Water Deficit Calculator
Estimate the free water deficit in patients with hypernatremia.
Patient Details
Calculation Results
Total Body Water
0 L
Water Deficit
0 L
This tool is intended for use by healthcare professionals. It is not a substitute for clinical judgment.
About the Water Deficit Calculator
This calculator is a clinical tool for healthcare professionals to estimate the free water deficit in patients with hypernatremia (high serum sodium). Accurately calculating the water deficit is a critical first step in formulating a safe and effective fluid repletion plan. This tool simplifies the process, helping to guide appropriate treatment strategies.
Formula Explained
The calculator uses the widely accepted Adrogue-Madias formula to estimate the water deficit:
- Estimate Total Body Water (TBW): TBW is estimated as a percentage of body weight. This percentage varies by sex and age, typically around 60% for men and 50% for women.
- Calculate Water Deficit: The deficit is then calculated based on the difference between the patient's current serum sodium and the desired normal level (140 mEq/L).
How to Safely Address a Water Deficit
Correcting hypernatremia must be done carefully to avoid neurological complications. Here are the key principles:
Correct Slowly
The serum sodium level should be lowered gradually. A typical goal is a correction rate of 0.5 mEq/L per hour, not to exceed 8-12 mEq/L in the first 24 hours.
Choose the Right Fluid
Free water is typically replaced using hypotonic fluids, such as 5% dextrose in water (D5W) or half-normal saline (0.45% NaCl), either intravenously or orally if the patient can drink.
Monitor Frequently
Serum sodium levels should be monitored every 2-4 hours initially to ensure the rate of correction is safe and appropriate, adjusting the fluid rate as needed.
Address the Underlying Cause
It is crucial to identify and treat the root cause of the water loss (e.g., fever, diabetes insipidus, gastrointestinal losses) to prevent recurrence.
Frequently Asked Questions
What is a water deficit? →
A water deficit, in a medical context, refers to the amount of free water that needs to be replaced in a patient to correct hypernatremia (high sodium levels in the blood). It represents the volume of water lost from the body that has led to the concentration of sodium.
What is hypernatremia? →
Hypernatremia is an electrolyte imbalance characterized by a high concentration of sodium in the blood, typically defined as a serum sodium level above 145 mEq/L. It is usually caused by a net loss of water or, less commonly, an excess of sodium.
How is the water deficit calculated? →
The water deficit is calculated using a standard formula: Water Deficit (L) = Total Body Water (TBW) × [(Serum Sodium / 140) - 1]. Total Body Water is estimated as a percentage of body weight (typically 60% for men and 50% for women). This calculator uses this formula to provide an estimate.
Why is it important to correct a water deficit slowly? →
Correcting hypernatremia too quickly can be dangerous and lead to cerebral edema (swelling of the brain), which can cause seizures, permanent brain damage, or death. The goal is to lower the serum sodium level gradually, typically by no more than 0.5 mEq/L per hour, or 8-12 mEq/L over a 24-hour period.
What is Total Body Water (TBW)? →
Total Body Water (TBW) is the total amount of fluid in a person's body. It is estimated as a percentage of their body weight. For this calculation, standard values are used: 60% for adult men, 50% for adult women, and slightly lower percentages for elderly individuals, as body water content decreases with age.