Urine Osmolal Gap (Estimated Urinary Ammonium)

Urine Osmolal Gap estimates urinary ammonium excretion to distinguish renal from extra-renal causes of normal anion gap metabolic acidosis. Estimate only — invalid if other unmeasured urine osmoles are present (ethanol, methanol, ethylene glycol, mannitol, contrast); not a substitute for direct urine NH4 measurement.