A 56-year-old man with a history of alcohol use disorder and decompensated cirrhosis is ad… — USMLE Step 2 CK

A 56-year-old man with a history of alcohol use disorder and decompensated cirrhosis is admitted to the hospital with worsening lethargy and diffuse abdominal swelling over the past week. His temperature is $37.2^\circ\text{C}$ ( $99.0^\circ\text{F}$ ), blood pressure is $102/64 \text{ mm Hg}$ , pulse is $108/\text{min}$ , and respiratory rate is $22/\text{min}$ . Physical examination reveals scleral icterus, shifting dullness to percussion, and bilateral pitting edema. Laboratory studies are obtained:

Laboratory Parameter	Value
Sodium ( $Na^+$ )	$138 \text{ mEq/L}$
Potassium ( $K^+$ )	$3.8 \text{ mEq/L}$
Chloride ( $Cl^-$ )	$107 \text{ mEq/L}$
Bicarbonate ( $HCO_3^-$ )	$17 \text{ mEq/L}$
Blood urea nitrogen	$32 \text{ mg/dL}$
Creatinine	$1.5 \text{ mg/dL}$
Albumin	$1.6 \text{ g/dL}$

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

Answer: 20 mEq/L

Answer

The correct calculation yields

20 \text{ mEq/L}

. The patient's observed anion gap is

14 \text{ mEq/L}

(

138 - [107 + 17]

). Because the patient has severe hypoalbuminemia (

1.6 \text{ g/dL}

), this observed value is misleadingly low. To find the true anion gap, a correction of

2.5 \text{ mEq/L}

is added for every

1.0 \text{ g/dL}

that the albumin is below the reference of

4.0 \text{ g/dL}

. The albumin deficit is

2.4 \text{ g/dL}

(

4.0 - 1.6

). Multiplying

2.4

2.5

gives a correction factor of

6.0 \text{ mEq/L}

. Adding this to the observed gap of

14 \text{ mEq/L}

gives a true, corrected anion gap of

20 \text{ mEq/L}

, revealing a hidden high anion gap metabolic acidosis (likely due to lactic acidosis, given his elevated heart rate, low blood pressure, and severe liver disease).

Step-by-Step Solution

Calculate the uncorrected serum anion gap.

138 - (107 + 17) = 138 - 124 = 14 \text{ mEq/L}

The standard anion gap estimates unmeasured anions in the blood, primarily driven by the difference between major measured cations and anions.

Identify the patient's albumin deficit.

4.0 \text{ g/dL} - 1.6 \text{ g/dL} = 2.4 \text{ g/dL}

Albumin is a negatively charged protein and represents a major portion of the baseline unmeasured anions. Hypoalbuminemia artificially lowers the baseline anion gap.

Calculate the albumin correction factor.

2.4 \times 2.5 = 6.0 \text{ mEq/L}

For every

1.0 \text{ g/dL}

drop in serum albumin below the normal

4.0 \text{ g/dL}

, the anion gap is falsely lowered by approximately

2.5 \text{ mEq/L}

Add the correction factor to the observed anion gap.

14 + 6.0 = 20 \text{ mEq/L}

Adding the lost unmeasured anion equivalent back to the observed gap reveals the true metabolic state, uncovering a high anion gap metabolic acidosis.

Key Concept

In patients with significant hypoalbuminemia (such as those with cirrhosis or nephrotic syndrome), the serum anion gap must be corrected upward (

2.5 \text{ mEq/L}

for every

1 \text{ g/dL}

decrease in albumin below

4.0 \text{ g/dL}

) to accurately identify an underlying high anion gap metabolic acidosis.

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