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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
| UAG | Interpretation | Differential Diagnosis |
|---|---|---|
| Negative (< 0) | Normal NH₄⁺ excretion | GI losses (diarrhea), Type II (proximal) RTA, external losses |
| Positive (> 0) | Impaired NH₄⁺ excretion | Type I (distal) RTA, Type IV RTA (hypoaldosteronism) |
| ≈ 0 (borderline) | Indeterminate | Consider 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
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.
