A 42-year-old man with known nephrotic syndrome is brought to the emergency department due… — USMLE Step 3

A 42-year-old man with known nephrotic syndrome is brought to the emergency department due to altered mental status and fever. His family reports he developed a spreading, painful, erythematous rash on his right lower extremity over the past three days. Vital signs reveal a temperature of $39.1^\circ\text{C}$ ( $102.4^\circ\text{F}$ ), heart rate of $118/\text{min}$ , blood pressure of $88/54\text{ mmHg}$ , and respiratory rate of $26/\text{min}$ .

Initial laboratory studies show:

Test	Result
Sodium ( $Na^+$ )	$140\text{ mEq/L}$
Potassium ( $K^+$ )	$4.2\text{ mEq/L}$
Chloride ( $Cl^-$ )	$108\text{ mEq/L}$
Bicarbonate ( $HCO_3^-$ )	$16\text{ mEq/L}$
Blood urea nitrogen	$45\text{ mg/dL}$
Creatinine	$2.1\text{ mg/dL}$
Albumin	$1.2\text{ g/dL}$
Lactate	$4.5\text{ mmol/L}$

What is the patient's albumin-corrected serum anion gap in mEq/L? (Assume a normal serum albumin concentration of $4.0\text{ g/dL}$ ).

Answer: 23 mEq/L

Answer

The patient has a profound high anion gap metabolic acidosis secondary to septic shock and lactic acidosis. However, his underlying nephrotic syndrome has caused severe hypoalbuminemia (1.2 g/dL). Since albumin normally accounts for a large portion of the unmeasured anions in the blood, its depletion artificially lowers the calculated anion gap. The measured gap is 140 - (108 + 16) = 16 mEq/L. To correct for hypoalbuminemia, the deficit (4.0 - 1.2 = 2.8 g/dL) is multiplied by 2.5 mEq/L to get a correction factor of 7 mEq/L. Adding this back to the measured gap (16 + 7) yields a true, corrected anion gap of 23 mEq/L.

Step-by-Step Solution

Calculate the uncorrected, measured serum anion gap.

16 mEq/L

The baseline anion gap evaluates the difference between primary measured cations and anions using the standard formula: Na - (Cl + HCO3).

Calculate the patient's albumin deficit.

2.8 g/dL

Because albumin is the primary unmeasured circulating anion, significant deficits must be quantified against the normal baseline of 4.0 g/dL.

Calculate the adjustment factor for the anion gap.

7 mEq/L

For every 1 g/dL decrease in serum albumin below 4.0 g/dL, the anion gap is falsely lowered by 2.5 mEq/L. Multiplying the 2.8 deficit by 2.5 yields the necessary correction.

Apply the correction factor to the measured anion gap.

23 mEq/L

Adding the 7 mEq/L correction to the 16 mEq/L measured gap reveals the true physiological anion gap.

Key Concept

Hypoalbuminemia artificially lowers the measured anion gap because albumin acts as a significant unmeasured anion. To correctly interpret acid-base status, the measured gap must be corrected by adding 2.5 mEq/L for every 1 g/dL decrease in serum albumin below the normal level of 4.0 g/dL.

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic