Corrected Sodium Calculation in Hyperglycemia
Add 1.6 mEq/L to the measured sodium for every 100 mg/dL of glucose above 100 mg/dL to calculate the corrected sodium. 1
The Formula
Corrected [Na⁺] = Measured [Na⁺] + 1.6 × ([Glucose in mg/dL - 100]/100) 1, 2
This correction accounts for the dilutional effect of hyperglycemia on serum sodium, as glucose draws water from the intracellular to the extracellular compartment, artificially lowering the measured sodium concentration. 3
Worked Example
For your specific case with measured sodium 130 mEq/L and glucose 500 mg/dL:
- Corrected [Na⁺] = 130 + 1.6 × ([500 - 100]/100) 1
- Corrected [Na⁺] = 130 + 1.6 × (4) 1
- Corrected [Na⁺] = 130 + 6.4 = 136.4 mEq/L 1
Why This Matters Clinically
The corrected sodium—not the measured sodium—determines your fluid choice after initial resuscitation in hyperglycemic crises. 1, 2 This distinction is critical because using measured sodium alone can lead to inappropriate fluid selection and dangerous osmolality shifts. 1
Fluid Selection Algorithm Based on Corrected Sodium:
- If corrected sodium is LOW: Continue 0.9% NaCl at 4-14 mL/kg/h 1, 2
- If corrected sodium is NORMAL or ELEVATED: Switch to 0.45% NaCl at 4-14 mL/kg/h 1, 2
In your example, the corrected sodium of 136.4 mEq/L is normal, so you would switch to half-normal saline (0.45% NaCl) after the initial hour of isotonic resuscitation. 1
Critical Safety Limit
Never allow serum osmolality to decrease faster than 3 mOsm/kg/h during correction, as this causes cerebral edema with significant mortality risk. 1, 2 This translates to approximately 8-10 mEq/L per 24 hours maximum sodium correction rate. 2
Common Pitfall to Avoid
Never use measured sodium to guide fluid choice—always calculate and use corrected sodium. 1 The measured sodium of 130 mEq/L in your example would incorrectly suggest continuing isotonic saline, when the corrected value of 136.4 mEq/L indicates hypotonic saline is appropriate. This error can worsen hypernatremia and increase osmolality too rapidly. 2
Important Distinction: Osmolality Calculation Uses Measured Sodium
While corrected sodium guides fluid therapy, effective serum osmolality is calculated using the measured (uncorrected) sodium: 2[measured Na] + glucose/18. 1 For your patient: 2(130) + 500/18 = 288 mOsm/kg. 1 This value assesses severity and monitors treatment progress. 1