During a routine follow-up, a 28-year-old female patient complains of new-onset fatigue and profound muscle weakness. She has a notable past medical history of recurrent nephrolithiasis. Her vital signs are within normal limits.
Laboratory testing reveals the following values:
| Serum Parameter | Value |
|---|---|
| Sodium () | 138 mEq/L |
| Potassium () | 2.8 mEq/L |
| Chloride () | 112 mEq/L |
| Bicarbonate () | 14 mEq/L |
| Urine Parameter | Value |
|---|---|
| Sodium () | 65 mEq/L |
| Potassium () | 25 mEq/L |
| Chloride () | 55 mEq/L |
| pH | 6.8 |
To help differentiate the cause of her normal anion gap metabolic acidosis, the attending physician requests calculation of the urine anion gap.
What is this patient's urine anion gap (in mEq/L)?
Answer: 35 mEq/L
Answer
35
The patient presents with a normal anion gap metabolic acidosis (NAGMA), likely secondary to type 1 (distal) renal tubular acidosis given her severe hypokalemia, nephrolithiasis, and alkaline urine pH. To confirm the renal etiology, the urine anion gap is calculated using the formula . Plugging in the patient's urine electrolyte values yields mEq/L. A positive urine anion gap indicates impaired renal ammonium excretion, which points toward a renal tubular acidosis rather than gastrointestinal bicarbonate loss (where the UAG would typically be negative).
Step-by-Step Solution
Key Concept
The urine anion gap (UAG) is calculated as and provides an indirect measurement of urinary ammonium excretion to help distinguish between renal and gastrointestinal causes of a normal anion gap metabolic acidosis.