Which parameter serves as a bedside surrogate for urinary ammonium (NH4+) excretion when assessing a patient with normal anion gap metabolic acidosis?

• A Urine pH alone
• B Urine anion gap = urine (Na+ + K+) minus urine Cl- ✓
• C Transtubular potassium gradient
• D Fractional excretion of sodium

Explanation

NH4+ is excreted with chloride as NH4Cl, so adequate ammonium excretion lowers the urine anion gap to a negative value (below zero). A positive urine anion gap indicates impaired NH4+ disposal, as seen in distal RTA, while diarrhoea produces a strongly negative gap. Urine pH alone is misleading because acidic urine can occur despite inadequate total acid excretion. TTKG and fractional excretion of sodium assess potassium handling and tubular function respectively.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.