Urine Anion Gap Calculator

The Urine Anion Gap Calculator differentiates between gastrointestinal and renal causes of non-anion gap (hyperchloremic) metabolic acidosis. The urine anion gap (UAG) serves as an indirect measure of urinary ammonium (NH4+) excretion, which is the kidney's primary mechanism for eliminating acid. By assessing whether the kidneys are appropriately excreting ammonium in response to acidosis, the UAG helps distinguish between diarrheal bicarbonate loss (where renal acidification is intact) and renal tubular acidosis (where renal acidification is impaired).

The UAG is calculated as: UAG = Urine Na+ + Urine K+ - Urine Cl-. The rationale is based on electroneutrality: in urine, the major measured cations (Na+, K+) and measured anion (Cl-) should balance, with unmeasured cations (primarily NH4+) accounting for any difference. When NH4+ excretion is high (as in appropriate renal response to acidosis), ammonium is excreted with chloride as NH4Cl, making urine chloride exceed the sum of sodium and potassium, producing a negative UAG. When NH4+ excretion is impaired (renal tubular acidosis), this chloride excess does not occur, and the UAG remains positive.

A negative urine anion gap (typically -20 to -50 mEq/L) indicates adequate renal ammonium excretion and points to an extrarenal cause of metabolic acidosis, most commonly diarrhea. During diarrhea, the intestine loses bicarbonate-rich fluid, causing metabolic acidosis. The kidneys respond appropriately by increasing ammonium production and excretion, which is reflected by the negative UAG. This appropriate renal response confirms that renal acidification mechanisms are intact.

A positive urine anion gap (typically 0 to +20 mEq/L or higher) indicates inadequate renal ammonium excretion and suggests renal tubular acidosis (RTA). In RTA, the kidney cannot adequately excrete ammonium despite systemic acidosis. Type 1 (distal) RTA results from impaired hydrogen ion secretion in the collecting duct. Type 4 RTA results from aldosterone deficiency or resistance, which impairs both potassium secretion and ammonium excretion. Type 2 (proximal) RTA results from impaired proximal bicarbonate reabsorption, but UAG can be negative in this type because distal acidification is intact.

Important limitations of the UAG include its unreliability when urine pH is very alkaline (above 6.5), when there are significant unmeasured urinary anions (ketoacids, hippurate from toluene exposure, penicillin-derived anions), or when sodium intake is very low. In these situations, the urine osmolal gap provides a more reliable estimate of urinary ammonium excretion. The urine osmolal gap is calculated as measured urine osmolality minus calculated urine osmolality, with values above 100 mOsm/kg suggesting adequate ammonium excretion.

The UAG is most diagnostically useful in the evaluation of hyperchloremic metabolic acidosis after the anion gap has been confirmed to be normal. The clinical scenario is typically a patient with metabolic acidosis and normal serum anion gap where the question is: is the kidney or the GI tract responsible? The UAG answers this question quickly and inexpensively using a spot urine sample, directing the subsequent workup toward gastrointestinal evaluation or renal tubular acidosis testing.