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Difficulty: MediumNephrology

A 3535-year-old woman with a history of Sjögren syndrome comes to the clinic due to progressive generalized weakness and fatigue for the past month. Her blood pressure is 110/70110/70 mm Hg and her physical examination is unremarkable except for dry oral mucosa.

Laboratory studies show:
- Serum sodium: 140140 mEq/L
- Serum potassium: 2.82.8 mEq/L
- Serum chloride: 115115 mEq/L
- Serum bicarbonate: 1313 mEq/L
- Urine sodium: 4545 mEq/L
- Urine potassium: 2525 mEq/L
- Urine chloride: 4040 mEq/L

The patient's serum anion gap is calculated to be 1212 mEq/L (normal: 10−1410-14 mEq/L), indicating a normal anion gap metabolic acidosis. To help determine whether the acidosis is due to renal or gastrointestinal etiology, what is this patient's calculated urine anion gap (in mEq/L)?

Answer: 30 mEq/L

Answer

30
The calculated urine anion gap (UAG) is 3030 mEq/L. The UAG is determined using the equation: UAG=UrineNa++UrineK+−UrineCl−UAG = Urine_{Na^+} + Urine_{K^+} - Urine_{Cl^-}. Plugging in the patient's urinary values yields 45+25−40=3045 + 25 - 40 = 30 mEq/L. In the setting of a normal anion gap metabolic acidosis, a positive UAG indicates that the kidneys are failing to excrete sufficient ammonium (NH4+NH_4^+) and chloride (Cl−Cl^-) in the urine. This points to a renal etiology, specifically distal (Type 1) renal tubular acidosis, which strongly correlates with this patient's history of Sjögren syndrome and hypokalemia.

Step-by-Step Solution

1
Select the correct formula for the urine anion gap (UAG).
UAG=UrineNa++UrineK+−UrineCl−UAG = Urine_{Na^+} + Urine_{K^+} - Urine_{Cl^-}
The UAG indirectly measures urinary ammonium (NH4+NH_4^+) excretion, which is essential for determining the cause of a non-anion gap metabolic acidosis.
2
Extract the appropriate urinary electrolyte values from the clinical vignette.
UrineNa+=45Urine_{Na^+} = 45 mEq/L, UrineK+=25Urine_{K^+} = 25 mEq/L, UrineCl−=40Urine_{Cl^-} = 40 mEq/L
Serum values must be ignored for this calculation to avoid computational errors.
3
Substitute the values into the formula.
UAG=45+25−40UAG = 45 + 25 - 40
Proper substitution ensures mathematical accuracy.
4
Complete the arithmetic calculation.
70−40=3070 - 40 = 30 mEq/L
A positive UAG (>0>0) confirms impaired renal ammonium excretion, pointing toward renal tubular acidosis (RTA) rather than gastrointestinal bicarbonate loss.

Key Concept

The urine anion gap (UAG=UrineNa++UrineK+−UrineCl−UAG = Urine_{Na^+} + Urine_{K^+} - Urine_{Cl^-}) helps differentiate causes of normal anion gap metabolic acidosis. A positive UAG indicates impaired renal acid excretion (e.g., Renal Tubular Acidosis), whereas a negative UAG indicates appropriate renal ammonium excretion in response to extrarenal bicarbonate loss (e.g., diarrhea).
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Nephrology

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