Sodium Deficit Calculation
The sodium deficit is calculated using the formula: Sodium deficit (mEq) = [Desired sodium (mEq/L) - Current sodium (mEq/L)] × Total Body Water (TBW), where TBW = 0.5 × ideal body weight in kg for most patients. 1
Understanding the Formula Components
Total Body Water (TBW) is estimated as 0.5 (or 50%) of ideal body weight in kilograms for the standard calculation, representing the proportion of body weight that is water 1, 2
The desired increase in sodium is multiplied by the estimated TBW to determine the total sodium deficit that needs correction 1
For example, if a 70 kg patient has a current sodium of 120 mEq/L and you want to increase it to 126 mEq/L: Sodium deficit = (126 - 120) × (0.5 × 70) = 6 × 35 = 210 mEq 1
Critical Correction Rate Limitations
The maximum correction should never exceed 8 mmol/L in 24 hours to prevent osmotic demyelination syndrome, regardless of the calculated deficit 1, 3, 4
For severe symptomatic hyponatremia (seizures, coma), the initial goal is correction of 6 mmol/L over 6 hours or until symptoms resolve, then limit additional correction to only 2 mmol/L in the following 18 hours 1, 3
High-risk patients (advanced liver disease, alcoholism, malnutrition, prior encephalopathy) require even slower correction at 4-6 mmol/L per day, maximum 8 mmol/L in 24 hours 1, 4
Practical Application Based on Clinical Scenario
For severe symptomatic hyponatremia: Administer 3% hypertonic saline as 100-150 mL boluses, which provides approximately 50 mEq of sodium per 100 mL, and monitor sodium every 2 hours 1, 3, 5
For asymptomatic or mild hyponatremia: Calculate the deficit but correct gradually over days, not hours, using oral sodium supplementation (100 mEq three times daily) or fluid restriction depending on volume status 1
The formula provides an estimate only—actual sodium response depends on ongoing losses, fluid intake, ADH activity, and renal function 6
Important Caveats
The Adrogué-Madias and similar predictive equations have significant variability (root mean square error 4.79-6.37 mmol/L) and cannot reliably predict sodium levels over 12-30 hour periods 6
Frequent monitoring (every 2-4 hours during active correction) is essential because calculated deficits do not account for ongoing physiologic changes 1, 6
The 0.5 factor for TBW may need adjustment: use 0.6 for children and young men, 0.45 for elderly patients and women, and 0.4 for elderly women 2
Never rely solely on calculated deficits to guide therapy—clinical response and frequent sodium measurements must drive treatment adjustments 1, 6