A 42-year-old man is brought to the emergency department by ambulance after being found unresponsive in his garage. His wife reports that he has a history of major depressive disorder and recently lost his job. On physical examination, he is unarousable. Blood pressure is 115/75 mm Hg, pulse is 112/min, and respirations are 24/min and deep.
Laboratory studies show:
Serum sodium: 140 mEq/L
Serum potassium: 4.6 mEq/L
Serum chloride: 105 mEq/L
Serum bicarbonate: 10 mEq/L
Blood urea nitrogen (BUN): 28 mg/dL
Serum creatinine: 1.8 mg/dL
Serum glucose: 108 mg/dL
Measured serum osmolality: 320 mOsm/kg H2O
What is this patient's serum osmolar gap in mOsm/kg H2O?
Answer: 24 mOsm/kg H2O
Answer
The correct osmolar gap is 24 mOsm/kg H2O.
The serum osmolar gap is the difference between the measured serum osmolality and the calculated serum osmolality. The calculated osmolality is determined using the formula: . For this patient, the calculated osmolality is mOsm/kg H2O. The measured osmolality is 320 mOsm/kg H2O. Therefore, the osmolar gap is mOsm/kg H2O. A normal osmolar gap is generally mOsm/kg H2O. An elevated osmolar gap in the setting of a high anion gap metabolic acidosis (anion gap = 140 - (105 + 10) = 25 mEq/L) strongly suggests toxic alcohol ingestion, such as ethylene glycol or methanol.
Step-by-Step Solution
Key Concept
Calculation of the serum osmolar gap in the evaluation of high anion gap metabolic acidosis and suspected toxic alcohol ingestion.