A 35-year-old automobile mechanic is evaluated in the emergency department for altered mental status. His spouse reports finding him confused and stumbling in their unventilated garage. Vital signs show a blood pressure of 132/82 mm Hg, a pulse of 98/min, and a respiratory rate of 26/min. He appears lethargic and is oriented only to person.
Laboratory studies reveal the following:
| Laboratory Study | Result |
|---|---|
| Sodium | 140 mEq/L |
| Potassium | 4.5 mEq/L |
| Chloride | 105 mEq/L |
| Bicarbonate | 12 mEq/L |
| Blood urea nitrogen (BUN) | 14 mg/dL |
| Creatinine | 1.2 mg/dL |
| Glucose | 90 mg/dL |
The patient's measured serum osmolality is 330 mOsm/kg H₂O.
What is this patient's calculated serum osmolar gap (in mOsm/kg H₂O)?
Answer: 40 mOsm/kg H₂O
Answer
The patient's calculated serum osmolar gap is 40 mOsm/kg H₂O.
The patient presents with altered mental status and a high anion gap metabolic acidosis (Anion Gap = ), raising high clinical suspicion for toxic alcohol ingestion, such as ethylene glycol or methanol. To further evaluate this, the serum osmolar gap must be calculated. The osmolar gap is the difference between the measured serum osmolality (provided by the laboratory) and the calculated serum osmolality.
Using conventional units (mg/dL), the calculated osmolality is determined by the formula:
Substituting the patient's values:
Next, the osmolar gap is computed:
A normal osmolar gap is typically less than 10 mOsm/kg H₂O. A significantly elevated osmolar gap in the setting of a high anion gap metabolic acidosis strongly supports the presence of an unmeasured osmotically active substance, aligning with a diagnosis of toxic alcohol poisoning.
Using conventional units (mg/dL), the calculated osmolality is determined by the formula:
Substituting the patient's values:
Next, the osmolar gap is computed:
A normal osmolar gap is typically less than 10 mOsm/kg H₂O. A significantly elevated osmolar gap in the setting of a high anion gap metabolic acidosis strongly supports the presence of an unmeasured osmotically active substance, aligning with a diagnosis of toxic alcohol poisoning.
Step-by-Step Solution
Key Concept
Calculation of the serum osmolar gap and its utility in identifying toxic alcohol ingestion.