Urine Anion Gap Calculator

Created by Małgorzata Koperska, MD
Reviewed by Bogna Szyk and Steven Wooding
Last updated: Feb 23, 2023

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 the 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 the subtraction of the concentration of chloride ions from those of sodium and potassium:

AG = [Na⁺] + [K⁺] − [Cl⁻]

What is a normal urine anion gap?

In healthy individuals, the urine anion gap is usually between 0 and 10 mEq/L, although this range is sometimes extended to include values from -10 to 20 mEq/L.

More broadly speaking, the urine anion gap is a measure used in the differential diagnosis of metabolic acidosis to determine whether the kidneys can appropriately excrete acid.

The urine anion gap represents the unmeasured ions in urine. Ammonium (NH₄⁺) is the kidney's most important form of acid excretion. It isn't easy to measure directly, but the chloride anion usually accompanies it. Therefore, we can use a negative anion gap 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).


How do I calculate urine anion gap?

To calculate the urine anion gap:

  1. Measure the urine concentrations of potassium (Na+), sodium (K+), and chloride (Cl-) ions.

  2. Add the concentrations of potassium and sodium together.

  3. To find the urine anion gap, subtract the value measured for chloride ions from the sum.

When do you calculate urine anion gap?

The urine anion gap is usually calculated to check the patient's renal (kidney) function. Since it also indicates if the ammonium is increased or decreased, it can help in the differential diagnosis of metabolic acidosis.

What is the urine anion gap for 30 mEq/L of sodium and 25 mEq/L of potassium?

If we assume that the urine chloride concentration is 15 mEq/L, that gives us a urine anion gap of 40 mEq/L.

The anion gap will be negative if the chlorine concentration exceeds 55 mEq/L.

Does urine anion gap use random or 24-hour values?

In determining the urine anion gap, you'd want to use concentrations measured over 24 hours. This accounts for any variations due to water intake, circadian rhythm, consumption of certain foods, etc.

Małgorzata Koperska, MD
Urine anion gap
In this case, the values in mEq/L are equal to those in mmol/L. If your results are in mmol/L, type in these values.
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