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Difficulty: MediumNephrology

A 28-year-old woman with a history of Sjögren syndrome is evaluated in the clinic for progressive muscle weakness over the past week. She reports no vomiting, diarrhea, or recent illnesses. Her vital signs are within normal limits. Physical examination reveals 3/5 strength in the proximal muscles of her lower extremities. Laboratory studies show:

Serum:
Na +^+: 139 mEq/L
K +^+: 2.8 mEq/L
Cl −^-: 114 mEq/L
HCO 3−_3^-: 14 mEq/L

Arterial blood gas (room air):
pH: 7.28
pCO 2_2: 32 mm Hg

Urine:
Na +^+: 62 mEq/L
K +^+: 30 mEq/L
Cl −^-: 55 mEq/L
pH: 6.0

What is the calculated urine anion gap for this patient in mEq/L?

Answer: 37 mEq/L

Answer

The calculated urine anion gap is 37 mEq/L.
The urine anion gap (UAG) is calculated using the formula: UAG=UNa+UK−UClUAG = U_{Na} + U_{K} - U_{Cl}. Using the patient's urine electrolytes, the calculation is 62+30−55=3762 + 30 - 55 = 37 mEq/L. In the context of normal anion gap metabolic acidosis, a positive UAG indicates impaired renal excretion of ammonium (NH 4+_4^+). This points to a renal etiology, specifically renal tubular acidosis (RTA). This patient's presentation with profound hypokalemia, progressive muscle weakness, and Sjögren syndrome is classic for Type 1 (distal) RTA.

Step-by-Step Solution

1
Identify the patient's primary acid-base disorder.
The patient has a normal anion gap metabolic acidosis (NAGMA).
Serum pH is 7.28 and HCO 3−_3^- is 14 mEq/L, indicating metabolic acidosis. The serum anion gap is calculated as 139 - (114 + 14) = 11 mEq/L, which is within the normal range.
2
Determine the appropriate diagnostic calculation to find the etiology.
Calculate the urine anion gap (UAG).
The UAG helps differentiate between renal (e.g., renal tubular acidosis) and gastrointestinal (e.g., diarrhea) causes of NAGMA by serving as a surrogate marker for unmeasured urinary ammonium (NH 4+_4^+) excretion.
3
Apply the UAG formula using the given laboratory values.
UAG=UNa+UK−UClUAG = U_{Na} + U_{K} - U_{Cl}
Unlike the serum anion gap, potassium is an essential component of the urine anion gap equation due to its significant variation and quantity in the urine.
4
Calculate the final numerical value.
UAG=62+30−55=37UAG = 62 + 30 - 55 = 37 mEq/L.
A positive UAG indicates impaired renal ammonium excretion, which is consistent with Type 1 (distal) renal tubular acidosis, a condition commonly associated with Sjögren syndrome.

Key Concept

Calculation and clinical utility of the urine anion gap in evaluating normal anion gap metabolic acidosis.
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Topics

Nephrology

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