American Board of Internal Medicine (ABIM) Certification Practice Exam

For every increase of 100 mg/dL in plasma glucose above what level, does serum sodium decrease by approximately 1.6 mEq/L?

1. 80 mg/dL

2. 100 mg/dL

3. 120 mg/dL

4. 140 mg/dL

The correct answer is: 100 mg/dL

The correct answer is based on the understanding of how hyperglycemia affects serum sodium levels, particularly in the context of hyperosmolality. When plasma glucose levels rise, as seen in conditions like diabetes mellitus, this elevation in glucose can draw water from cells into the extracellular fluid, diluting serum sodium levels—a phenomenon quantitatively described by the glucose-sodium relationship. For every increase of 100 mg/dL above a baseline level of plasma glucose, serum sodium levels tend to decrease by approximately 1.6 mEq/L. This relationship assumes a normal physiological environment, where 100 mg/dL is a level commonly seen in clinical settings. Therefore, 100 mg/dL serves as the pivotal reference point from which significant changes in sodium can be observed due to hyperglycemia. Understanding this principle is crucial for clinicians when evaluating electrolyte abnormalities in patients with uncontrolled diabetes or other states of hyperglycemia, as it emphasizes the importance of assessing glucose levels when interpreting sodium homeostasis. This relationship does not apply accurately to lower or excessively high baseline glucose levels, which is why the other options do not provide the same context.