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Difficulty: MediumNephrology

During a routine follow-up at the nephrology clinic, a 42-year-old woman is evaluated for persistent hypokalemia and mild muscle weakness. She has a history of Sjögren syndrome and has had two episodes of calcium phosphate nephrolithiasis in the past five years. She reports no diarrhea, vomiting, or recent dietary changes. Her blood pressure is 118/76 mm Hg. Laboratory studies show:

Serum sodium: 140 mEq/L
Serum potassium: 2.9 mEq/L
Serum chloride: 115 mEq/L
Serum bicarbonate: 15 mEq/L
Blood urea nitrogen: 12 mg/dL
Serum creatinine: 0.8 mg/dL

Urine studies obtained simultaneously show:
Urine sodium: 55 mEq/L
Urine potassium: 25 mEq/L
Urine chloride: 60 mEq/L

What is this patient's urine anion gap in mEq/L? (Provide your answer as a number only)

Answer: 20 mEq/L

Answer

20
The patient has a normal anion gap metabolic acidosis (NAGMA), indicated by a serum anion gap of 10 mEq/L (140−(115+15)140 - (115 + 15)). To evaluate the underlying cause of a NAGMA, the urine anion gap (UAG) is calculated to estimate renal ammonium (NH4+NH_4^+) excretion. The formula is: UAG=(UNa+UK)−UClUAG = (U_{Na} + U_{K}) - U_{Cl}. Using the patient's values: UAG=(55+25)−60=80−60=20UAG = (55 + 25) - 60 = 80 - 60 = 20 mEq/L. In normal physiology and gastrointestinal bicarbonate loss (e.g., diarrhea), the kidneys increase NH4+NH_4^+ production, which is excreted with chloride, resulting in a negative UAG. A positive UAG, as seen in this patient, indicates a failure of the kidneys to excrete adequate acid, which is diagnostic for renal tubular acidosis (RTA). Her history of Sjögren syndrome and calcium phosphate stones further points to Type 1 (distal) RTA.

Step-by-Step Solution

1
Calculate the serum anion gap.
Serum AG = 140−(115+15)=10140 - (115 + 15) = 10 mEq/L.
Confirms the presence of a normal anion gap metabolic acidosis (NAGMA), prompting the need for a urine anion gap.
2
Set up the urine anion gap formula.
UAG=(UNa+UK)−UClUAG = (U_{Na} + U_{K}) - U_{Cl}
The formula estimates unmeasured urinary ammonium excretion.
3
Insert the provided urine electrolyte values.
UAG=(55+25)−60UAG = (55 + 25) - 60
Accurate substitution of variables is required for the calculation.
4
Perform the final calculation.
80−60=2080 - 60 = 20 mEq/L
Yields the correct numerical answer indicating impaired ammonium excretion.

Key Concept

The urine anion gap (UAG=UNa+UK−UClUAG = U_{Na} + U_{K} - U_{Cl}) differentiates renal from gastrointestinal causes of normal anion gap metabolic acidosis. A positive value reflects impaired urinary ammonium excretion, characteristic of distal renal tubular acidosis.
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Topics

Nephrology

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