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Difficulty: MediumInterpreting Laboratory Results

A 34-year-old woman with a history of systemic lupus erythematosus and Sjögren syndrome is evaluated in the emergency department for progressive muscle weakness over the past 3 days. She reports difficulty climbing stairs and getting out of chairs. She has not had diarrhea or vomiting. Her blood pressure is 112/76 mm Hg, and her pulse is 88/min. Physical examination shows 3/5 motor strength in the proximal lower extremities. Laboratory results are as follows:

Serum:
Sodium: 138138 mEq/L
Potassium: 2.42.4 mEq/L
Chloride: 115115 mEq/L
Bicarbonate: 1313 mEq/L

Arterial blood gas (room air):
pH: 7.277.27
pCO2: 2828 mm Hg

Urine:
Sodium: 4242 mEq/L
Potassium: 3535 mEq/L
Chloride: 5151 mEq/L

To help determine the etiology of this patient's acid-base disturbance, the physician evaluates her urinary electrolytes. What is this patient's urine anion gap in mEq/L?

Answer: 26 mEq/L

Answer

26
The correct calculation for the urine anion gap (UAG) relies on the formula: UAG=Urine Na++Urine K+−Urine Cl−UAG = Urine\ Na^+ + Urine\ K^+ - Urine\ Cl^-. Substituting the provided urine electrolyte values yields: 42 mEq/L+35 mEq/L−51 mEq/L=26 mEq/L42\ mEq/L + 35\ mEq/L - 51\ mEq/L = 26\ mEq/L. In the setting of a normal anion gap metabolic acidosis (confirmed by a serum anion gap of 10), the UAG is used as a surrogate marker for unmeasured urinary ammonium (NH4+NH_4^+) excretion. A positive UAG (as seen here) indicates impaired ammonium excretion, pointing toward a renal cause such as renal tubular acidosis (RTA). This patient's profound hypokalemia, systemic lupus erythematosus, and Sjögren syndrome strongly suggest type 1 (distal) RTA.

Step-by-Step Solution

1
Analyze the serum electrolytes and arterial blood gas to confirm the primary acid-base disorder.
The patient has a normal anion gap metabolic acidosis. The serum pH is 7.27, bicarbonate is low at 13 mEq/L, and the serum anion gap is 10 mEq/L (Calculated as 138 - (115 + 13)).
Establishing the presence of a normal anion gap metabolic acidosis is the clinical prerequisite for utilizing the urine anion gap.
2
Determine the appropriate formula for calculating the urine anion gap (UAG).
UAG = Urine Na++Urine K+−Urine Cl−Urine\ Na^+ + Urine\ K^+ - Urine\ Cl^-
The UAG indirectly estimates urinary ammonium excretion to differentiate between gastrointestinal bicarbonate loss (negative UAG) and renal tubular acidosis (positive UAG).
3
Substitute the patient's measured urine electrolyte values into the UAG formula and calculate the result.
UAG = 42 + 35 - 51 = 77 - 51 = 26 mEq/L.
Applying the values correctly yields the final diagnostic index necessary to evaluate the etiology of the acidosis.

Key Concept

The urine anion gap is calculated using the formula (Urine Na++Urine K+)−Urine Cl−(Urine\ Na^+ + Urine\ K^+) - Urine\ Cl^- and is essential for differentiating between gastrointestinal and renal causes of normal anion gap metabolic acidosis.
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Interpreting Laboratory Results

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