Nate’s Corner: Where Does the Hyperglycemia Correction Factor for Sodium Come From?

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Margaret DeOliveira, MD
Renal Fellow Network Co-Editor

Sodium correction for hyperglycemia is a common topic but what is the physiology behind it? When patients are hyperglycemic, the serum osmolarity is higher as a result of glucose being an effective osmole. This elevation in osmolarity results in water shifting from the intracellular to  extracellular space. Though there is a shift, the total body water has not changed –  neither has the actual amount of sodium. Thus, the serum sodium reported in a basic metabolic panel is in fact the concentration of extracellular sodium based on the current glucose level. Once hyperglycemia has improved, the serum sodium returns to within the normal range. Thus, keep serum glucose in mind when a patient presents with mild hyponatremia. 

The correction factor used to determine the serum sodium based on hyperglycemia has changed over the years. Dr. Katz commented on this in 1973, illustrating that the correction factor should be 1.6 meq/L decrease in serum sodium for every 100 mg/dL glucose above 400 mg/dL – based on the fact that the serum osmolarity will never completely equilibrate. In 1999, a study demonstrated that the correction factor is not a 1.6 meq/L decrease in serum sodium, but actually a 2.4 meq/L decrease for every 100 mg/dL glucose above 400 mg/dL based on six healthy subjects. These subjects were given somatostatin to block endogenous insulin production, 20% dextrose infusion, and finally insulin was given. Labs were drawn every 10 minutes.

Figure 1. Panels A & B illustrate the relationship between serum sodium and glucose levels.

The figure above illustrates the relationship between serum glucose and serum sodium from this study, with the solid black line demonstrating linear regression (Panel A) with a slope of 2.4 sodium per 100 mg/dL glucose. Panel B demonstrates that the correction factor of 1.6 meq/L works well up to 400 mg/dL, but is considerably higher for glucose levels over 400 mg/dL. This is how we’ve arrived at our current correction factor today! 

Reviewed by Samira Farouk, Matthew A. Sparks

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