American Board of Internal Medicine (ABIM) Certification Practice Exam

What is the approximate decrease in serum sodium levels for every 100 mg/dL increase in plasma glucose above 100 mg/dL?

1. 0.5 mEq/L

2. 1.0 mEq/L

3. 1.6 mEq/L

4. 2.0 mEq/L

The correct answer is: 1.6 mEq/L

The approximate decrease in serum sodium levels with an increase in plasma glucose levels is a vital concept in understanding how hyperglycemia affects sodium concentration. When plasma glucose levels exceed 100 mg/dL, for every 100 mg/dL rise in glucose, there's a corresponding decrease in serum sodium of about 1.6 mEq/L. This is primarily due to the phenomenon known as "glucose-induced hyponatremia," which occurs because of osmotic effects; as the glucose level rises, water moves into the extracellular space to dilute the glucose concentration, leading to a dilutional drop in serum sodium. This relationship emphasizes the importance of monitoring sodium levels in patients with hyperglycemia and understanding that serum sodium may not accurately reflect true body sodium stores in such cases. Recognizing this can prevent misinterpretation of hyponatremia in diabetic patients. The choices offer variations of the expected drop in sodium levels, with 1.6 mEq/L being the most commonly accepted value based on clinical observations and studies that examine plasma glucose's effect on serum sodium concentrations.