American Board of Internal Medicine (ABIM) Certification Practice Exam

How is plasma osmolality (mOsm/kg H2O) calculated?

1. Serum sodium (mEq/L) + glucose (mg/dL)/18

2. 2 × serum sodium (mEq/L) - blood urea nitrogen (mg/dL)/2.8

3. 2 × serum sodium (mEq/L) + plasma glucose (mg/dL)/18 + blood urea nitrogen (mg/dL)/2.8

4. Serum sodium (mEq/L) + blood urea nitrogen (mg/dL)/2.8

The correct answer is: 2 × serum sodium (mEq/L) + plasma glucose (mg/dL)/18 + blood urea nitrogen (mg/dL)/2.8

The calculation of plasma osmolality is crucial in understanding various clinical scenarios, especially in the context of fluid and electrolyte management. The correct formula incorporates the contributions of different solutes in the plasma, specifically sodium, glucose, and blood urea nitrogen (BUN). Option C is the correct answer as it provides a comprehensive formula for calculating plasma osmolality: it includes two times the serum sodium concentration, the plasma glucose concentration adjusted for milligrams per deciliter, and BUN adjusted in a similar way. The reasoning behind this is as follows: 1. **Sodium** is a major determinant of plasma osmolality; hence, it is essential to multiply the serum sodium by two to account for its dissociation into two ions (Na+ and Cl−). 2. **Glucose** contributes to osmotic pressure, and converting glucose from mg/dL to mOsm/kg requires dividing it by 18, as 1 mEq of glucose corresponds to 18 mg. 3. **BUN** also contributes but is less significant compared to sodium and glucose; hence, it is included with a conversion factor of 2.8 which converts its measurement into an equivalent of mOsm. This formula reflects the actual