In normal anion gap metabolic acidosis, which statement about the urine anion gap is correct?

In normal anion gap metabolic acidosis, which statement about the urine anion gap is correct?

• A. Negative urine anion gap indicates GI bicarbonate loss
• B. Positive urine anion gap indicates GI bicarbonate loss
• C. Urine anion gap is not helpful in this differentiation
• D. Urine pH alone differentiates GI from GU

Answer: (A)

In normal anion gap metabolic acidosis, the urine anion gap reflects how much ammonium (NH4+) the kidneys are excreting to compensate for bicarbonate loss. NH4+ is excreted with chloride, so when GI bicarbonate loss (like diarrhea) drives the kidney to dump lots of NH4+ with Cl-, urine chloride rises relative to sodium and potassium. Since the urine anion gap is calculated as Na+ plus K+ minus Cl-, this increased chloride makes the gap negative. So a negative urine anion gap points to a GI source of bicarbonate loss. If the problem were renal (kidney) in origin, NH4+ excretion would be reduced, keeping urine chloride lower relative to Na+ and K+, and the urine anion gap would be positive or near zero. Urine pH alone doesn’t reliably distinguish between GI and renal causes, so the urine anion gap provides the functional clue needed in this question. Thus, a negative urine anion gap indicates GI bicarbonate loss as the cause of the metabolic acidosis.

In normal anion gap metabolic acidosis, the urine anion gap reflects how much ammonium (NH4+) the kidneys are excreting to compensate for bicarbonate loss. NH4+ is excreted with chloride, so when GI bicarbonate loss (like diarrhea) drives the kidney to dump lots of NH4+ with Cl-, urine chloride rises relative to sodium and potassium. Since the urine anion gap is calculated as Na+ plus K+ minus Cl-, this increased chloride makes the gap negative. So a negative urine anion gap points to a GI source of bicarbonate loss.

If the problem were renal (kidney) in origin, NH4+ excretion would be reduced, keeping urine chloride lower relative to Na+ and K+, and the urine anion gap would be positive or near zero. Urine pH alone doesn’t reliably distinguish between GI and renal causes, so the urine anion gap provides the functional clue needed in this question.

Thus, a negative urine anion gap indicates GI bicarbonate loss as the cause of the metabolic acidosis.