# Urine Anion Gap (UAG) Calculator

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.

> Canonical page: https://elysiatools.com/en/tools/anion-gap-renal

- **Category:** Health

- **Keywords:** urine anion gap, UAG, urine net charge, renal tubular acidosis, RTA, hyperchloremic metabolic acidosis, normal anion gap acidosis, ammonium, NH4, diarrhea, nephrology

## 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.

## Inputs

- **Urine Sodium U_Na (mmol/L)** (number): Urine sodium U_Na (mmol/L). Must be ≥0. Reliability drops below 25 mmol/L.
- **Urine Potassium U_K (mmol/L)** (number): Urine potassium U_K (mmol/L). Must be ≥0.
- **Urine Chloride U_Cl (mmol/L)** (number): Urine chloride U_Cl (mmol/L). Must be ≥0.
- **Decimal Places** (number)

## 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

- Enter urine sodium (U_Na), urine potassium (U_K), and urine chloride (U_Cl) in mmol/L.
- The calculator applies the formula UAG = (U_Na + U_K) − U_Cl.
- The result is reported in mmol/L with a category and interpretation related to urinary NH₄⁺ excretion.
- Choose 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.

## Frequently asked questions

### What is the urine anion gap formula?

UAG = (urine sodium + urine potassium) − urine chloride, reported in mmol/L.

### What does a negative UAG suggest?

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.

## Related tools

- [Serum Anion Gap (AG) Calculator](https://elysiatools.com/en/tools/anion-gap-calculator): Calculate the serum Anion Gap (AG) to classify metabolic acidosis. Standard formula AG = Na⁺ − Cl⁻ − HCO₃⁻ (mmol/L), or AG = Na⁺ + K⁺ − Cl⁻ − HCO₃⁻ with optional potassium. A high AG (≈ >12, or >20 with K⁺) indicates accumulation of unmeasured anions (lactic acidosis, ketoacidosis, renal failure, and toxins such as methanol/ethylene glycol, salicylates — mnemonic GOLD-MARK). A normal AG (8–12) during metabolic acidosis points to a hyperchloremic (normal-anion-gap) acidosis: diarrhea, renal tubular acidosis, saline resuscitation. A low AG (<3–6) is usually hypoalbuminemia, also hypercalcemia/hypermagnesemia, lithium, cationic IgG paraproteins, or bromide pseudo-hyperchloraemia. Optional albumin correction AG_corrected = AG + 2.5 × (4.0 − albumin g/dL) avoids missing a high-AG acidosis in hypoalbuminemia. Derived from Kraut CJASN 2007, Figge 1998, and MDCalc. Interpret with blood gas and full clinical context. Not medical advice.
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