Renal tubular acidosis
Renal tubular acidosis is a normal anion gap metabolic acidosis caused by impaired renal bicarbonate reabsorption (type 2, proximal) or impaired distal acid excretion (type 1, hypokalemic distal, or type 4, hyperkalemic distal from aldosterone deficiency or resistance). The urine anion gap, serum potassium, and urine pH distinguish the three types, which are treated respectively with alkali replacement plus a thiazide, potassium citrate, and correction of the underlying cause plus mineralocorticoid replacement when appropriate.
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What this article covers
- Overview
Renal tubular acidosis (RTA) is a normal anion gap metabolic acidosis from a kidney defect in bicarbonate reabsorption or acid excretion, distinct from gastrointestinal bicarbonate loss (diarrhea)…
- Epidemiology
Type 2 (proximal) RTA most commonly occurs with myeloma light chains or nephrotoxic medications, including ifosfamide, carbonic anhydrase inhibitors (acetazolamide, topiramate), and tenofovir…
- Risk factors
Multiple myeloma or a dysproteinemia (type 2 RTA, from light-chain proximal tubular toxicity).
- Pathophysiology
Type 2 (proximal) RTA: the proximal tubule fails to reabsorb filtered bicarbonate, causing bicarbonate wasting until a new resorptive threshold is reached and serum bicarbonate stabilizes around…
- Clinical features
Normal anion gap (hyperchloremic) metabolic acidosis on a basic metabolic panel, often found incidentally or during evaluation of an unexplained low bicarbonate.
- Differential diagnosis
Diarrhea or laxative abuse: the most common cause of normal anion gap acidosis, distinguished by a negative urine anion gap (like type 2 RTA) but normal, not low, serum potassium and a clear GI history.
- Diagnosis
RTA is diagnosed by pairing a normal anion gap metabolic acidosis with a renal source, confirmed by urine anion gap, serum potassium, and urine pH: ammonium chloride ingestion and diarrhea show a…
- Management
First, identify and treat the underlying cause where possible (stop an offending drug such as topiramate, acetazolamide, lithium, or amphotericin B, or an ACE inhibitor/ARB/NSAID driving type 4 RTA; treat myeloma; relieve obstruction).
- Complications
Nephrocalcinosis and recurrent calcium phosphate kidney stones, the dominant complication of untreated type 1 RTA.
- Prognosis
Generally favorable with appropriate alkali/citrate replacement, which corrects the acidosis and, in type 1 RTA, substantially reduces stone and nephrocalcinosis risk.
- Prevention
Screen with a basic metabolic panel and urine studies in any patient with recurrent calcium phosphate stones or unexplained nephrocalcinosis, since undiagnosed type 1 RTA is a preventable cause of progressive stone disease.
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