Question

30 y/F, 70 kg man with serum sodium = 120 mEq/L. Calculate total sodium deficit.

• A. 420
• B. 840
• C. 1400
• D. 280

Hint:

Use the formula: Na deficit = 0.6 x weight (kg) x (desired Na - actual Na) with desired Na = 140 mEq/L.

Answer 1

The Correct Option is B

Step 1: Recall the sodium deficit formula.
Sodium deficit = Total Body Water (TBW) x (Desired Na⁺ - Actual Na⁺)

Step 2: Calculate TBW.
TBW = 0.6 x body weight (kg) [using the factor applied in the PDF explanation].
TBW = 0.6 x 70 = 42 litres

Step 3: Apply the formula.
Desired Na⁺ = 140 mEq/L (normal serum sodium)
Actual Na⁺ = 120 mEq/L
Sodium deficit = 0.6 x 70 x (140 - 120)
= 42 x 20
= 840 mEq

Step 4: Conclusion.
Total sodium deficit = 840 mEq. Correction should not exceed 8-10 mEq/L per day to avoid osmotic demyelination syndrome (central pontine myelinolysis).