Calculate the Urine Anion Gap (UAG) to differentiate causes of normal-anion-gap (hyperchloremic) metabolic acidosis: UAG = (U_Na + U_K) − U_Cl, in mmol/L. The UAG is an inverse surrogate for urinary NH₄⁺ excretion. A negative UAG (typically −20 to −50) indicates appropriate high NH₄⁺ excretion → extrarenal (GI) bicarbonate loss (diarrhea, fistulae, ureteral diversions). A positive/high UAG indicates low NH₄⁺ excretion → renal tubular acidosis (distal/type 1, type 4 hypoaldosteronism, proximal/type 2 variable). Reference range ≈ 0 to +40 mmol/L. Caveats: unreliable with unmeasured urine anions (ketoacidosis, toluene/hippurate, D-lactate) or when urine sodium < 25 mmol/L (use urine osmolar gap instead). Derived from Batlle CJASN 2012, Ito 2025, Bonner AJKD 2025. Interpret with blood gas and full clinical context. Not medical advice.
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Key facts
Category
Health
Input types
number
Output type
json
Sample coverage
4
API ready
Yes
Overview
The Urine Anion Gap (UAG) Calculator computes UAG = (U_Na + U_K) − U_Cl in mmol/L and provides a concise interpretation for normal-anion-gap metabolic acidosis. Negative values suggest appropriate urinary NH₄⁺ excretion and an extrarenal bicarbonate loss, while positive or high values suggest reduced NH₄⁺ excretion and a possible renal cause. Interpret results with blood gas findings and the full clinical context; this tool is not medical advice.
When to use
Assess the renal response in suspected normal-anion-gap or hyperchloremic metabolic acidosis.
Compare a negative UAG pattern associated with gastrointestinal bicarbonate loss, such as diarrhea, with a positive pattern suggesting renal tubular acidosis.
Calculate UAG quickly from urine sodium, potassium, and chloride values while noting important reliability limitations.
How it works
1Enter urine sodium (U_Na), urine potassium (U_K), and urine chloride (U_Cl) in mmol/L.
2The calculator applies the formula UAG = (U_Na + U_K) − U_Cl.
3The result is reported in mmol/L with a category and interpretation related to urinary NH₄⁺ excretion.
4Choose the number of decimal places for the displayed result; interpret the value alongside clinical data.
Use cases
Nephrology evaluation of suspected distal, type 4, or variable proximal renal tubular acidosis.
Assessment of diarrhea or other gastrointestinal bicarbonate loss in hyperchloremic metabolic acidosis.
Teaching and case review using measured urine electrolyte values and calculated urinary net charge.
UAG = −60 mmol/L. The negative result is categorized as consistent with appropriate NH₄⁺ excretion and an extrarenal, gastrointestinal bicarbonate loss.
2. Possible distal renal tubular acidosis
Nephrology trainee reviewing urine electrolytes
Background
A patient with normal-anion-gap metabolic acidosis has urine sodium of 30 mmol/L, urine potassium of 20 mmol/L, and urine chloride of 40 mmol/L.
Problem
Calculate the UAG and assess whether urinary NH₄⁺ excretion appears reduced.
How to use
Enter urine sodium 30, urine potassium 20, urine chloride 40, and select 1 decimal place.
A negative UAG suggests high NH₄⁺ excretion and an appropriate renal response, often consistent with extrarenal bicarbonate loss such as diarrhea.
What does a positive UAG suggest?
A positive or high UAG suggests low NH₄⁺ excretion and may indicate impaired renal acid excretion, such as renal tubular acidosis.
When is UAG less reliable?
UAG is less reliable with unmeasured urine anions, including ketoacidosis, toluene or hippurate exposure, and D-lactate, or when urine sodium is below 25 mmol/L.
What should be used when urine sodium is below 25 mmol/L?
The tool description recommends using the urine osmolar gap instead because UAG reliability decreases below 25 mmol/L urine sodium.