Urine Anion Gap Calculator

Q: When is the urine anion gap used?

It is used as a second-line tool in the workup of a non-anion-gap (normal serum anion gap) metabolic acidosis, to help localize the cause to a renal versus gastrointestinal source.

Q: What units does the urine anion gap use?

All three inputs — urine sodium, potassium, and chloride — and the resulting gap are in mEq/L (milliequivalents per liter).

Q: What are the limitations of the urine anion gap?

Values near zero don't reliably discriminate a renal from a GI cause. The urine anion gap can also be unreliable with a high urine pH, ketonuria, or unmeasured urine anions — in those settings the urine osmotic gap is a more direct alternative.

Result	Range	Suggests
Negative	< −5 mEq/L	GI bicarbonate loss (e.g. diarrhea)
Near zero	−5 to 5 mEq/L	Indeterminate — not reliably discriminating
Positive	> 5 mEq/L	Renal tubular acidosis

Finding	Mechanism
Positive UAG	Urine chloride is relatively low compared to sodium + potassium — the kidney is not pairing enough chloride with ammonium, implying impaired NH4+ excretion (renal cause).
Negative UAG	Urine chloride is relatively high — the kidney is excreting plenty of ammonium chloride, implying an appropriately responding kidney (extrarenal/GI cause).

Dimension	Serum Anion Gap	Urine Anion Gap
Formula	Na − (Cl + HCO3)	UNa + UK − UCl
Sample	Venous blood (basic metabolic panel)	Random or spot urine
Question answered	Is there an elevated-anion-gap metabolic acidosis (unmeasured anions accumulating in blood)?	Given a confirmed non-anion-gap acidosis, is the kidney excreting ammonium (NH4+) appropriately?
High/positive result	Unmeasured anions present (MUDPILES differential)	Kidney is not excreting enough NH4Cl → renal cause (RTA)
Low/negative result	Usually not diagnostically meaningful alone	Kidney is appropriately excreting NH4Cl → extrarenal/GI cause
When ordered	First step in any metabolic acidosis workup	Second step — only after acidosis is confirmed non-anion-gap

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Walk-through

How to Use This Calculator

3 steps

1

Enter urine sodium, potassium, and chloride

Enter the patient's urine sodium (UNa), urine potassium (UK), and urine chloride (UCl) from a random or spot urine sample, all in mEq/L. The calculator updates instantly.

2

Read the urine anion gap and interpretation

The result card shows the urine anion gap and classifies it as negative, near zero, or positive. A positive result suggests renal tubular acidosis; a negative result suggests GI bicarbonate loss.

3

Confirm it's the right test to run

The urine anion gap only makes sense once a non-anion-gap (normal serum-AG) metabolic acidosis has already been confirmed. Check the vs Serum AG tab if you're not sure which test applies.

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Reference

Formula & Methodology

1 formula

Urine anion gap

UAG = UNa + UK − UCl

Urine sodium (UNa), urine potassium (UK), and urine chloride (UCl) are entered in mEq/L from a random or spot urine sample. The result is also in mEq/L. A positive UAG suggests renal tubular acidosis (impaired ammonium excretion); a negative UAG suggests GI bicarbonate loss.

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High YMYL

Trust, Methodology & Sources

Reviewed by Calculover Clinical Team · Updated 2026-07-23 · 1 sources

Editorial accountability

Author: Calculover Clinical Team - Clinical content team

Owner: Calculover Editorial - Editorial oversight

Reviewer: Calculover Clinical Team - Clinical reviewer

Last reviewed: 2026-07-23

Last verified: 2026-07-23

Data effective: 2026-07-23

Methodology

Implements the urine anion gap as described by MDCalc ("Urine Anion Gap"): UAG = UNa + UK − UCl, an indirect estimate of urinary ammonium (NH4+) excretion used to help localize the cause of an already-confirmed non-anion-gap metabolic acidosis. The calculator additionally classifies results within ±5 mEq/L of zero as 'near zero — indeterminate,' which is an editorial readability heuristic added for clarity, not a value sourced from MDCalc or the underlying literature — the core clinical rule is the sign of the result (positive vs negative), not a three-tier cutoff.

Assumption: Urine sodium, potassium, and chloride are assumed to be from the same random or spot urine sample.

