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Calculates the urine anion gap: UAG = (urine sodium + urine potassium) − urine chloride. It is used to distinguish renal from extrarenal (gastrointestinal) causes of a normal anion gap (hyperchloremic) metabolic acidosis with hypokalemia and normal renal function.
Clinical decision-support tool for qualified healthcare professionals. Results are not a diagnosis and should not replace professional clinical judgment.
In normal urine, ammonium (an unmeasured cation) balances the chloride excess, so the gap is usually negative in states of GI bicarbonate loss where the kidney is excreting ammonium appropriately.
Specialty
Internal MedicineCategory
NephrologyDifficulty
Basic
Estimated Time
30 seconds
Version
1.0
Last Updated
2026-08-16
Formula
Keywords
Normal Range
Negative in extrarenal (GI) bicarbonate loss; positive in renal tubular acidosis
| Classification | Range |
|---|---|
Negative extrarenal (GI) bicarbonate loss | < 0 |
Zero / equivocal no reliable discrimination | ≈ 0 |
Positive impaired renal acidification (e.g., distal RTA) | > 0 |
Calculates the urine anion gap (UAG = [urine sodium + urine potassium] − urine chloride) to distinguish renal from extrarenal causes of a normal anion gap (hyperchloremic) metabolic acidosis.
Urine anion gap = (urine sodium + urine potassium) - urine chloride, all in mEq/L.
Urine sodium in mEq/L, added to urine potassium.
Urine potassium in mEq/L, added to urine sodium.
Urine chloride in mEq/L, subtracted from the sum of sodium and potassium.
A negative UAG (typically −20 to −50 mEq/L) indicates appropriate renal ammonium excretion, consistent with extrarenal (GI) bicarbonate loss. A positive or near-zero UAG suggests impaired renal acidification, as in distal renal tubular acidosis.
The urine anion gap uses routine urine electrolytes to approximate ammonium excretion, helping localize the cause of hyperchloremic acidosis without specialized testing.
A 30-year-old woman with severe diarrhea and hyperchloremic metabolic acidosis has urine sodium 10 mEq/L, potassium 20 mEq/L, and chloride 110 mEq/L.
Inputs:
Expected Result: UAG = (10 + 20) − 110 = −80 mEq/L — negative, consistent with appropriate renal ammonium excretion and gastrointestinal bicarbonate loss.
A negative gap reflects unmeasured cations (mainly ammonium) in the urine, indicating the kidney is excreting acid appropriately — consistent with extrarenal bicarbonate loss such as diarrhea.
Kidney disease itself impairs ammonium excretion, so the urine anion gap can no longer distinguish renal from extrarenal causes of acidosis.
High anion gap acidosis
Acute kidney injury evaluation
Evaluating the PaCO2 response
Disclaimer: This calculator is intended for educational and clinical decision support purposes only. The urine anion gap is an indirect estimate of ammonium excretion and must be interpreted with renal function and clinical context.