What does a negative urine anion gap mean?

A negative UAG (urine Cl > urine Na + urine K) means the kidneys are excreting significant ammonium (NH4+), which carries chloride along with it. This is the expected response to metabolic acidosis and points to a GI source of bicarbonate loss, most commonly diarrhea.

What does a positive urine anion gap mean?

A positive UAG suggests the kidneys are not excreting adequate ammonium despite systemic acidosis. This is abnormal and indicates a renal cause, most commonly renal tubular acidosis (RTA type 1 or type 4). It can also be seen in early renal failure.

Why is the urine anion gap an indirect measure of ammonium?

Urine ammonium (NH4+) is positively charged but is not routinely measured in clinical labs. Because NH4+ is excreted with Cl−, a rise in urine Cl beyond what Na and K account for signals high ammonium. The UAG formula captures this imbalance as a proxy.

When is the urine anion gap unreliable?

The UAG is unreliable when urine sodium is very low (< 25 mEq/L), indicating a volume-depleted state where sodium avidity limits Cl delivery. It is also confounded by ketoaciduria, where ketoanions (unmeasured anions) are excreted with Na and K, falsely elevating the UAG.

How does the urine anion gap differ from the serum anion gap?

The serum anion gap (Na − Cl − HCO3) identifies unmeasured serum anions and classifies metabolic acidosis as high-AG or non-AG. The urine anion gap is only used once non-AG acidosis is confirmed, to pinpoint whether the problem is renal or GI in origin.

Urine Anion Gap Calculator

Enter urine sodium, potassium, and chloride to calculate the urine anion gap and interpret the likely cause of non-anion-gap metabolic acidosis.

Urine Electrolyte Values

Spot urine sample — all values in mEq/L

Urine Sodium (Na⁺) mEq/L

Urine Potassium (K⁺) mEq/L

Urine Chloride (Cl⁻) mEq/L

Results

Enter urine electrolytes and click Calculate

UAG Reference & Clinical Context

UAG Result	Ammonium (NH4+)	Likely Diagnosis
< 0 (negative)	High — renal response intact	GI bicarbonate loss: diarrhea, ileostomy, fistula
0 – +20	Borderline / low	Equivocal; consider urine pH, clinical context, or urine osmolal gap
> +20 (positive)	Low — impaired renal excretion	Renal tubular acidosis (type 1 distal or type 4); early renal failure

When UAG may be unreliable

Urine Na < 25 mEq/L (volume depletion limits Cl delivery)
Ketoaciduria — ketoanions replace Cl, falsely lowers UAG
High-anion-gap acidosis (UAG not indicated)
Toluene toxicity — hippurate excreted with Na, falsely negative UAG

RTA sub-types at a glance

Type 1 (distal): positive UAG, urine pH > 5.5
Type 2 (proximal): variable UAG, urine pH < 5.5 when acidemic
Type 4: positive UAG, hyperkalemia, low aldosterone effect

For educational and reference purposes only. Clinical decisions require a qualified healthcare professional.

Summary

The urine anion gap (UAG) is calculated as urine Na + urine K − urine Cl and serves as a surrogate for urine ammonium (NH4+) excretion. A negative UAG (urine Cl exceeds Na + K) indicates adequate renal acid excretion, pointing to a GI cause of non-anion-gap metabolic acidosis such as diarrhea. A positive or zero UAG suggests impaired ammonium excretion, consistent with renal tubular acidosis (RTA) or other renal causes.

How it works

Collect a spot urine sample and measure urine sodium (Na+), potassium (K+), and chloride (Cl−).
Enter each value in mEq/L into the calculator.
The tool applies the formula: UAG = urine Na + urine K − urine Cl.
A negative UAG (< 0) indicates high urine ammonium — GI bicarbonate loss is likely (e.g., diarrhea).
A positive UAG (> 0) suggests low urine ammonium — impaired renal acid excretion points to RTA.
Review the interpretation panel for clinical guidance and next steps.

Use cases

Differentiating renal tubular acidosis from diarrhea-induced non-anion-gap metabolic acidosis.
Evaluating a patient with persistent hyperchloremic metabolic acidosis.
Bedside acid-base workup in the emergency department or nephrology consult.
Medical education and USMLE preparation for acid-base physiology.
Monitoring response to treatment in type 1 (distal) or type 4 RTA.
Screening for ammonium excretion defects in chronic kidney disease.
Teaching tool for nephrology fellows reviewing urine electrolyte interpretation.

Frequently Asked Questions

Last updated: 2026-06-15 · Reviewed by Nham Vu