Assumption: The urine anion gap is assumed to be applied only after a non-anion-gap (normal serum anion gap) metabolic acidosis has already been confirmed — it is not a general screening test for acidosis.

Assumption: The ±5 mEq/L 'near zero' band shown in the Interpretation tab is an editorial heuristic for readability, not a clinically validated cutoff; values in that band should be interpreted primarily by their sign, correlated with urine pH and the clinical picture.

Limitations & guidance

The urine anion gap loses reliability with a high urine pH (bicarbonaturia), ketonuria, or unmeasured urine anions (e.g. toluene exposure) — the urine osmotic gap is a more direct alternative in those settings.

It does not replace the serum anion gap, which must already be normal (non-anion-gap acidosis) before the urine anion gap is a meaningful next step.

Results are for clinical decision support and education only and do not replace a treating clinician's full assessment, including urine pH and the complete electrolyte panel.

Professional guidance: This calculator is for clinical decision support and educational purposes only and is not a substitute for professional medical judgment, diagnosis, or treatment. Always consult a qualified clinician for patient care decisions.

Primary sources

MDCalc. Urine Anion Gap.

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Glossary

Key Terms Explained

6 terms

Urine anion gap The difference between measured urine cations (sodium and potassium) and measured urine anion (chloride): UAG = UNa + UK − UCl. It is an indirect estimate of urinary ammonium (NH4+) excretion, used to help localize the cause of a non-anion-gap metabolic acidosis.

Renal tubular acidosis A group of disorders in which the kidney fails to properly acidify the urine or reclaim bicarbonate, producing a non-anion-gap metabolic acidosis. A positive urine anion gap, reflecting impaired ammonium excretion, is a classic finding.

Non-anion gap acidosis A metabolic acidosis in which the serum anion gap is normal — meaning bicarbonate is lost or chloride is retained without accumulation of unmeasured anions. Also called hyperchloremic metabolic acidosis. The urine anion gap is used specifically to work up this type of acidosis.

Ammonium excretion The kidney's primary mechanism for excreting excess acid, by pairing ammonium (NH4+) with chloride in the urine. Ammonium is not directly measured on a routine urine panel, so the urine anion gap serves as an indirect proxy for it.

GI bicarbonate loss Loss of bicarbonate-rich fluid from the gastrointestinal tract, most commonly from diarrhea, which produces a non-anion-gap metabolic acidosis. The kidney responds appropriately by increasing ammonium chloride excretion, producing a negative urine anion gap.

Urine chloride The chloride concentration in a urine sample, in mEq/L. Because urinary ammonium is excreted paired with chloride, a relatively high urine chloride (compared to sodium + potassium) drives the urine anion gap negative and signals appropriate renal ammonium excretion.

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Scenarios

Real-World Examples

3 worked examples

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Positive — RTA-suggestive

Urine sodium 50, urine potassium 30, urine chloride 40 mEq/L

Urine sodium 50 mEq/L Urine potassium 30 mEq/L Urine chloride 40 mEq/L

50 + 30 − 40 = 40 mEq/L. A markedly positive urine anion gap suggests impaired renal ammonium excretion, consistent with renal tubular acidosis as the cause of the non-anion-gap acidosis.

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Negative — GI loss

Urine sodium 20, urine potassium 30, urine chloride 70 mEq/L

Urine sodium 20 mEq/L Urine potassium 30 mEq/L Urine chloride 70 mEq/L

20 + 30 − 70 = −20 mEq/L. A negative urine anion gap suggests the kidney is appropriately excreting ammonium chloride, consistent with GI bicarbonate loss such as diarrhea.

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Near zero — indeterminate

Urine sodium 40, urine potassium 20, urine chloride 58 mEq/L

Urine sodium 40 mEq/L Urine potassium 20 mEq/L Urine chloride 58 mEq/L

40 + 20 − 58 = 2 mEq/L. A urine anion gap this close to zero doesn't reliably distinguish a renal from a GI cause — consider the urine osmotic gap or repeat testing alongside the clinical picture.

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Deep Dive

Understanding the Urine Anion Gap

3 chapters · 3 min read

The urine anion gap is a quick screening calculation from a random or spot urine sample — urine sodium plus urine potassium minus urine chloride — used to help localize the cause of a non-anion-gap metabolic acidosis. This calculator computes the gap and classifies the result as negative, near zero, or positive.

