If metabolic acidosis is present and the urine anion gap is positive, which renal tubular acidosis types should be considered first, and what test helps differentiate?

• A. Proximal/type 2 RTA
• B. Distal/type 1 RTA and type 4 RTA; check serum potassium
• C. Diabetic ketoacidosis
• D. Normal anion gap metabolic acidosis due to diarrhea

Answer: (B)

Metabolic acidosis with a positive urine anion gap points to a problem with renal ammonium excretion, not a gut loss of bicarbonate. That makes renal tubular acidosis the leading consideration, specifically distal (type 1) RTA and type 4 RTA, both of which impair ammonium excretion and produce a positive UAG. To tell them apart, check the serum potassium: distal RTA typically has low potassium (hypokalemia), while type 4 RTA has high potassium (hyperkalemia) due to aldosterone deficiency or resistance. Proximal (type 2) RTA can occur with acidosis, but a positive UAG in this context most strongly suggests distal or type 4 RTA. Diarrheal acidosis would usually have a negative UAG because the kidney increases ammonium excretion to compensate, and diabetic ketoacidosis is an extrarenal cause of acidosis not primarily defined by a positive UAG.