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Distal RTA Nephrocalcinosis Diagnosis — ESENeph MCQ

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HardTubular DisordersDistal RTA Nephrocalcinosis DiagnosisESENeph

A patient has potassium 3.0 mmol/L, bicarbonate 15 mmol/L and chloride 116 mmol/L. During systemic acidosis, urine pH is 6.2 and urine sodium, potassium and chloride are 25, 15 and 12 mmol/L respectively. Which interpretation is correct?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: A — A positive urine anion gap of +28 mmol/L supports impaired distal ammonium excretion

Explanation lettering: B = shown as A · E = shown as B · A = shown as C · C = shown as D · D = shown as E

B is correct. The urine anion gap is sodium plus potassium minus chloride: 25 + 15 − 12 = +28 mmol/L. During systemic metabolic acidosis, intact renal ammonium excretion increases urinary ammonium chloride and usually makes the gap negative. A positive gap therefore supports inadequate ammonium excretion. Together with urine pH 6.2 and hypokalaemic non-anion-gap acidosis, this points to distal renal tubular acidosis. Proximal RTA can acidify urine once plasma bicarbonate has fallen below the reabsorptive threshold, while type 4 RTA is usually hyperkalaemic.

Reference: ERKNet/ESPN clinical practice points: distal renal tubular acidosis: https://www.erknet.org/fileadmin/Guidelines/Trepiccione_et_al.__2021__CPR_dRTA.pdf