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Urine Anion Gap (UAG) Calculator

· Last updated：2026-08-06

This tool is for informational purposes only and does not constitute medical, financial, or legal advice; consult a qualified professional for advice.

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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Tool usage guide

Learn when to use this tool, what it supports, and how real users apply it.

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

  1. 1Enter urine sodium (U_Na), urine potassium (U_K), and urine chloride (U_Cl) in mmol/L.
  2. 2The calculator applies the formula UAG = (U_Na + U_K) − U_Cl.
  3. 3The result is reported in mmol/L with a category and interpretation related to urinary NH₄⁺ excretion.
  4. 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.

Examples

1. Diarrhea-associated bicarbonate loss

Clinician evaluating hyperchloremic metabolic acidosis

Background

A patient with diarrhea has urine sodium of 25 mmol/L, urine potassium of 35 mmol/L, and urine chloride of 120 mmol/L.

Problem

Determine whether the kidneys are appropriately increasing NH₄⁺ excretion.

How to use

Enter urine sodium 25, urine potassium 35, urine chloride 120, and select 1 decimal place.

U_Na = 25 mmol/L; U_K = 35 mmol/L; U_Cl = 120 mmol/L; decimal places = 1

Outcome

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.

FAQ

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.

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Serum Anion Gap (AG) 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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Fractional Excretion of Sodium (FENa)

Calculate the Fractional Excretion of Sodium (FENa) to differentiate prerenal from intrinsic acute kidney injury: FENa (%) = [U_Na × S_Cr] / [S_Na × U_Cr] × 100. In oliguric AKI, FENa <1% suggests prerenal azotemia (sodium-avid state: volume depletion, heart failure, hepatorenal syndrome), while FENa ≥1% suggests intrinsic injury (typically ATN). FENa >4% is sometimes cited as supportive of post-obstructive pathology. Caveats: unreliable on diuretics (use FEUrea <35%), contrast/sepsis ATN can present with low FENa, chronic CKD and glucosuria elevate FENa independently. Derived from Espinel CH, JAMA 1976. Most useful in oliguric AKI; interpret with full clinical context. Not medical advice.

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Free Water Clearance Calculator

Calculate solute-free water clearance (C_H₂O) and electrolyte-free water clearance (eC_H₂O) to differentiate hypo-/hypernatremia. Classic: C_H₂O = V·(1 − U_osm/P_osm); positive = dilute urine excreting free water (diabetes insipidus, polydipsia, water-load recovery), negative = concentrated urine conserving free water (dehydration, SIADH). Electrolyte-free: eC_H₂O = V·(1 − (U_Na + U_K)/P_Na), which better predicts the direction of serum sodium change — a negative eC_H₂O means the urine is electrolyte-free-water-rich and tends to raise serum Na⁺, informing IV fluid choice. Optional Na/K inputs enable the electrolyte-free calculation. Derived from Rose, Shimizu 2002 (Nephron), and NephSIM. Point estimate; interpret with full clinical context. Not medical advice.

U_Na = 30 mmol/L; U_K = 20 mmol/L; U_Cl = 40 mmol/L; decimal places = 1

Outcome

UAG = 10 mmol/L. The positive result is categorized as suggesting low NH₄⁺ excretion and a possible renal tubular acidosis pattern.