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Difficulty: MediumNephrology

A 35-year-old woman with a history of systemic lupus erythematosus presents to the clinic with progressive muscle weakness and fatigue over the past 2 weeks. She denies any recent illnesses, diarrhea, or vomiting. Current medications include hydroxychloroquine. Vital signs are within normal limits. Laboratory testing reveals the following:

TestValue
Serum sodium138 mEq/L138 \text{ mEq/L}
Serum potassium2.9 mEq/L2.9 \text{ mEq/L}
Serum chloride114 mEq/L114 \text{ mEq/L}
Serum bicarbonate14 mEq/L14 \text{ mEq/L}
Serum creatinine0.8 mg/dL0.8 \text{ mg/dL}
Urine sodium38 mEq/L38 \text{ mEq/L}
Urine potassium22 mEq/L22 \text{ mEq/L}
Urine chloride15 mEq/L15 \text{ mEq/L}
Urine pH6.86.8

What is this patient's calculated urine anion gap?

Answer: 45 mEq/L

Answer

45
The correct answer is 45.

The urine anion gap (UAG) is calculated using the formula:
UAG=Naurine++Kurine+−Clurine−UAG = Na^+_{urine} + K^+_{urine} - Cl^-_{urine}

Plugging in the values from the vignette:
UAG=38 mEq/L (sodium)+22 mEq/L (potassium)−15 mEq/L (chloride)=45UAG = 38 \text{ mEq/L (sodium)} + 22 \text{ mEq/L (potassium)} - 15 \text{ mEq/L (chloride)} = 45

In a patient with normal anion gap metabolic acidosis (NAGMA), the UAG helps distinguish between extrarenal and renal causes of bicarbonate loss. A positive UAG (>0>0) suggests impaired renal ammonium (NH4+NH_4^+) excretion, pointing to a renal etiology such as renal tubular acidosis (RTA). In this patient with systemic lupus erythematosus, hypokalemia, and a urine pH >5.5> 5.5, the presentation is highly consistent with Type 1 (distal) RTA.

In contrast, a negative UAG (<−10<-10) would suggest appropriately increased ammonium excretion, pointing to an extrarenal cause like gastrointestinal bicarbonate loss from profuse diarrhea.

Step-by-Step Solution

1
Calculate the serum anion gap to classify the acid-base disturbance.
Serum anion gap = 138−(114+14)=10 mEq/L138 - (114 + 14) = 10 \text{ mEq/L}.
This confirms the presence of a normal anion gap metabolic acidosis (NAGMA), which indicates the need for a urine anion gap evaluation.
2
Select the correct formula for the urine anion gap (UAG).
The formula is UAG=Naurine++Kurine+−Clurine−UAG = Na^+_{urine} + K^+_{urine} - Cl^-_{urine}.
The UAG estimates the unmeasured urinary cations (primarily ammonium, NH4+NH_4^+) and anions to help differentiate renal versus gastrointestinal causes of NAGMA.
3
Substitute the patient's urine electrolyte values into the formula.
UAG=38+22−15=45UAG = 38 + 22 - 15 = 45.
This yields a positive UAG, indicating impaired renal ammonium excretion, consistent with Type 1 distal renal tubular acidosis.

Key Concept

The urine anion gap (UAG=Naurine++Kurine+−Clurine−UAG = Na^+_{urine} + K^+_{urine} - Cl^-_{urine}) is used to evaluate normal anion gap metabolic acidosis; a positive value indicates a renal etiology (like renal tubular acidosis), whereas a negative value indicates extrarenal bicarbonate loss (like diarrhea).
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Topics

Nephrology

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