The dilution illusion

Glucose stays mostly outside cells and pulls water with it. When blood glucose climbs into the hundreds, water floods from cells into the bloodstream and dilutes everything dissolved there, sodium included. The lab faithfully reports the diluted concentration, but the low number is partly an artifact. Once insulin brings glucose back down, water returns to cells and sodium rises without a single grain of salt being added. Treating the falsely low reading with extra sodium would overshoot.

The 1.6 rule and its challenger

Katz described the correction in 1973. For every 100 mg/dL of glucose above 100, add 1.6 mEq/L to the measured sodium. Decades later Hillier reanalyzed larger datasets and argued the true factor sits closer to 2.4, especially at extreme glucose levels. Many hospital protocols still use 1.6 while some nephrologists prefer the higher figure, so this calculator shows the classic correction and the article flags the debate. Either way, the direction never changes. High glucose always hides some sodium.

Why the distinction matters

In diabetic ketoacidosis and hyperosmolar states, corrected sodium separates two very different problems. A patient whose sodium normalizes after correction needs fluids and insulin. One whose sodium stays high after correction has a profound free water deficit that insulin alone will not fix, and management shifts accordingly. The number also forecasts risk, since a rising corrected sodium during treatment warns that fluids are falling behind.

A worked example

A glucose of 600 with a measured sodium of 130 corrects to 130 plus 1.6 times 5, which is 138. That patient is not hyponatremic at all. The same glucose with a measured 140 corrects to 148, flagging true hypernatremia that demands attention alongside the sugar.

References

• Katz MA. Hyperglycemia induced hyponatremia. N Engl J Med 1973
• Hillier TA et al. Hyponatremia evaluating the correction factor for hyperglycemia. Am J Med 1999