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Difficulty: HardNephrology

An ambulatory care physician evaluates a 33-year-old woman who reports a one-month history of persistent fatigue and severe muscle weakness. She denies any recent illnesses, diarrhea, vomiting, or medication use. Her vital signs are within normal limits. Physical examination reveals diffusely decreased muscle strength in all extremities with diminished deep tendon reflexes.

Laboratory studies reveal the following:

SerumValue
Sodium138 mEq/L
Potassium2.8 mEq/L
Chloride115 mEq/L
Bicarbonate13 mEq/L
Blood urea nitrogen12 mg/dL
Creatinine0.8 mg/dL
UrineValue
Sodium70 mEq/L
Potassium30 mEq/L
Chloride55 mEq/L
pH6.5
Specific gravity1.010

Based on the laboratory data provided, what is this patient's urine anion gap (in mEq/L)?

Answer: 45 mEq/L

Answer

The urine anion gap is calculated as 45 mEq/L.
The urine anion gap (UAG) is a critical calculation used in the evaluation of a normal anion gap metabolic acidosis (hyperchloremic acidosis) to distinguish between renal and extrarenal (usually gastrointestinal) etiologies. It provides an indirect estimate of urinary ammonium (NH4+NH_4^+) excretion. The formula is UAG=UNa+UK−UClUAG = U_{Na} + U_{K} - U_{Cl}. In a healthy physiologic response to acidemia, the kidneys increase acid excretion in the form of NH4+NH_4^+. Because ammonium is positively charged, it is accompanied by chloride (Cl−Cl^-) into the urine to maintain electroneutrality. This leads to a high urinary chloride concentration and a negative UAG (typically −20-20 to −50-50 mEq/L), an appropriate compensatory response classically seen in severe diarrhea. Conversely, a positive UAG (as seen in this patient with a value of 45 mEq/L) indicates an inability of the kidneys to appropriately excrete NH4+NH_4^+, which is a hallmark of Renal Tubular Acidosis (RTA). Given this patient's concurrent hypokalemia, inappropriately basic urine pH (6.5), and positive UAG, Distal (Type 1) RTA is the definitive diagnosis.

Step-by-Step Solution

1
Identify the clinical indication and the appropriate diagnostic formula.
The formula for Urine Anion Gap is UAG=UNa+UK−UClUAG = U_{Na} + U_{K} - U_{Cl}.
The UAG helps differentiate between gastrointestinal (negative UAG) and renal (positive UAG) causes of a normal anion gap metabolic acidosis.
2
Extract the necessary variables from the provided urinalysis data panel.
UNaU_{Na} = 70 mEq/L, UKU_{K} = 30 mEq/L, and UClU_{Cl} = 55 mEq/L.
Only urine electrolyte concentrations are utilized in this formula; serum electrolytes serve as distractors for this specific calculation step.
3
Perform the calculation using the extracted values.
UAG=70+30−55=45UAG = 70 + 30 - 55 = 45 mEq/L.
This yields a positive integer, indicating impaired renal ammonium excretion consistent with Distal (Type 1) Renal Tubular Acidosis.

Key Concept

Calculation and clinical utility of the Urine Anion Gap (UAG) in the differential diagnosis of normal anion gap metabolic acidosis.
Estimated Time:1m 30s
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Topics

Nephrology

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