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?

Cevap: 20 mEq/L

Cevap

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).

Adım Adım Çözüm

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.

Anahtar Kavram

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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