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Nephrology / Renal
Urine Anion Gap

Urine Anion Gap (UAG)

Differentiates causes of non-anion gap metabolic acidosis

Urine Electrolytes
Required (0–400)
Required (0–300)
Required (0–500)
— mEq/L (UAG)
Non-Anion Gap Metabolic Acidosis — Differential
UAGInterpretationDifferential Diagnosis
Negative (< 0)Normal NH₄⁺ excretionGI losses (diarrhea), Type II (proximal) RTA, external losses
Positive (> 0)Impaired NH₄⁺ excretionType I (distal) RTA, Type IV RTA (hypoaldosteronism)
≈ 0 (borderline)IndeterminateConsider urine osmolal gap for NH₄⁺ estimation
UAG = UNa + UK − UCl
Reflects unmeasured urinary anions (primarily NH₄⁺ as NH₄Cl)
Negative UAG → ↑ NH₄⁺ excretion → appropriate renal response
Positive UAG → ↓ NH₄⁺ excretion → renal tubular defect
Clinical Use: Apply only in the context of non-anion gap metabolic acidosis (hyperchloremic). Ensure the patient is not volume-depleted severely (very low UCl can confound). If UAG is equivocal, calculate the urine osmolal gap (NH₄⁺ ≈ Uosm gap / 2) for more accurate NH₄⁺ estimation.
⚕️ For qualified healthcare professionals only. Not a substitute for clinical judgment.

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