What is the formula to estimate the serum sodium increase when giving hypertonic saline to a 70‑kg man with a serum sodium of 120 mmol/L, and what are the safe limits for correcting hyponatremia?

Hyponatremia Correction Formula and Safe Limits

For a 70-kg man with serum sodium of 120 mmol/L receiving hypertonic saline, use the Adrogue-Madias formula to estimate sodium increase: Change in serum Na = [(Infusate Na - Serum Na) / (Total Body Water + 1)], where 3% saline contains 513 mmol/L sodium and TBW = 0.5 × body weight for men, yielding an expected increase of approximately 1 mmol/L per 100 mL of 3% saline in this patient. 1, 2

Calculation Formula

The Adrogue-Madias formula provides the most accurate prediction for sodium correction:

• Change in serum Na (mmol/L) = [(Infusate Na concentration - Patient's serum Na) / (Total Body Water + 1)] 2
• For this 70-kg man: TBW = 0.5 × 70 kg = 35 liters 1
• 3% hypertonic saline contains 513 mmol/L sodium 1
• Expected change per liter = (513 - 120) / (35 + 1) = 393/36 = approximately 10.9 mmol/L per liter of 3% saline 2
• Per 100 mL bolus: approximately 1.1 mmol/L increase 1, 3

The formula has been validated and predicts changes with relative accuracy in most patients, though special attention is needed in volume-depleted patients after euvolemia restoration 2.

Safe Correction Limits

The absolute maximum correction rate is 8 mmol/L in any 24-hour period to prevent osmotic demyelination syndrome, with even slower rates (4-6 mmol/L per day) required for high-risk patients. 4, 1

Standard Risk Patients

• Maximum 8 mmol/L in 24 hours 4, 1, 5
• Target rate: 4-8 mmol/L per day 4, 1
• Do not exceed 10-12 mmol/L in 24 hours under any circumstances 4, 5

High-Risk Patients (Advanced Liver Disease, Alcoholism, Malnutrition, Prior Encephalopathy)

• Maximum 4-6 mmol/L per day 4, 1
• Absolute ceiling of 8 mmol/L in 24 hours 4, 1
• These patients have 0.5-1.5% risk of osmotic demyelination syndrome even with careful correction 4

Severe Symptomatic Hyponatremia (Seizures, Coma, Altered Mental Status)

• Initial goal: 6 mmol/L increase over first 6 hours OR until symptoms resolve 1, 3
• After symptom resolution, total correction must still not exceed 8 mmol/L in 24 hours 1, 3
• If 6 mmol/L corrected in first 6 hours, only 2 mmol/L additional correction allowed in next 18 hours 1

Practical Hypertonic Saline Dosing

For severe symptomatic hyponatremia, administer 250 mL of 3% saline as initial bolus, which is more effective than 100 mL without increasing overcorrection risk. 3

• 250 mL bolus of 3% saline: 52% of patients achieved ≥5 mmol/L rise within 4 hours 3
• 100 mL bolus of 3% saline: Only 32% achieved target rise (p=0.018) 3
• Both volumes had identical 21% overcorrection rates 3
• Can repeat 100 mL boluses every 10 minutes up to three times if needed for severe symptoms 1

Monitoring Requirements

Check serum sodium every 2 hours during initial correction for severe symptoms, then every 4 hours after symptom resolution. 1

• Severe symptoms: Every 2 hours initially 1
• After symptom resolution: Every 4 hours 1
• Asymptomatic/mild symptoms: Every 4-6 hours during active correction 1
• Continue frequent monitoring for at least 24-48 hours 1

Strategy to Prevent Overcorrection

Concurrent administration of desmopressin (1-2 mcg IV every 6-8 hours) with hypertonic saline prevents inadvertent overcorrection from spontaneous water diuresis. 5

• This combination achieved mean correction of 5.8 ± 2.8 mmol/L in first 24 hours 5
• No patient exceeded 12 mmol/L in 24 hours or 18 mmol/L in 48 hours 5
• Prevents unpredictable water diuresis that commonly causes overcorrection 5

Critical Pitfalls to Avoid

• Never correct chronic hyponatremia faster than 8 mmol/L in 24 hours - this causes osmotic demyelination syndrome with dysarthria, dysphagia, oculomotor dysfunction, and quadriparesis appearing 2-7 days later 4, 1
• Never correct to normonatremia acutely - target is 125-130 mmol/L, not normal values 6
• Never use the formula without accounting for ongoing losses or gains - the formula assumes stable fluid balance 2
• Never ignore high-risk features - patients with liver disease, alcoholism, malnutrition require 4-6 mmol/L per day maximum 4, 1

If Overcorrection Occurs

Immediately discontinue hypertonic saline and administer D5W (5% dextrose in water) or desmopressin to relower sodium levels. 4, 1

• Target: bring total 24-hour correction back to ≤8 mmol/L from starting point 1
• Desmopressin can slow or reverse rapid sodium rise 4
• Tromethamine may reduce osmotic demyelination risk 4