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Urine Anion Gap Calculator

Calculate urine anion gap (Na+ + K+ - Cl-) to differentiate renal tubular acidosis from non-renal causes of metabolic acidosis.

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Inputs

Urine Sodium (Na+)

Urine Potassium (K+)

Urine Chloride (Cl-)

Urine Anion Gap

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Urine Anion Gap—mEq/L

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Urine Anion Gap: Formula, Derivation, and Clinical Interpretation

The urine anion gap (UAG) is a calculated index that estimates urinary ammonium (NH4⁺) excretion — the kidney's principal mechanism for eliminating excess acid. Clinicians apply the UAG to distinguish renal from non-renal causes of normal anion gap (hyperchloremic) metabolic acidosis, making it a cornerstone tool in the diagnosis of renal tubular acidosis (RTA) and unexplained acidemia.

The UAG Formula

UAG = (Na⁺ + K⁺) − Cl⁻

All three variables are measured in milliequivalents per liter (mEq/L), which is numerically identical to mmol/L for monovalent ions. A standard urine electrolyte panel ordered from any clinical laboratory provides these values from a single urine specimen.

Variable Definitions

Urine Sodium (Na⁺) — The dominant measured urinary cation. Concentration varies with dietary sodium intake and renal tubular handling. Values below 20 mEq/L suggest volume depletion.
Urine Potassium (K⁺) — The second major measured cation. Included because potassium contributes meaningfully to total cation load, particularly in patients taking diuretics or with hyperaldosteronism.
Urine Chloride (Cl⁻) — The primary measured urinary anion and the most diagnostically critical variable. As ammonium excretion rises, chloride rises proportionally to maintain urinary electroneutrality, driving the UAG negative.

Physiological Basis: Why UAG Reflects Ammonium

Urine must remain electrically neutral — total cations equal total anions. The measured cations (Na⁺ + K⁺) and the measured anion (Cl⁻) leave a gap that represents unmeasured ions. In clinical practice, the most important unmeasured cation is ammonium (NH4⁺), which the proximal tubule synthesizes from glutamine and the collecting duct secretes into the urine.

When systemic acid load rises, healthy kidneys dramatically increase NH4⁺ output. Because NH4⁺ is excreted paired with Cl⁻, urinary chloride climbs, shrinking — and eventually reversing — the gap. A strongly negative UAG therefore signals intact renal acidification. Impaired NH4⁺ secretion, as occurs in distal RTA, leaves Cl⁻ low relative to Na⁺ and K⁺, yielding a positive UAG. This mechanism is detailed in Kaplan LJ et al., The Urine Anion Gap in Context (PMC/NIH, 2018).

Interpreting the Result

Negative UAG (typically −20 to −50 mEq/L)

A negative UAG confirms robust NH4⁺ excretion and an intact renal response. Causes of normal anion gap acidosis with a negative UAG include:

Severe diarrhea with bicarbonate loss — the most common scenario globally
Proximal (Type 2) RTA with preserved distal acidification
Exogenous acid ingestion (e.g., ammonium chloride, acetazolamide)
Urinary diversion procedures (e.g., ileal conduit)

Positive UAG (greater than 0 mEq/L)

A positive UAG in the setting of confirmed metabolic acidosis indicates defective renal ammonium excretion. Key diagnoses include:

Distal (Type 1) RTA — inability to acidify urine below pH 5.5; UAG often exceeds +20 mEq/L
Type 4 RTA (hyporeninemic hypoaldosteronism) — commonly seen with diabetes mellitus, ACE-inhibitor use, or chronic kidney disease
Advanced chronic kidney disease — reduced nephron mass limits NH4⁺ synthesis

According to StatPearls: Biochemistry, Anion Gap (NCBI Bookshelf), a UAG persistently above +20 mEq/L carries high specificity for distal RTA when serum pH is below 7.35 and the serum anion gap is normal. The UCSF Hospitalist Handbook Algorithm for Acid-Base Disorders incorporates UAG as a branch-point decision in evaluating non-anion gap acidosis.