How the urine anion gap works

The urine anion gap is calculated as UAG = UNa + UK − UCl, using electrolyte values from a random or spot urine sample, all in mEq/L. It is an indirect proxy for urinary ammonium (NH4+) excretion — ammonium is not measured directly on a routine urine panel, but it is excreted paired with chloride. A relatively high urine chloride (compared to sodium plus potassium) drives the calculated gap negative and implies high ammonium chloride excretion, meaning the kidney is responding appropriately to the acidosis.

A positive urine anion gap means urine chloride is relatively low, implying impaired ammonium excretion — a renal cause such as renal tubular acidosis (RTA). A negative urine anion gap points to an extrarenal, typically gastrointestinal, cause such as diarrhea-driven bicarbonate loss.

Inputs and what they mean

Urine sodium (UNa) and urine potassium (UK) are the measured urine cations, both in mEq/L. Urine chloride (UCl) is the measured urine anion and the value most directly tied to ammonium excretion.

All three values should come from the same urine sample — typically a random or spot collection rather than a timed collection, since the urine anion gap is a ratio-style screening estimate rather than an absolute excretion rate.

Limits and edge cases

The urine anion gap is a screening tool, not a diagnosis, and it is only meaningful once a non-anion-gap (normal serum-AG) metabolic acidosis has already been confirmed — see the vs Serum AG tab for how the two tests differ. Values close to zero don't reliably discriminate a renal from a GI cause. The urine anion gap can also be unreliable in patients with a high urine pH (bicarbonaturia), ketonuria, or unmeasured urine anions (e.g. from toluene exposure) — in those settings the urine osmotic gap is a more direct estimate of ammonium excretion.

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Questions

Frequently Asked Questions

6 questions

What is the urine anion gap formula?+

UAG = urine sodium (UNa) + urine potassium (UK) − urine chloride (UCl), all in mEq/L, from a spot or random urine sample.

What does a positive urine anion gap mean?+

A positive urine anion gap suggests renal tubular acidosis — the kidney is not excreting enough ammonium (NH4+), pointing to a renal cause of the non-anion-gap acidosis.

What does a negative urine anion gap mean?+

A negative urine anion gap suggests the kidney is appropriately excreting ammonium chloride, pointing to an extrarenal (e.g. GI) cause such as diarrhea-driven bicarbonate loss.

When is the urine anion gap used?+

It's used as a second-line tool in the workup of a non-anion-gap (normal serum anion gap) metabolic acidosis, to help localize the cause to a renal versus gastrointestinal source. It is not a first-line screen for metabolic acidosis in general — that's the role of the serum anion gap.

What units does the urine anion gap use?+

All three inputs — urine sodium, urine potassium, and urine chloride — and the resulting gap are in mEq/L (milliequivalents per liter).

What are the limitations of the urine anion gap?+

Values close to zero don't reliably discriminate a renal from a GI cause. The urine anion gap can also be unreliable with a high urine pH, ketonuria, or unmeasured urine anions — in those settings the urine osmotic gap is a more direct alternative.

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Urine Anion Gap Calculator

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Urine Anion Gap & Interpretation

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Why the Sign Matters

Serum Anion Gap vs Urine Anion Gap

How to Use This Calculator

Enter urine sodium, potassium, and chloride

Read the urine anion gap and interpretation

Confirm it's the right test to run

Formula & Methodology

Trust, Methodology & Sources

Key Terms Explained

Real-World Examples

Positive — RTA-suggestive

Negative — GI loss

Near zero — indeterminate

Understanding the Urine Anion Gap

How the urine anion gap works

Inputs and what they mean

Limits and edge cases

Frequently Asked Questions

Get a branded PDF of your results

Urine Anion Gap Calculator

Urine Electrolytes

Urine Anion Gap & Interpretation

Urine Anion Gap Reference Ranges

Why the Sign Matters

Serum Anion Gap vs Urine Anion Gap

How to Use This Calculator

Enter urine sodium, potassium, and chloride

Read the urine anion gap and interpretation

Confirm it's the right test to run

Formula & Methodology

Trust, Methodology & Sources

Key Terms Explained

Real-World Examples

Understanding the Urine Anion Gap

How the urine anion gap works

Inputs and what they mean

Limits and edge cases

Frequently Asked Questions

Get a branded PDF of your results

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