A 24-year-old man is brought to the emergency department by paramedics after being found u… — USMLE Step 2 CK

A 24-year-old man is brought to the emergency department by paramedics after being found unresponsive in his garage. An open bottle of antifreeze was found nearby. His temperature is $36.5^\circ\text{C}$ ( $97.7^\circ\text{F}$ ), blood pressure is $110/70\text{ mm Hg}$ , pulse is $112\text{/min}$ , and respirations are $28\text{/min}$ and deep. Physical examination reveals a comatose patient with no focal neurologic deficits.

Laboratory studies show the following:
Serum sodium: $138\text{ mEq/L}$
Serum potassium: $5.0\text{ mEq/L}$
Serum chloride: $104\text{ mEq/L}$
Serum bicarbonate: $10\text{ mEq/L}$
Blood urea nitrogen: $14\text{ mg/dL}$
Serum creatinine: $1.2\text{ mg/dL}$
Serum glucose: $90\text{ mg/dL}$
Measured serum osmolality: $315\text{ mOsm/kg H}_2\text{O}$

Arterial blood gas on room air:
pH: $7.20$
pCO $_2$ : $24\text{ mm Hg}$
pO $_2$ : $95\text{ mm Hg}$

What is this patient's serum osmolar gap? (Enter the value only)

Cevap: 29 mOsm/kg

Cevap

The patient's serum osmolar gap is 29 mOsm/kg.

The serum osmolar gap is calculated by subtracting the calculated serum osmolality from the measured serum osmolality. The calculated serum osmolality is

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

. For this patient, the calculated osmolality is

2 \times 138 + \frac{90}{18} + \frac{14}{2.8} = 286 \text{ mOsm/kg}

. The osmolar gap is

315 - 286 = 29 \text{ mOsm/kg}

. A normal osmolar gap is typically less than 10 mOsm/kg. An elevated osmolar gap in the setting of a high anion gap metabolic acidosis is highly suggestive of toxic alcohol ingestion (e.g., ethylene glycol or methanol).

Adım Adım Çözüm

Identify the need to calculate the osmolar gap.

The patient has a high anion gap metabolic acidosis and suspected toxic alcohol ingestion, which requires calculation of the osmolar gap to support the diagnosis.

Ethylene glycol and methanol are unmeasured osmotically active substances that increase the osmolar gap.

Calculate the expected serum osmolality.

Expected Osmolality =

2 \times 138 + \frac{90}{18} + \frac{14}{2.8} = 276 + 5 + 5 = 286 \text{ mOsm/kg H}_2\text{O}

The calculated serum osmolality accounts for the primary normal osmotically active solutes in the blood: sodium, glucose, and urea.

Calculate the osmolar gap.

Osmolar Gap = Measured Osmolality - Calculated Osmolality =

315 - 286 = 29 \text{ mOsm/kg}

The difference between measured and calculated osmolality represents the concentration of unmeasured osmoles (e.g., ethylene glycol).

Anahtar Kavram

The serum osmolar gap is used to detect the presence of unmeasured osmotically active substances (such as toxic alcohols) in the blood. It is calculated as the difference between the measured serum osmolality and the calculated serum osmolality.

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