Worked Example

Case 1 — Distal RTA: A 34-year-old woman has serum pH 7.28, serum HCO3⁻ 14 mEq/L, serum anion gap 10 mEq/L, and urine pH 6.2. Urine electrolytes: Na⁺ = 45 mEq/L, K⁺ = 22 mEq/L, Cl⁻ = 18 mEq/L. UAG = (45 + 22) − 18 = +49 mEq/L. This strongly positive result with high urine pH confirms distal RTA.

Case 2 — Diarrhea: A 19-year-old man has serum pH 7.30, HCO3⁻ 17 mEq/L after three days of profuse diarrhea. Urine: Na⁺ = 35 mEq/L, K⁺ = 30 mEq/L, Cl⁻ = 95 mEq/L. UAG = (35 + 30) − 95 = −30 mEq/L. The negative UAG confirms appropriate renal NH4⁺ excretion and a GI bicarbonate loss etiology.

Limitations

The UAG loses reliability when urinary Na⁺ falls below 20 mEq/L (volume depletion), as sodium avidity limits chloride delivery independent of ammonium. It is also distorted by ketonuria, hippurate excretion (toluene ingestion), and heavy proteinuria — all of which add unmeasured anions. In these scenarios, the urine osmol gap provides a more accurate surrogate for NH4⁺ excretion.

Reference

Frequently asked questions

What is a normal urine anion gap value?

In healthy individuals without an acid load, the urine anion gap is typically near zero or slightly positive, ranging from 0 to +10 mEq/L. When the body faces metabolic acidosis, healthy kidneys dramatically increase ammonium excretion, shifting the UAG to a clearly negative value — commonly between -20 and -50 mEq/L. A UAG that stays at zero or positive despite confirmed systemic acidosis indicates impaired renal acid excretion and warrants further evaluation for renal tubular acidosis.

What does a positive urine anion gap indicate?

A positive UAG (greater than 0 mEq/L) during normal anion gap metabolic acidosis indicates the kidneys are failing to excrete adequate ammonium in response to the acid load. This pattern is characteristic of distal renal tubular acidosis (Type 1 RTA), Type 4 RTA caused by hypoaldosteronism (common in diabetics and CKD patients), or severely reduced nephron mass in advanced chronic kidney disease. A UAG above +20 mEq/L is considered highly specific for distal RTA.

What does a negative urine anion gap mean?

A negative UAG — typically -20 mEq/L or more negative — means the kidneys are excreting large quantities of ammonium chloride, representing an appropriate and intact renal response to systemic acidosis. This pattern implicates a non-renal source of bicarbonate loss, most commonly severe secretory diarrhea. Other causes include proximal RTA (Type 2), exogenous acid ingestion such as ammonium chloride, or urinary diversion. Kidney function itself is preserved in these cases.

How is the urine anion gap different from the serum anion gap?

The serum anion gap (Na − [Cl + HCO3], normal 8–12 mEq/L) identifies unmeasured anions in the blood — such as lactate, ketoacids, or uremic acids — and classifies metabolic acidosis as either high or normal anion gap. The urine anion gap is a second-step tool applied only after a normal serum anion gap is found. It then determines whether the bicarbonate deficit stems from renal tubular dysfunction or from gastrointestinal bicarbonate loss, guiding entirely different management pathways.

When should clinicians order a urine anion gap?

The urine anion gap is most informative when a patient has confirmed normal anion gap (hyperchloremic) metabolic acidosis — specifically a serum pH below 7.35 and serum bicarbonate under 22 mEq/L with a serum anion gap under 12 mEq/L. It is a key step in differentiating diarrhea-induced bicarbonate loss from renal tubular acidosis, evaluating nephrolithiasis risk in patients with distal RTA, and guiding decisions about alkali therapy. It should not be applied in high anion gap acidosis, where its interpretation is not valid.

What are the main limitations of the urine anion gap calculation?

The UAG becomes unreliable when urinary sodium falls below 20 mEq/L, as occurs in severe volume depletion — low sodium delivery restricts chloride excretion independently of ammonium output, producing a falsely positive result. Significant ketonuria, hippurate excretion from toluene poisoning, and heavy proteinuria all add unmeasured urinary anions that distort the calculation. In these scenarios, the urine osmol gap — estimated ammonium = (measured urine osmolality minus the calculated osmolality) divided by 2 — offers a more accurate reflection of NH4+ excretion.