Urine Anion Gap Calculator
Urine Anion Gap Calculator
Use urine electrolytes as a proxy for ammonium excretion, distinguishing gastrointestinal bicarbonate loss from renal tubular acidosis.
Urine Anion Gap
Na + K − ClUrine sodium 40, potassium 25, chloride 80 mmol/L
Formula
- Na, K, Cl
- mmol/L, spot or 24-hour urine electrolytes
- negative gap
- appropriate ammonium (as NH₄Cl) excretion
- positive gap
- impaired ammonium excretion
Worked example
Urine sodium 40, potassium 25, chloride 80 mmol/L
40 + 25 − 80 = −15 mmol/L
Below −10 → negative, extrarenal bicarbonate loss
Interpreting the gap
| Urine anion gap (mmol/L) | Pattern | Consider |
|---|---|---|
| < −10 | Negative | Extrarenal HCO₃⁻ loss, e.g. diarrhoea — ammonium excretion is appropriate |
| −10 to 10 | Near zero | Indeterminate — consider the urine osmolal gap |
| > 10 | Positive | Renal tubular acidosis — ammonium excretion is impaired |
A proxy for what cannot be measured directly
Ammonium is the kidney’s main mechanism for excreting a daily acid load, but most clinical analysers cannot measure urinary ammonium directly. The urine anion gap is a workaround: ammonium is excreted paired with chloride as ammonium chloride, so a rise in unmeasured urinary chloride relative to sodium and potassium indirectly signals a rise in ammonium excretion, and the gap turns negative.
In a normal anion gap (hyperchloraemic) metabolic acidosis, this distinction does real clinical work. A negative gap — sometimes remembered as “neGUTive” — shows the kidney is excreting ammonium appropriately in response to the acid load, pointing to an extrarenal cause such as diarrhoea, where bicarbonate is lost from the gut and the kidney is responding correctly. A positive gap shows the kidney has failed to raise ammonium excretion despite the acidosis, pointing to renal tubular acidosis, where the defect lies in the kidney itself.
The gap becomes invalid whenever unmeasured anions are present in the urine, because they are excreted with a cation other than ammonium and confound the calculation. Ketones, hippurate from toluene inhalation (as in glue-sniffing), and penicillin derivatives are the classic examples. In any of these settings the urine osmolal gap — which estimates ammonium from the difference between measured and calculated urine osmolality — should be used instead.
Frequently asked questions
What does a negative urine anion gap mean?
It indicates appropriate ammonium excretion in response to a metabolic acidosis, pointing to an extrarenal cause of bicarbonate loss such as diarrhoea.
What does a positive urine anion gap mean?
It indicates the kidney is failing to raise ammonium excretion despite an acid load, pointing to renal tubular acidosis.
Why is it called the neGUTive gap trick?
It is a memory aid: a negative gap points to a gut (gastrointestinal) cause of bicarbonate loss, such as diarrhoea, rather than a renal one.
When is the urine anion gap unreliable?
When unmeasured urinary anions are present — ketones, hippurate from toluene exposure, or penicillins — since these are excreted with cations other than ammonium and invalidate the calculation. Use the urine osmolal gap instead.
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References
- Batlle DC, Hizon M, Cohen E, Gutterman C, Gupta R. The use of the urinary anion gap in the diagnosis of hyperchloremic metabolic acidosis. N Engl J Med. 1988;318(10):594–9.
- Rose BD, Post TW. Clinical Physiology of Acid-Base and Electrolyte Disorders. 5th ed. McGraw-Hill; 2001.