Question

A 38-year-old mechanic presents to the urgent care clinic complaining of severe flank pain, nausea, and blurry vision that started earlier this morning. He admits to accidentally ingesting a few mouthfuls of a liquid from an unmarked container in his garage while working late last night. His temperature is 36.8°C (98.2°F), blood pressure is 142/88 mm Hg, pulse is 98/min, and respiratory rate is 22/min. Cardiopulmonary examination is unremarkable. Initial laboratory tests reveal the following:

* Serum sodium: $138 \text{ mEq/L}$
* Serum potassium: $4.5 \text{ mEq/L}$
* Serum chloride: $104 \text{ mEq/L}$
* Serum bicarbonate: $14 \text{ mEq/L}$
* Blood urea nitrogen (BUN): $28 \text{ mg/dL}$
* Serum glucose: $126 \text{ mg/dL}$
* Measured serum osmolality: $315 \text{ mOsm/kg H}_2\text{O}$

Based on these findings, what is the patient's calculated serum osmolar gap?