Glucose is a simple sugar that is readily transported through your bloodstream and used by your cells for fuel. If your blood glucose increases significantly – a condition called hyperglycemia – it acts like a chemical sponge, drawing water from inside your cells and diluting the sodium outside your cells. While this does not change the total amount of sodium in your body, it can interfere with its accurate measurement. "The Merck Manual of Diagnosis and Therapy" reports that for every 100 mg/dL your blood glucose rises above normal, your sodium level falls about 1.6 mEq/L.
Subtract 200 – the upper limit of normal blood glucose – from your blood glucose reading. For example, if your blood glucose reading is 350 mg/dL, the difference between your blood glucose and normal – your “glucose excess” – is 350 minus 200, or 150.
Determine your “dilution factor” by dividing your glucose excess by 100. If your glucose excess is 150, your dilution factor is 150 divided by 100, or 1.5.
Multiply your dilution factor by 1.6 to find your “sodium deficit.” If your dilution factor is 1.5, your sodium deficit is 1.6 multiplied by 1.5, or 2.4.
Add your sodium deficit to your serum sodium measurement to determine your corrected sodium level. If your sodium deficit is 2.4 and your serum sodium level is 135 mEq/L, your corrected sodium is 135 plus 2.4, or 137.4 mEq/L.
If your blood glucose is measured in mmol/L, you can still find your corrected sodium. Subtract 11 – the upper limit of normal – from your blood glucose level, divide the result by 5.55 to obtain your dilution factor, then multiply your dilution factor by 1.6 and add the result to your measured sodium level to determine your corrected sodium. The Society for Biomedical Diabetes Research offers a conversion table for switching between mmol/L and mg/dL.
Hyperglycemia causes a shift in body fluids from one "compartment" to another, leading to dilution of the sodium in your bloodstream. Taking extra sodium is not useful in this situation. If you correct your hyperglycemia, your body fluids will return to their usual state and your sodium level will return to normal.