Hyperglycemia lowers serum sodium levels; the degree to which has been updated from the previous dogma of:

Conventional Units: For each 100 mg/dL (over 100 mg/dL) the blood glucose rises; the measured serum sodium should have 1.6 mEq/L added to it in order to correct for the effect of the hyperglycemia.

SI Units: For each 5.6 mmol/L (over 5.6 mmol/L) the blood glucose rises; the measured serum sodium should have 1.6 mmol/L added to it in order to correct for the effect of the hyperglycemia.

In an interesting trial where the effects of hyperglycemia on sodium levels were actually measured, Hillier, et al. demonstrate that the correction factor of 1.6 is inaccurate and leads to serious underestimation of serum sodium levels; especially in those patients with blood glucose concentrations of >500 mg/dL. The clinical information from this; study, is that a factor of 2.4 is more appropriate.

We are now utilizing the 2.4 adjustment factor. The formulas for each are below:

Conventional Units:

Glucose = mg/dL Sodium = mEq/L For each 100 mg/dL (greater than 100 mg/dL) the blood glucose rises; the measured serum sodium should have 2.4 mEq/L added to it in order to correct for the effect of the hyperglycemia.

• Corrected Serum Sodium = Measured Serum Sodium + [(Glucose measured - 100)/100]×2.4

SI Units: For each 5.6 mmol/L (greater than 5.6 mmol/L) the blood glucose rises; the measured serum sodium should have 2.4 mmol/L added to it in order to correct for the effect of the hyperglycemia.

• Corrected Serum Sodium = Measured Serum Sodium + [(Glucose measured - 5.6)/5.6]×2.4

Reference:

• Hillier TA, Abbott RD, Barrett EJ. Hyponatremia: evaluating the correction factor for hyperglycemia. Am J Med. 1999;106(4):399-403.
• Huffman, GB. Adjusting Sodium Levels in Patients with Hyperglycemia. Am Fam Phy. 1999;60(6):1798.