Sodium Correction Formula
To increase serum sodium by 1 mEq/L, you need approximately 0.5 mEq/kg of sodium per 1 mEq/L desired increase in serum sodium concentration.
The Calculation Formula
The neurosurgical guidelines provide the specific formula for calculating sodium deficit 1:
Sodium deficit = Desired increase in Na (mEq/L) × (0.5 × ideal body weight in kg)
This means:
- For a 70 kg patient wanting to increase sodium by 1 mEq/L: 1 × (0.5 × 70) = 35 mEq of sodium needed
- The 0.5 factor represents the estimated total body water as a fraction of body weight (conservative estimate)
Practical Application with 3% Hypertonic Saline
3% NaCl contains 513 mEq/L of sodium
Using the formula above:
- 100 mL of 3% NaCl = approximately 51 mEq sodium
- In clinical practice, 100 mL of 3% NaCl typically raises serum sodium by approximately 2 mEq/L in an average adult 2
- This translates to roughly 50 mL of 3% NaCl per 1 mEq/L increase in serum sodium
Critical Correction Rate Limits
Never exceed these correction rates to avoid osmotic demyelination syndrome:
- Acute symptomatic hyponatremia: Correct 6 mEq/L over first 6 hours OR until severe symptoms resolve 1
- Maximum 24-hour correction: 8 mEq/L total 1
- If you correct 6 mEq/L in first 6 hours: Only increase by 2 mEq/L more over the remaining 18 hours 1
Important Caveats
Monitor for overcorrection risk factors:
- Check serum sodium within 6 hours of any bolus administration 3
- Monitor urine specific gravity every 4 hours - a sudden decrease ≥0.010 indicates water diuresis and risk of overcorrection 2
- Do not re-administer hypertonic saline until serum sodium is <155 mEq/L 3
- Overcorrection occurs more frequently in severely symptomatic patients (38% vs 6% in moderate symptoms) 4
The formula uses 0.5 as a conservative multiplier because it assumes total body water is 50% of body weight. In reality, this varies by sex, age, and body composition, but using 0.5 prevents overcorrection 1.
Chronic hyponatremia requires slower correction - rapid correction at >1 mEq/L/hour should be reserved only for acute (<48 hours) and severely symptomatic cases (seizures, coma) 1.