A 45-year-old man with chronic diarrhea has: serum Na 138, K 2.8 mEq/L, Cl 112 mEq/L, HCO3 12 mEq/L, pH 7.28. Urine pH is 5.2, urine Na 18, urine K 12, urine Cl 38 mEq/L. Calculated serum anion gap = 138 − (112+12) = 14 mEq/L. Urine anion gap = 18+12−38 = −8 mEq/L (negative). The acid-base diagnosis is:

• A Normal anion gap metabolic acidosis due to GI bicarbonate loss (diarrhea) ✓
• B Normal anion gap metabolic acidosis due to renal tubular acidosis (Type 1)
• C High anion gap metabolic acidosis (lactic acidosis)
• D Distal RTA (Type 1) with inability to acidify urine

Explanation

Serum anion gap = 138 − (112+12) = 14 mEq/L — normal, confirming a normal anion gap metabolic acidosis. A negative urine anion gap (UAG = urine Na + urine K − urine Cl = 18+12−38 = −8 mEq/L) reflects high urinary ammonium excretion (NH4+ accompanies Cl−), indicating an intact renal response to acidosis — i.e., the kidney is correctly excreting acid. This is consistent with extrarenal bicarbonate loss (diarrhea). Renal tubular acidosis (Type 1 or Type 2) produces a positive or zero UAG because the kidney fails to excrete sufficient acid.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.