Sodium Deficit Calculation Formula
The sodium deficit is calculated using the formula: Sodium deficit (mEq) = Desired increase in Na (mEq/L) × (0.5 × ideal body weight in kg). 1
Understanding the Formula Components
- Desired increase in Na represents the target correction amount, typically 6 mEq/L over 6 hours for severe symptomatic hyponatremia, or the difference between current and target sodium levels 1, 2
- The multiplier 0.5 represents the estimated total body water as a fraction of body weight in adults, accounting for the distribution volume of sodium 1
- Ideal body weight in kg should be used rather than actual body weight, particularly in obese patients where actual weight would overestimate the sodium deficit 1
Critical Correction Rate Limitations
Regardless of the calculated deficit, total sodium correction must never exceed 8 mmol/L in 24 hours to prevent osmotic demyelination syndrome. 1, 2, 3
- For severe symptomatic hyponatremia (seizures, altered mental status, coma), the initial target is 6 mmol/L correction over 6 hours or until symptoms resolve 1, 2
- After achieving 6 mmol/L correction in the first 6 hours, only 2 mmol/L additional correction is permitted in the remaining 18 hours 1
- High-risk patients (advanced liver disease, alcoholism, malnutrition, severe hyponatremia <120 mmol/L) require even slower correction at 4-6 mmol/L per day, maximum 8 mmol/L in 24 hours 1, 3
Practical Application Example
For a 70 kg patient with sodium of 118 mEq/L requiring correction to 124 mEq/L:
- Desired increase = 124 - 118 = 6 mEq/L
- Sodium deficit = 6 mEq/L × (0.5 × 70 kg) = 6 × 35 = 210 mEq 1
Treatment Implementation Based on Calculated Deficit
- For severe symptomatic hyponatremia: Administer 3% hypertonic saline (513 mEq/L sodium) as 100-150 mL boluses, which provides approximately 50-75 mEq sodium per bolus 2, 4
- For mild-moderate symptomatic hyponatremia: Use oral sodium chloride 100 mEq three times daily combined with fluid restriction to 1 L/day 1, 5
- For hypovolemic hyponatremia: Isotonic saline (154 mEq/L) at 15-20 mL/kg/h initially, then 4-14 mL/kg/h based on response 1
Essential Monitoring Requirements
- Check serum sodium every 2 hours during initial correction for severe symptoms 1, 2
- Switch to every 4-hour monitoring after severe symptoms resolve 1, 2
- Recalculate deficit and adjust therapy if correction rate approaches or exceeds 8 mmol/L in 24 hours 1, 4
Common Pitfalls to Avoid
- Never use the formula to justify rapid overcorrection - the 8 mmol/L per 24-hour limit supersedes any calculated deficit 1, 3
- Do not use actual body weight in obese patients - this overestimates total body water and sodium deficit 1
- Avoid applying this formula to hypervolemic hyponatremia (cirrhosis, heart failure) where fluid restriction, not sodium supplementation, is primary therapy 1, 6
- Never correct chronic hyponatremia (>48 hours duration) at rates exceeding 1 mmol/L/hour except in acute severe symptomatic cases 1, 4