What is the urine anion gap formula?

The urine anion gap equals urine sodium plus urine potassium minus urine chloride, all in mmol per litre. It is an indirect marker of how much ammonium the kidney is excreting, because ammonium is excreted alongside chloride.

How do I interpret a positive versus negative gap?

A negative gap, often around minus 20 to minus 50, means the kidney is excreting plenty of ammonium and the bicarbonate loss is extrarenal, typically diarrhoea. A positive gap means the kidney is failing to excrete ammonium, pointing to renal tubular acidosis, classically the distal or type 4 forms.

What is a better alternative when the gap is unreliable?

The urine osmolar gap is preferred when conditions invalidate the anion gap, because it estimates ammonium directly from measured versus calculated urine osmolality. Many nephrologists use both together in difficult cases.

Does a near-zero gap mean anything?

A gap between about minus 10 and plus 10 is indeterminate. It does not reliably separate a renal from a gastrointestinal cause, so you should not draw a firm conclusion and may need the urine osmolar gap or further testing.

Urine Anion Gap Calculator

Q: When is the urine anion gap valid?

It is only meaningful in a normal anion gap, hyperchloraemic metabolic acidosis. It becomes unreliable when urine pH is above 6.5, when ketones or other unmeasured anions are present, or in volume depletion, because those situations change urinary chloride independently of ammonium.

The urine anion gap is a simple bedside calculation that helps work out why a patient has a normal anion gap metabolic acidosis. Because the kidney’s main acid-handling response is to excrete ammonium, and ammonium leaves with chloride, the gap acts as an indirect window onto that response.

How it works

The calculation adds the two main urinary cations measured and subtracts the main measured anion:

UAG = U(Na+) + U(K+) - U(Cl-)     (all in mmol/L)

When the kidney excretes a lot of ammonium, urine chloride climbs to balance it, so chloride exceeds sodium plus potassium and the gap turns negative. When the kidney cannot make ammonium, chloride stays relatively low and the gap is positive.

Interpretation and caveats

A clearly negative gap signals an appropriate renal response and therefore an extrarenal cause of bicarbonate loss, most often diarrhoea. A positive gap signals impaired ammonium excretion and points to renal tubular acidosis, especially distal type 1 or hyperkalaemic type 4. The tool only applies inside a hyperchloraemic, normal-anion-gap acidosis. It becomes unreliable when urine pH is above 6.5, when ketones or other unmeasured anions are present, or in volume depletion, and in those situations the urine osmolar gap is the better estimate of ammonium excretion.