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Difficulty: MediumNephrology

A 35-year-old woman presents for follow-up of generalized weakness and worsening fatigue over the last two weeks. Her medical history is notable for Sjögren syndrome, for which she takes hydroxychloroquine. She appears comfortable and her vital signs are stable. Muscle strength testing demonstrates 4/5 strength in the bilateral quadriceps and hip flexors. Given her symptoms, laboratory testing is obtained:

Serum:
Sodium: 140 mEq/L
Potassium: 2.8 mEq/L
Chloride: 115 mEq/L
Bicarbonate: 15 mEq/L
Blood urea nitrogen: 14 mg/dL
Creatinine: 0.8 mg/dL
Glucose: 95 mg/dL

Urine:
Sodium: 60 mEq/L
Potassium: 25 mEq/L
Chloride: 45 mEq/L
pH: 6.0

To determine the etiology of this patient's metabolic acidosis, the physician decides to calculate the urine anion gap. What is the value of this patient's urine anion gap?

Answer: 40 mEq/L

Answer

40 mEq/L
The patient has a normal anion gap metabolic acidosis (NAGMA), as her serum anion gap is 140−(115+15)=10140 - (115 + 15) = 10 mEq/L (normal: 8-12 mEq/L). In the setting of NAGMA, the urine anion gap (UAG) helps differentiate between renal loss of bicarbonate (e.g., renal tubular acidosis) and extrarenal loss (e.g., diarrhea). The formula for UAG is UNa++UK+−UCl−U_{Na^+} + U_{K^+} - U_{Cl^-}. Plugging in this patient's urinary values: 60 mEq/L+25 mEq/L−45 mEq/L=40 mEq/L60 \text{ mEq/L} + 25 \text{ mEq/L} - 45 \text{ mEq/L} = 40 \text{ mEq/L}. A positive UAG indicates impaired renal NH4+NH_4^+ excretion, which is consistent with renal tubular acidosis (Type 1 RTA in this patient with Sjögren syndrome and severe hypokalemia).

Step-by-Step Solution

1
Identify the formula for urine anion gap.
UAG=UNa++UK+−UCl−UAG = U_{Na^+} + U_{K^+} - U_{Cl^-}
The urine anion gap is used to indirectly measure urinary ammonium (NH4+NH_4^+) excretion in a patient with a normal anion gap metabolic acidosis.
2
Extract the appropriate urinary electrolyte values from the laboratory data.
UNa+=60U_{Na^+} = 60 mEq/L, UK+=25U_{K^+} = 25 mEq/L, UCl−=45U_{Cl^-} = 45 mEq/L
These are the measured urinary cations and anions needed for the calculation. Care must be taken not to mistakenly use serum values.
3
Calculate the urine anion gap.
60+25−45=4060 + 25 - 45 = 40 mEq/L
Adding the cations and subtracting the chloride yields a positive value of 40 mEq/L, indicating impaired renal ammonium excretion (typical of distal renal tubular acidosis).

Key Concept

The urine anion gap (UNa++UK+−UCl−U_{Na^+} + U_{K^+} - U_{Cl^-}) is calculated to differentiate between gastrointestinal and renal causes of normal anion gap metabolic acidosis; a positive value suggests a renal etiology such as renal tubular acidosis.
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Topics

Nephrology

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