How is corrected sodium calculated in hyperglycemia?

• A. Corrected Na+ = measured Na+ - 1.6 mEq/L for every 100 mg/dL of glucose above normal.
• B. Corrected Na+ = measured Na+ plus 0.5 mEq/L for every 100 mg/dL above normal.
• C. Corrected Na+ = measured Na+ + 1.6 mEq/L for every 100 mg/dL of glucose above normal.
• D. Corrected Na+ equals measured Na+. No adjustment.

Answer: (C)

When glucose is high, water shifts from inside cells into the extracellular space, diluting the plasma sodium. The sodium level you measure can look lower than it really is because of this dilution. To estimate the true sodium, you add a correction that accounts for the amount of glucose elevation. The standard rule is to add 1.6 mEq/L of sodium for every 100 mg/dL that glucose is above the normal level (about 100 mg/dL). So the corrected sodium is measured Na+ plus 1.6 times the number of 100 mg/dL increments above normal glucose.

For example, if you measure a Na+ of 132 mEq/L when glucose is 280 mg/dL, the glucose excess is 180 mg/dL, which is 1.8 increments of 100 mg/dL. The correction is 1.8 × 1.6 ≈ 2.9, so the corrected Na+ ≈ 134.9 mEq/L. This approach explains why the sodium appears low and provides a more accurate sense of the patient’s true sodium status.

The other options either subtract the adjustment, use a wrong correction factor, or apply no correction at all, which would misrepresent the actual sodium level.