Urine anion gap and osmolal gap - use in localising cause of metabolic acidosis
Core concept
- Used to localise the cause of a normal anion gap (hyperchloraemic) metabolic acidosis once identified via the serum anion gap
- Urine anion gap (UAG) = (Urine Na+ + Urine K+) - Urine Cl-
- Estimates urinary NH4+ excretion indirectly (NH4+ is excreted with Cl-, not measured directly)
- Negative UAG -> appropriate renal NH4+ excretion -> acidosis is extrarenal (e.g. diarrhoea)
- Positive UAG -> inappropriately low NH4+ excretion -> acidosis is renal (renal tubular acidosis)
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