This calculator helps you estimate the urine anion gap based on sodium (Na⁺), potassium (K⁺), and chloride (Cl⁻) levels measured in urine. Read on to learn what is a normal urine anion gap.

Please note that this calculator estimates the urine anion gap. For serum anion gap based on anions found in blood serum, check out our anion gap calculator.

If this is helpful to you, you might like our GFR calculator or corrected calcium calculator.

How does the urine anion gap calculator work?

To estimate the anion gap, it is necessary to measure electrolyte levels in the urine. It is the difference between the measured cations and anions, which in this case is subtraction of the concentration of chloride ions from those of sodium and potassium:

AG = [Na⁺] + [K⁺] − [Cl⁻]

What is a normal urine anion gap?

The urine anion gap is a measure used in the differential diagnosis of metabolic acidosis to determine whether the kidneys are capable of appropriately excreting acid. The urine anion gap represents the unmeasured ions in urine. Ammonium (NH₄⁺) is the most important form of acid excretion from the kidney. It's difficult to measure directly, but it's usually accompanied by the chloride anion. Therefore, a negative anion gap can be used as evidence of increased NH₄⁺ excretion. In a metabolic acidosis without a serum anion gap:

  • A positive urine AG suggests a low urinary NH₄⁺ (renal tubular acidosis).
  • A negative urine AG suggests a high urinary NH₄⁺ (diarrhea).
Małgorzata Koperska, MD

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