Urine anion gap (urine Na+ + urine K+ minus urine Cl-) is used to distinguish extrarenal from renal causes of normal anion gap metabolic acidosis. In a patient with acidosis due to chronic diarrhoea, the urine anion gap will be:

• A Strongly positive because urinary bicarbonate excretion is high
• B Strongly positive because urinary NH4+ excretion is high
• C Zero because sodium and potassium balance chloride exactly
• D Strongly negative because urinary NH4+ excretion is high ✓

Explanation

The urine anion gap is an indirect index of ammonium excretion, since NH4+ is excreted with chloride but is not measured. In diarrhoea, intact kidneys respond to acidosis by increasing NH4+ excretion as NH4Cl, so unmeasured chloride dominates and the gap becomes strongly negative. In distal RTA, impaired H+ and NH4+ secretion keeps the gap positive despite systemic acidosis, which is the discriminating finding.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.