When the kidneys respond normally to acidosis they excrete lots of ammonium chloride, making the urine anion gap negative. A positive gap suggests renal tubular acidosis. The test is unreliable with high urine ketones, very low urine sodium or volume depletion.
How to use the Urine Anion Gap Calculator
- Urine sodium
- Urine potassium
- Urine chloride
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter urine sodium, potassium and chloride from a spot urine sample.
Inputs explained
| Field | What to enter |
|---|---|
| Urine sodium | In mEq/L |
| Urine potassium | In mEq/L |
| Urine chloride | In mEq/L |
Example values
How the main result changes when Urine sodium changes and everything else stays at the example values above:
| Urine sodium (mEq/L) | Urine anion gap (mEq/L) |
|---|---|
| 20 | -40 |
| 30 | -30 |
| 40 | -20 |
| 50 | -10 |
| 60 | 0 |
| 80 | 20 |
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
Urine anion gap = urine Na⁺ + urine K⁺ − urine Cl⁻
Negative: kidney excreting acid (diarrhoea likely) · Positive: impaired acid excretion (renal tubular acidosis)
Worked example
Inputs — Urine sodium: 40 mEq/L; Urine potassium: 30 mEq/L; Urine chloride: 90 mEq/L.
Results — Urine anion gap = -20 mEq/L; Interpretation in normal anion gap acidosis = Negative – gastrointestinal bicarbonate loss likely (e.g. diarrhoea).
Key terms
- RTA
- Renal tubular acidosis.
Frequently asked questions
What does a negative urine anion gap mean?
The kidney is excreting ammonium appropriately – think diarrhoea.
When is the test unreliable?
With ketones, very low urine sodium (< 25 mEq/L) or polyuria.
What is the alternative?
The urine osmolal gap.