An unresponsive 52-year-old male is transported to the triage area by paramedics after bei… — USMLE Step 2 CK

An unresponsive 52-year-old male is transported to the triage area by paramedics after being found in a mechanic's garage. According to bystanders, he had been drinking an unidentified liquid from a jug. His temperature is 36.5°C (97.7°F), blood pressure is 150/92 mm Hg, pulse is 108/min, and respirations are 24/min and deep. He responds only to painful stimuli.

Laboratory studies show the following:
Sodium: $139$ mEq/L
Potassium: $4.8$ mEq/L
Chloride: $104$ mEq/L
Bicarbonate: $10$ mEq/L
Blood urea nitrogen (BUN): $14$ mg/dL
Creatinine: $1.8$ mg/dL
Glucose: $90$ mg/dL
Measured serum osmolality: $318$ mOsm/kg $H_2O$

Arterial blood gas (room air):
pH: $7.22$
$PCO_2$ : $23$ mm Hg
$PO_2$ : $98$ mm Hg

Based on the provided laboratory data, what is the patient's serum osmolar gap?

Answer: 30 mOsm/kg

Answer

The patient presents with an unexplained high anion gap metabolic acidosis (anion gap =

139 - (104 + 10) = 25

) and altered mental status in a context suspicious for toxic alcohol ingestion (e.g., ethylene glycol). To confirm the presence of an unmeasured osmotically active substance, the serum osmolar gap must be calculated. The standard formula for calculated serum osmolality is

2 \times [Na^+] + \frac{BUN}{2.8} + \frac{Glucose}{18}

. Plugging in the patient's laboratory values yields

2(139) + \frac{14}{2.8} + \frac{90}{18} = 278 + 5 + 5 = 288

mOsm/kg. The osmolar gap is the measured osmolality minus the calculated osmolality:

318 - 288 = 30

mOsm/kg. A normal osmolar gap is typically less than 10 mOsm/kg; an elevated gap of 30 mOsm/kg strongly supports the diagnosis of toxic alcohol poisoning.

Step-by-Step Solution

Calculate the expected serum osmolality.

288

mOsm/kg

Expected osmolality must be determined using the formula

2 \times [Na^+] + \frac{BUN}{2.8} + \frac{Glucose}{18}

to serve as a baseline for identifying unmeasured osmoles.

Determine the osmolar gap.

30

mOsm/kg

The osmolar gap is the difference between the measured serum osmolality and the calculated expected serum osmolality (

318 - 288

Key Concept

Calculation of the serum osmolar gap to evaluate high anion gap metabolic acidosis associated with suspected toxic alcohol ingestion.

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