Sodium deficit calculation for goal serum sodium increase
Measured serum sodium is 125 mmol/L and the desired serum sodium is 135 mmol/L. [1]
- Change in serum sodium (ΔNa) = 135 − 125 = 10 mmol/L (10 mEq/L). [1]
Total body water (TBW) is estimated as 0.6 × body weight for newborn fluid-electrolyte calculations that aim to reduce risk of overly rapid sodium shifts. [2]
- TBW = 0.6 × 2.5 kg = 1.5 L. [2]
Sodium deficit (mEq) is calculated as TBW × (desired Na − current Na). [1]
- Sodium deficit = 1.5 L × 10 mEq/L = 15 mEq of sodium. [1]
Required 3% NaCl volume for sodium deficit correction
3% NaCl contains 513 mEq of sodium per liter (≈0.513 mEq/mL). [3]
Required volume of 3% NaCl (mL) = sodium deficit (mEq) ÷ 0.513 (mEq/mL). [3]
- Volume = 15 mEq ÷ 0.513 mEq/mL = 29 mL (rounded). [3]
Practical cross-check using a neonatal dosing rule
A neonatal dosing rule states that 1 mL/kg of 3% NaCl increases serum sodium by ~1 mmol/L. [4]
- Estimated volume = 10 mmol/L × 2.5 kg = 25 mL. [4]
Correction limits requiring reassessment during administration
After symptoms resolve, the infusion should be adjusted so that serum sodium increases by no more than 8–10 mmol/L per day. [4]
Final computed quantities for the stated example
- Sodium deficit to reach 135 mmol/L from 125 mmol/L (using TBW = 0.6 × weight): 15 mEq. [1]
- Required 3% NaCl volume using 3% NaCl = 513 mEq/L: ~29 mL. [3]
- Alternate neonatal rule-of-thumb cross-check: ~25 mL. [4]
Safety note for symptomatic hyponatremia management
Frequent serum sodium monitoring is required because overly rapid correction increases risk of osmotic demyelination. [4]