypernatremia is caused by people who can’t consume water on demand, such as the very old, infants, or people with disabilities. Not treating an underlying problem that causes a patient to lose a lot of water, such as excessive diarrhea or a disease like diabetes insipidus, can also cause hypernatremia.

There is a standard calculation to determine the amount of free water needed to reach the proper balance of sodium in the blood. Blood samples are taken to determine the ratio of sodium to blood, usually expressed as the amount of sodium in milligrams per deciliter of blood (NA+ mg/dl). Once this is found, that value is divided by a reference number that is indicative of the normal amount of sodium in the blood, usually 140 mg/dl, minus one.

The free water deficit is then calculated by taking the resulting value and multiplying it by the total weight of the body in kilograms. It is then further multiplied by a reference number, typically 0.6 for men and 0.5 for women, to attain the free water deficit. The reference number is multiplied by the body weight because it determines how much total body water there is. Using a man as an example, the formula would then look like this: 0.6 x Body weight (kg) x (current Na+ level/140 – 1) = free water deficit.

Once the value is found, the proper amount of water can be delivered either orally or intravenously. The key factor in rehydrating someone with hypernatremia is to go at the proper pace. If done too quickly, it can cause damage to the body, in particular the cells of the brain. Brain cells can easily swell and this can lead to seizures, brain damage, or death. The procedure to correct a free water deficit is best done in a hospital setting or with an experienced medical practitioner.

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