A 62-year-old man with a history of alcohol-associated cirrhosis is brought to the emergen… — USMLE Step 2 CK

A 62-year-old man with a history of alcohol-associated cirrhosis is brought to the emergency department due to progressive confusion, lethargy, and fever over the past 2 days. His temperature is $38.6^\circ\text{C}$ ( $101.5^\circ\text{F}$ ), blood pressure is $95/60\text{ mm Hg}$ , heart rate is $112/\text{min}$ , and respiratory rate is $22/\text{min}$ . Physical examination is notable for scleral icterus, spider angiomas, a distended abdomen with a positive fluid wave, and bilateral lower extremity pitting edema.

Laboratory studies show:
Sodium: $136\text{ mEq/L}$
Potassium: $3.8\text{ mEq/L}$
Chloride: $104\text{ mEq/L}$
Bicarbonate: $18\text{ mEq/L}$
BUN: $32\text{ mg/dL}$
Creatinine: $1.5\text{ mg/dL}$
Glucose: $98\text{ mg/dL}$
Albumin: $2.0\text{ g/dL}$
Total bilirubin: $4.5\text{ mg/dL}$

Arterial blood gas on room air reveals a pH of $7.32$ and a $PaCO_2$ of $34\text{ mm Hg}$ .

Assuming a reference normal serum albumin of $4.0\text{ g/dL}$ , what is this patient's albumin-corrected serum anion gap in mEq/L?

Answer: 19 mEq/L

Answer

This patient has a metabolic acidosis (pH 7.32, low bicarbonate). To accurately assess the etiology, the serum anion gap must be calculated and corrected for his severe hypoalbuminemia. The uncorrected anion gap is

Na^+ - (Cl^- + HCO_3^-) = 136 - (104 + 18) = 14\text{ mEq/L}

. Because albumin is negatively charged, it constitutes a significant portion of the baseline 'unmeasured anions'. A decrease in albumin falsely lowers the calculated anion gap. The standard correction requires adding

2.5\text{ mEq/L}

to the calculated gap for every

1\text{ g/dL}

decrease in albumin below

4.0\text{ g/dL}

. Here, the deficit is

4.0 - 2.0 = 2.0\text{ g/dL}

. The correction is

2.0 \times 2.5 = 5\text{ mEq/L}

. Adding this to the uncorrected gap yields

14 + 5 = 19\text{ mEq/L}

. This reveals a high anion gap metabolic acidosis, likely secondary to lactic acidosis from sepsis.

Step-by-Step Solution

Calculate the uncorrected anion gap.

14\text{ mEq/L}

Using the standard formula:

AG = 136 - (104 + 18) = 14\text{ mEq/L}

Calculate the albumin deficit.

2.0\text{ g/dL}

Subtract the patient's albumin from normal:

4.0\text{ g/dL} - 2.0\text{ g/dL} = 2.0\text{ g/dL}

Calculate the albumin-corrected anion gap.

19\text{ mEq/L}

Add the correction factor to the uncorrected gap:

14 + (2.0 \times 2.5) = 14 + 5 = 19\text{ mEq/L}

Key Concept

Albumin is a major unmeasured anion; in hypoalbuminemic states, the calculated anion gap is falsely lowered and must be corrected by adding

2.5\text{ mEq/L}

for every

1\text{ g/dL}

decrease in serum albumin.

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