In hyperglycemia, the measured serum sodium concentration decreases by approximately how much for every 100 mg/dL increase in plasma glucose?

• A. 1.6 mmol/L
• B. 2.0 mmol/L
• C. 0.8 mmol/L
• D. 2.4 mmol/L

Answer: (A)

When glucose in the blood is elevated, water shifts from inside cells to the extracellular space to balance the osmotic difference. This extra water dilutes the sodium in the serum, making the measured sodium appear lower than it truly is. The practical correction used in clinical practice is that for every 100 mg/dL increase in plasma glucose above normal, the measured serum sodium decreases by about 1.6 mmol/L. This is the common, widely used factor for adjusting Na in hyperglycemia.

So, if glucose rises by 100 mg/dL, expect roughly a 1.6 mmol/L drop in measured sodium; with larger glucose elevations, multiply accordingly (for example, a 200 mg/dL rise would lower Na by about 3.2 mmol/L). A simple way to estimate the true sodium level is: corrected Na ≈ measured Na + 1.6 × ((glucose − 100) / 100). While some sources cite 2.0 or 2.4, 1.6 is the standard value used here.