American Board of Internal Medicine (ABIM) Certification Practice Exam

Which of the following correctly describes plasma osmolality calculation?

1. 2 × Serum Potassium + BUN/2.8

2. 2 × Serum Sodium + BUN/2.8

3. 2 × Serum Sodium + Plasma Glucose/18 + BUN/18

4. 2 × Serum Sodium + Plasma Glucose/18 + BUN/2.8

The correct answer is: 2 × Serum Sodium + Plasma Glucose/18 + BUN/2.8

The calculation of plasma osmolality is important in clinical practice for assessing various conditions such as dehydration, hyponatremia, and other electrolyte imbalances. The correct formula for estimating plasma osmolality is: 2 × Serum Sodium + Plasma Glucose/18 + BUN/2.8 This formula takes into account the contributions of sodium, glucose, and urea nitrogen to the total osmolality of plasma. Serum sodium is multiplied by 2 because sodium ions are typically the most significant contributors to the osmolality of plasma. The division of glucose by 18 and BUN by 2.8 allows for their concentrations to be converted from mg/dL to mmol/L. By including both glucose and BUN in the formula, this calculation recognizes that these substances also play a role in the osmolality of the plasma. If glucose levels are elevated, this will increase osmolality, and the same applies to elevated BUN levels. The other options do not provide the correct application of the components needed for accurate plasma osmolality assessment. For instance, some of them inaccurately handle the units or fail to include all the necessary components, such as glucose or use incorrect conversion factors. Therefore, the analytical structure and the