Anion Gap, Urine

Diagnostic Use

The urine anion gap (UAG) is a calculated parameter that indirectly gives information about the amount of ammonium excreted by the kidney.

Urine anion gap = Urine Na⁺ + Urine K⁺ - Urine Cl^-

UAG may be measured to differentiate between the causes of a normal anion gap (hyperchloraemic) metabolic acidosis.

Department

Core Biochemistry

Delphic Registration Code

UGAP

Constituent Tests

• Sodium, Urine
• Potassium, Urine
• Chloride, Urine

Synonyms

Ammonia, urine
Ammonium, urine

Turnaround Time

3 hours

Test Code

8370

Specimen Type

• Urine

Specimen Requirements

Tube/Container Type:

Preferred

• Pottle

Aliquot Instructions

Sample Type

• Urine

Minimum Volume (ml)

5.0

Transport to Lab

Ambient (8 - 24 degrees Celsius)

Interpretation

In a patient with a normal anion gap metabolic acidosis, a positive UAG indicates reduced renal ammonium (acid) excretion. Causes include type 1 (distal) renal tubular acidosis, which is associated with hypokalaemia, or hypoaldosteronism (type 4 RTA) which is associated with hyperkalaemia.

A negative UAG in the setting of a normal anion gap metabolic acidosis indicates appropriate renal ammonium excretion. Causes include GI loss of bicarbonate and type 2 (proximal) renal tubular acidosis.

Reference Intervals

N/A – see interpretation for more details.

Test Method

Calculated test, reported in mmol/L:

Urine anion gap = Urine Na⁺ + Urine K⁺ - Urine Cl^-

Contact Canterbury Health Laboratories on +64 3 364 0484 or email LabInfo@cdhb.health.nz

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