Free Water Deficit Calculator
Use this free water deficit calculator to estimate the amount of free water a person is missing in cases of hypernatremia. This medical calculator helps clinicians in the correction of high sodium levels.
Free Water Deficit
What is Free Water Deficit?
The free water deficit is the total amount of water that a person with hypernatremia requires to bring their serum sodium back to normal levels. Hypernatremia is a condition characterized by abnormally high levels of sodium in the blood (more than 145meq/L). It can be triggered by dehydration, too much salt, or failure to drink enough fluids.
Our water deficit calculator has been designed to aid in the estimation of the formula for free water deficit. The tool is primarily used by medical professionals to deal with fluid and electrolyte imbalance in the human body.
How to Use the Water Deficit Calculator?
Using this calculator involves the following steps:
- Gender Selection: Pick your gender (Male or Female) because it will affect the percentage of total water in the body.
- Weight Input: Enter your weight in kilograms (kg). In case your weight is in pounds (lbs), use this conversion formula: 1 kg = 2.2 lbs.
- Serum Sodium: Enter the sodium level in milliequivalents per liter (mEq/L).
- After completing the above steps, click the button ‘Calculate Free Water Deficit’ to obtain your results.
The Free Water Deficit Formula
The free water deficit formula is given below:
Free Water Deficit = Total Body Water × (Serum Sodium / 140 - 1)
Where total body water is given by:
- Body Water (men) = 0.6 × Weight (kg)
- Body Water (women) = 0.5 × Weight (kg)
In the above formula, 140 represents the normal level of serum sodium in the human body. If one's sodium levels are above 140, they are considered to have a free water deficit. On the other hand, those with serum sodium levels below 140 have free water excess.
Example: Let's say we have a 70 kg male with a serum sodium level of 155. Then,
Total Body Water = 0.6 × 70 = 0.6 × 70 = 42 L
Free Water Deficit = 42 × (155 / 140 - 1) = 42 × (1.107 - 1) = 42 × 0.107 = 4.5 L
Hence, the person needs about 4.5 L of free water to correct their hypernatremia.
Normal Serum Sodium Levels
Based on the table below, a person's blood contains normal levels of sodium when it is between 135 and 145 mEq/L.
| Category | Serum Sodium (mEq/L) |
|---|---|
| Normal | 135 - 145 |
| Hyponatremia (low) | Below 135 |
| Hypernatremia (high) | Above 145 |
| Severe Hypernatremia | Above 155 |
Clinical Uses of Free Water Deficit
The free water deficit is primarily associated with the management of hypernatremia. It is applicable in the following areas of medicine:
- Clinical management of hypernatremia: Calculating the amount of water needed to correct high serum sodium levels.
- Dehydration: It can also be used to determine the amount of water that has been lost in cases of dehydration.
- Fluid resuscitation: Managing the rate at which fluids are administered in hospitals.
- Critical care: Free water deficit plays a role in the management of fluid and electrolyte imbalances among critically ill patients.
- Diabetes insipidus: This is a disorder in which the body is unable to control water retention. It is usually managed by a free water deficit.
Risks of Rapid Correction
The correction of a free water deficit should be done gradually. The gradual correction allows the brain to slowly adjust to the rapid decrease in osmolarity. Under normal conditions, the hypothalamus encourages the production of idiogenic osmoles in cases of hypernatremia. The osmoles then prevent excessive water from leaving the cells. However, if one increases their water levels rapidly, the brain might swell. This complication is known as osmotic demyelination. The recommended rate of decline for serum sodium is no more than 10-12meq/L/day. If you are interested in the calculation of the body surface area of a human being, you can use our body surface area calculator.
Causes of Hypernatremia (Why Free Water Deficit Happens)
As mentioned earlier, hypernatremia is not a disease. Instead, it is an indicator of either a water deficit or gain in the human body. However, one must be aware of the underlying causes of hypernatremia before determining the free water deficit. The commonest causes of hypernatremia include:
- Inadequate water intake: It is the major contributor to hypernatremia. It is particularly common among the elderly, infants, and people who are too sick to drink fluids.
- Increased water loss: Excessive loss of fluids through vomiting, diarrhea, and having a high fever. In some cases, the condition can be brought about by large amounts of fluid loss from the skin due to severe burns.
- Diabetes insipidus: This happens when the kidneys are unable to concentrate the urine. As a result, a person will pass a large amount of urine.
- Osmotic diuresis: It occurs when there is an increase in the amount of glucose in the blood. This condition is common among diabetics.
- Increased salt intake: It is uncommon but can be brought about by too much salt in the diet or in the intravenous fluids in the hospital.
How is Free Water Deficit Administered? (Enteral vs. Parenteral)
After the calculation of the free water deficit, the fluid administration route becomes significant. The choice of enteral or parenteral fluid administration is a clinical decision made by the attending physician:
- Oral/enteral route: In this case, water is administered orally or through a feeding tube. This is the preferred method because it is comfortable and safe.
- Intravenous route: It is given when a person is unconscious, semi-conscious, or too ill to swallow. The fluid is given directly into the bloodstream through a catheter. Dextrose 5% in water (D5W) and half normal saline (0.45% NaCl) are the most common fluids used in this procedure.
- Fluid monitoring: A person's serum sodium levels should be monitored to ensure that the fluid is being administered at the right rate.
Special Populations to Consider
Some populations are more susceptible to water deficits and require special consideration when using the free water deficit formula:
- The elderly: They are at the highest risk of developing hypernatremia because of their low perception of thirst and reduced kidney function. Free water deficit is, therefore, critical in this population.
- Infants and children: They have an elevated metabolism and larger body surface area. Due to these factors, they lose fluids faster than adults. The free water deficit calculation for children is slightly different from that of adults.
- Athletes: They are likely to experience excessive sweating during exercise. Unless they are hydrated appropriately, they may suffer from exercise-associated hyponatremia. It is important to differentiate between exercise-associated hyponatremia and hypernatremia before using our calculator.
- Patients with neurological problems: Stroke patients and those with mental illness have a higher chance of developing fluid imbalances due to the inability to express thirst.