Renal Tubular Acidosis
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Direct answer
A normal anion gap (hyperchloraemic) metabolic acidosis with an inappropriately alkaline urine pH is renal tubular acidosis until proven otherwise, and the three types sort by potassium and urine pH: distal (type 1) RTA cannot acidify the urine below pH 5.5 at any level of plasma bicarbonate, runs with hypokalaemia and predisposes to calcium phosphate stones and nephrocalcinosis; proximal (type 2) RTA wastes bicarbonate with a threshold near 12-15 mmol/L, so urine pH falls below 5.5 once plasma bicarbonate drops under that threshold, and it travels with Fanconi-type proximal tubular dysfunction (glycosuria, aminoaciduria, phosphaturia); type 4 RTA reflects hypoaldosteronism or tubular resistance to it, presenting with hyperkalaemia and mild acidosis. Treatment follows type: distal RTA needs 1-2 mEq/kg/day of sodium-potassium bicarbonate and potassium repletion; proximal RTA demands larger bicarbonate doses (10-20 mEq/kg/day) with potassium and activated vitamin D; type 4 RTA is managed by withdrawing culprit drugs and treating hyperkalaemia, with fludrocortisone in selected aldosterone-deficient states.
What you must remember
- Master clue: normal anion gap acidosis plus urine pH above 5.5 (with low plasma bicarbonate) equals renal tubular acidosis; the urinary anion gap is positive in RTA and negative in diarrhoea.
- Type 1 (distal): hypokalaemia, urine pH stubbornly above 5.5, hypercalciuria with calcium phosphate stones and nephrocalcinosis on imaging; causes include Sjögren syndrome, SLE, amphotericin B, and hereditary forms.
- Type 2 (proximal): bicarbonate threshold lowered; urine pH high while plasma bicarbonate sits above the threshold but falls below 5.5 at steady state; part of Fanconi syndrome — ifosfamide, tenofovir (also causes a distal-type defect), valproate, myeloma, cystinosis; no stones, because citrate excretion is preserved.
- Type 4 (hypoaldosteronism): hyperkalaemia with mild acidosis — diabetic nephropathy with hyporeninaemic hypoaldosteronism is the classic; drugs include ACE inhibitors, ARBs, spironolactone, NSAIDs and calcineurin inhibitors.
- Potassium is the fork: hypokalaemia in types 1 and 2, hyperkalaemia in type 4 — the fastest discriminator in a stem.
- Treatment doses: distal RTA citrate/bicarbonate 1-2 mEq/kg/day; proximal RTA 10-20 mEq/kg/day plus potassium; type 4 corrects hyperkalaemia first with dietary restriction, loop diuretics, binders and fludrocortisone for aldosterone deficiency.
- Paediatric-Indian angle: distal RTA is a recognised cause of failure to thrive and rickets in Indian children, and oral bicarbonate transforms growth — a favourite paediatric crossover viva.
Working the algorithm on one patient
A 26-year-old woman presents with recurrent flank pain; evaluation shows serum bicarbonate 15 mmol/L, potassium 3.1 mEq/L, chloride high, anion gap normal, and urine pH 6.5 despite the acidosis; a plain film shows medullary nephrocalcinosis. The diagnosis assembles itself: normal-gap acidosis with an alkaline urine and hypokalaemia is distal (type 1) RTA, and the stones are calcium phosphate — the chemistry of an alkaline urine. Screen for Sjögren (dry eyes, anti-Ro/La) as the commonest acquired culprit, start sodium-potassium citrate (which binds calcium and raises urinary citrate, doubly protecting against stones), replete potassium, and monitor bicarbonate toward 20-22 mmol/L.
Change one laboratory line and the diagnosis pivots: potassium 5.6 with mild acidosis brings type 4 — check glucose and the drug list, treat potassium first. Urine pH 5.2 at a bicarbonate of 14 with normoglycaemic glycosuria moves the lesion proximally: Fanconi syndrome from tenofovir or myeloma, needing the big bicarbonate doses of type 2. Same three chemicals — bicarbonate, potassium, pH — three different machines broken.
Where students slip
The recurring error is reading the urine pH without checking the plasma bicarbonate: in proximal RTA the urine is eventually acid (pH under 5.5) at steady state, so a stem quoting "acidic urine" does not exclude RTA — it excludes only distal RTA. Second, the stone question: distal RTA forms calcium phosphate stones because urine is alkaline, whereas every other common stone former trends to calcium oxalate; MCQs bury that as "staghorn vs phosphate" trivia. Third, the urinary anion gap is positive in RTA (little urinary ammonium) and negative in diarrhoea; the logic is that RTA cannot excrete NH4+, so urine chloride is low, making the gap positive. Finally, tenofovir appears in modern option lists as a cause of both proximal and distal tubular injury.
Frequently asked questions
How does urine pH distinguish distal from proximal RTA?
Distal RTA never acidifies urine below 5.5 even at low plasma bicarbonate; proximal RTA urine pH falls below 5.5 once plasma bicarbonate drops beneath the lowered threshold.
Which RTA presents with hyperkalaemia?
Type 4, from aldosterone deficiency or resistance — typically diabetic hyporeninaemic hypoaldosteronism or drugs such as spironolactone, ACE inhibitors and NSAIDs.
Why does distal RTA cause calcium phosphate stones?
Chronic urine alkalinisation plus hypercalciuria and low urinary citrate favours calcium phosphate precipitation, producing stones and medullary nephrocalcinosis.
How much bicarbonate does proximal RTA require?
Large doses, 10-20 mEq/kg/day (against 1-2 in distal RTA), because administered bicarbonate is simply wasted in urine until plasma levels exceed the lowered threshold; potassium supplementation accompanies it.
What does a positive urinary anion gap indicate in normal-gap acidosis?
Impaired ammonium excretion — pointing to renal tubular acidosis rather than gastrointestinal bicarbonate loss, which produces a negative gap from abundant urinary ammonium.