A 62-year-old man is admitted to the intensive care unit with septic shock secondary to se… — USMLE Step 2 CK

A 62-year-old man is admitted to the intensive care unit with septic shock secondary to severe pneumonia. He remains hypotensive despite adequate intravenous fluid resuscitation and requires vasopressor support. Laboratory studies are obtained to evaluate his acid-base status.

Laboratory Parameter	Value
Sodium ( $Na^+$ )	$138 \text{ mEq/L}$
Potassium ( $K^+$ )	$4.2 \text{ mEq/L}$
Chloride ( $Cl^-$ )	$98 \text{ mEq/L}$
Bicarbonate ( $HCO_3^-$ )	$12 \text{ mEq/L}$

Based on the provided laboratory results, what is the patient's calculated serum anion gap?

Answer: 28 mEq/L

Answer

The serum anion gap is calculated using the formula

AG = Na^+ - (Cl^- + HCO_3^-)

. Substituting the patient's values yields

138 - (98 + 12) = 138 - 110 = 28 \text{ mEq/L}

. An elevated anion gap (normal is typically 8-12 mEq/L) in this patient with septic shock suggests a high anion gap metabolic acidosis, most likely due to lactic acidosis from poor tissue perfusion.

Step-by-Step Solution

Identify the formula for serum anion gap.

AG = Na^+ - (Cl^- + HCO_3^-)

The anion gap is essential for determining the presence and etiology of a metabolic acidosis.

Substitute the patient's laboratory values into the formula.

AG = 138 - (98 + 12)

Sodium is the primary measured extracellular cation, while chloride and bicarbonate are the primary measured extracellular anions.

Calculate the sum of the measured anions and subtract from the measured cation.

138 - 110 = 28

This yields the concentration of unmeasured anions in the serum.

Key Concept

Calculation of the serum anion gap

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