Expected Sodium Increase with 100 mL of 3% Hypertonic Saline
In a 70-kg adult with a serum sodium of 120 mmol/L, administering 100 mL of 3% hypertonic saline typically raises serum sodium by approximately 2 mmol/L. 1
Calculation Method
The Adrogue-Madias formula provides a theoretical estimate: Change in serum Na = (Infusate Na − Serum Na) / (Total Body Water + 1), where 3% saline contains 513 mEq/L sodium and TBW = 0.5 × body weight (kg) for adults 2
For a 70-kg patient: TBW = 0.5 × 70 = 35 L, so predicted increase = (513 − 120) / (35 + 1) = approximately 11 mEq per liter of 3% saline, or roughly 1.1 mEq per 100 mL 2
Real-world data shows the actual increase is closer to 2 mmol/L per 100 mL bolus, as demonstrated in a prospective observational trial of 58 patients with severe symptomatic hyponatremia (mean baseline sodium 114 mEq/L) 1
Clinical Application
For severe symptomatic hyponatremia (seizures, altered mental status), administer 100 mL of 3% saline over 10 minutes, repeating up to three times at 10-minute intervals until symptoms resolve 2
Target correction is 6 mmol/L over the first 6 hours or until severe symptoms abate, which typically requires 300 mL of 3% saline (three 100-mL boluses) 2
Check serum sodium 4–6 hours after each bolus to guide further dosing and prevent overcorrection 2, 3
Critical Safety Limits
Never exceed 8 mmol/L correction in any 24-hour period to prevent osmotic demyelination syndrome 2
High-risk patients (cirrhosis, alcoholism, malnutrition) require even more cautious correction at 4–6 mmol/L per day maximum 2
Monitor for spontaneous water diuresis by checking urine specific gravity every 4 hours; a sudden drop ≥0.010 signals risk of overcorrection and mandates immediate sodium rechecking 1
Important Caveats
The Adrogue-Madias formula systematically underestimates the actual sodium rise because it assumes stable fluid balance and does not account for ongoing losses or gains 2
Patients with SIADH may develop brisk water diuresis once hypertonic saline is administered, leading to unexpectedly rapid sodium correction; concurrent desmopressin (1–2 µg IV every 6–8 hours) can prevent this overcorrection 4
Hypovolemic patients may absorb sodium more efficiently than predicted, while hypervolemic patients (heart failure, cirrhosis) may show blunted responses 2
Renal function, volume status, and ongoing fluid administration all influence the final sodium increment, making frequent monitoring essential 1, 3