A 34-year-old man is brought to the emergency department after being found unresponsive in… — USMLE Step 2 CK

A 34-year-old man is brought to the emergency department after being found unresponsive in his garage. Emergency medical personnel found a half-empty container of commercial antifreeze nearby. His blood pressure is 145/90 mm Hg, pulse is 112/min, and respirations are 24/min. Physical examination reveals deep, rapid breathing.

Laboratory studies show the following:
Serum sodium: 142 mEq/L
Serum potassium: 4.5 mEq/L
Serum chloride: 104 mEq/L
Serum bicarbonate: 12 mEq/L
Blood urea nitrogen: 28 mg/dL
Serum creatinine: 1.4 mg/dL
Serum glucose: 126 mg/dL
Measured serum osmolality: 345 mOsm/kg $H_2O$

Arterial blood gas on room air shows a pH of 7.22 and $pCO_2$ of 26 mm Hg.

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

Answer: 44 mOsm/kg

Answer

The patient's calculated serum osmolar gap is 44 mOsm/kg.

The correct osmolar gap is derived by subtracting the calculated serum osmolality from the measured serum osmolality. Calculated osmolality is found using the formula:

2 \times Sodium + (BUN / 2.8) + (Glucose / 18)

. For this patient,

2(142) + (28 / 2.8) + (126 / 18) = 284 + 10 + 7 = 301

mOsm/kg. Subtracting this expected value from the measured osmolality of 345 mOsm/kg yields a gap of 44 mOsm/kg, strongly suggesting toxic alcohol (ethylene glycol) poisoning.

Step-by-Step Solution

Calculate the patient's expected serum osmolality using the standard formula.

Calculated Osmolality =

284 + 10 + 7 = 301

mOsm/kg.

Serum osmolality is primarily determined by sodium and its accompanying anions, urea, and glucose. The formula

(2 \times Na) + (BUN / 2.8) + (Glucose / 18)

converts US units (mg/dL) to SI units (mmol/L) to find the total expected osmoles.

Subtract the calculated expected osmolality from the measured osmolality provided in the laboratory results.

345 - 301 = 44

mOsm/kg.

The osmolar gap represents unmeasured osmotically active particles in the blood. A gap greater than 10-15 mOsm/kg is highly suggestive of toxic alcohol ingestion, such as ethylene glycol.

Key Concept

Diagnosis of toxic alcohol ingestion using the osmolar gap, calculated by subtracting the expected osmolality from the measured osmolality.

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