A 56-year-old male with long-standing Crohn's disease and a history of multiple small bowe… — USMLE Step 3

A $56$ -year-old male with long-standing Crohn's disease and a history of multiple small bowel resections is brought to the emergency department by his wife. She reports that he has been increasingly lethargic and confused over the past $24$ hours. He has had significantly increased, high-volume watery output from his ileostomy for the last three days. Physical examination reveals dry mucous membranes, poor skin turgor, and a blood pressure of $88/54\text{ mmHg}$ with a heart rate of $118\text{ /min}$ .

Laboratory studies are obtained and reveal the following:

Laboratory Parameter	Result	Reference Range
Sodium, serum	$136\text{ mEq/L}$	$136-145\text{ mEq/L}$
Potassium, serum	$3.4\text{ mEq/L}$	$3.5-5.0\text{ mEq/L}$
Chloride, serum	$106\text{ mEq/L}$	$95-105\text{ mEq/L}$
Bicarbonate, serum	$16\text{ mEq/L}$	$22-28\text{ mEq/L}$
Blood urea nitrogen	$48\text{ mg/dL}$	$7-18\text{ mg/dL}$
Creatinine, serum	$2.1\text{ mg/dL}$	$0.6-1.2\text{ mg/dL}$
Albumin, serum	$1.6\text{ g/dL}$	$3.5-5.0\text{ g/dL}$

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

Answer: 20 mEq/L

Answer

The patient is experiencing a high anion gap metabolic acidosis that is visually masked by his profound hypoalbuminemia. Albumin is a major unmeasured anion in the serum. In states of severe malnutrition or protein loss (such as short bowel syndrome), reduced serum albumin artificially shrinks the apparent anion gap. The formula for the uncorrected gap is

\text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-)

, which gives

136 - (106 + 16) = 14\text{ mEq/L}

. To find the corrected anion gap,

2.5\text{ mEq/L}

is added for every

1.0\text{ g/dL}

the albumin is below the normal reference of

4.0\text{ g/dL}

. The albumin deficit is

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

. Multiplying

2.4

2.5

yields a correction factor of

6

. Adding

6

to the uncorrected gap of

14

gives a true anion gap of

20\text{ mEq/L}

. This indicates a significant occult high anion gap metabolic acidosis, likely resulting from acute kidney injury (uremia) and poor tissue perfusion (lactic acidosis) secondary to profound hypovolemia.

Step-by-Step Solution

Calculate the standard (uncorrected) anion gap.

136 - (106 + 16) = 136 - 122 = 14\text{ mEq/L}

The basic anion gap must first be derived from the measured primary electrolytes to establish a baseline for correction.

Calculate the deficit in serum albumin from the normal expected value.

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

Albumin constitutes the majority of unmeasured anions in the blood. A deficiency in albumin falsely lowers the measured anion gap, masking potential acid-base disorders.

Calculate the correction value and add it to the uncorrected anion gap.

Correction =

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

. Corrected AG =

14 + 6 = 20\text{ mEq/L}

For every

1.0\text{ g/dL}

decrease in albumin below

4.0\text{ g/dL}

, the anion gap must be increased by

2.5\text{ mEq/L}

to reveal the true metabolic gap.

Key Concept

Patients with severe hypoalbuminemia have a falsely lowered baseline anion gap; an albumin correction factor must be calculated to accurately identify an occult high anion gap metabolic acidosis.

Estimated Time:1m 30s

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic