Free Water Deficit Calculator – Hypernatremia Correction
Calculate free water deficit for hypernatremia correction
This calculator is for healthcare professionals only and not a substitute for clinical judgment.
Table of Contents
How to Use
- Enter patient weight in kilograms or pounds
- Enter patient age
- Select patient sex
- Enter current serum sodium level (mEq/L)
- Enter desired sodium level (typically 140 mEq/L)
- Click calculate to see free water deficit
What is Free Water Deficit?
Free water deficit is the calculated amount of water needed to correct hypernatremia (elevated serum sodium). Hypernatremia occurs when sodium levels exceed 145 mEq/L and indicates a relative water deficit compared to sodium content.
The calculation is based on total body water (TBW), which varies by sex, age, and body composition. TBW is approximately 60% of body weight in men, 50% in women, and decreases with age and obesity.
Calculation Formula
Free Water Deficit (L) = TBW × [(Current Na / Desired Na) - 1]
Where TBW is calculated as:
- Men: 0.6 × body weight (kg)
- Women: 0.5 × body weight (kg)
- Elderly men (≥65): 0.5 × body weight (kg)
- Elderly women (≥65): 0.45 × body weight (kg)
Correction Guidelines
Safe sodium correction is critical to prevent complications:
- Maximum correction rate: 10-12 mEq/L per 24 hours
- For chronic hypernatremia: correct over 48 hours minimum
- Monitor sodium levels every 2-4 hours during correction
- Consider underlying causes (diabetes insipidus, dehydration, etc.)
- Account for ongoing losses and insensible water losses
- Use hypotonic fluids (5% dextrose in water, half-normal saline)
Clinical Considerations
- This calculator provides an estimate only
- Individual patient factors may require adjustments
- Monitor neurological status during correction
- Too rapid correction can cause cerebral edema
- Consider underlying conditions and comorbidities
- Adjust for ongoing fluid losses or gains
Frequently Asked Questions
- What causes hypernatremia?
- Hypernatremia results from water loss exceeding sodium loss (dehydration, diabetes insipidus, osmotic diuresis) or excess sodium intake. Common causes include inadequate water intake in elderly or hospitalized patients, excessive sweating, diarrhea, and certain medications.
- Why is slow correction important?
- Rapid correction of chronic hypernatremia can cause cerebral edema due to osmotic shifts. Neurons adapt to high sodium by generating organic osmolytes, and rapid correction causes water to rush into brain cells, potentially causing seizures, brain damage, or death.
- What fluids are used for correction?
- Hypotonic fluids like 5% dextrose in water (D5W) or half-normal saline (0.45% NaCl) are typically used. The choice depends on the degree of hypernatremia, volume status, and glucose levels. D5W provides pure free water after glucose metabolism.
- How do I monitor correction?
- Check serum sodium every 2-4 hours during active correction. Monitor vital signs, neurological status, and urine output. Adjust infusion rates based on sodium trends. Consider ICU monitoring for severe hypernatremia (>160 mEq/L).
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