type 2 RTA
Summary
Type 2 (proximal) Renal Tubular Acidosis is caused by impaired bicarbonate reabsorption in the proximal convoluted tubule, leading to a normal anion gap (hyperchloremic) metabolic acidosis with a urine pH that can be <5.5 once serum bicarbonate falls below the reduced reabsorptive threshold. It is often part of Fanconi syndrome, associated with glucosuria, phosphaturia, aminoaciduria, and hypokalemia.
Detail
Pathophysiology: The proximal tubule normally reabsorbs ~85-90% of filtered HCO3-. In type 2 RTA, this reabsorption is impaired due to dysfunction of the Na+/H+ exchanger (NHE3) and/or carbonic anhydrase activity, causing HCO3- wasting in the urine. As serum HCO3- falls, less is filtered, and eventually the amount filtered matches the reduced reabsorptive capacity, allowing the distal nephron (which is intact) to acidify urine appropriately (urine pH <5.5) once a new steady state is reached—distinguishing it from type 1 (distal) RTA where urine pH remains >5.5 despite acidosis.
Etiologies: Often part of generalized proximal tubular dysfunction (Fanconi syndrome), which can be caused by: multiple myeloma (light chain nephropathy), Wilson disease, cystinosis (most common cause in children), heavy metal toxicity (lead, cadmium), drugs (carbonic anhydrase inhibitors like acetazolamide, tenofovir, ifosfamide, expired tetracyclines), and vitamin D deficiency. Isolated type 2 RTA is rare.
Clinical Features: Hypokalemia (due to increased distal Na+ delivery enhancing K+ secretion via increased aldosterone activity and flow-dependent K+ secretion), normal anion gap metabolic acidosis, growth failure/rickets in children (due to associated Fanconi syndrome and phosphate wasting), and features of Fanconi syndrome (glucosuria despite normal serum glucose, phosphaturia, aminoaciduria, uricosuria).
Diagnosis: Fractional excretion of bicarbonate >15% during bicarbonate loading test; urine pH <5.5 when serum HCO3- is significantly reduced (distinguishing from type 1 RTA); evidence of other proximal tubule dysfunction (glucosuria, phosphaturia, aminoaciduria) supports Fanconi syndrome.
Treatment: Large doses of oral bicarbonate or citrate (much higher doses needed than in type 1 RTA due to ongoing bicarbonate wasting), potassium supplementation, thiazide diuretics (paradoxically can help by inducing mild volume depletion, increasing proximal reabsorption), and treatment of underlying cause. Vitamin D and phosphate supplementation may be needed if rickets/osteomalacia present.
Comparison: Type 1 RTA (distal) - impaired H+ secretion, urine pH always >5.5, severe hypokalemia, nephrocalcinosis/kidney stones common. Type 4 RTA - hypoaldosteronism, hyperkalemia (not hypokalemia), commonly seen in diabetic nephropathy.
Sources
- Harrison's Principles of Internal Medicine, 21st ed.
- First Aid for the USMLE Step 1, 2024
- Brenner and Rector's The Kidney, 11th ed.
- UpToDate: Etiology and diagnosis of distal (type 1) and proximal (type 2) renal tubular acidosis
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