Renal tubular acidosis — viva

Opening (must-hit)

"This is a normal anion gap (hyperchloraemic) metabolic acidosis in an infant with failure to thrive. The potassium is low at 2.6 and the urine pH is 6.8, persistently above 5.5 despite acidosis, with nephrocalcinosis on ultrasound. That combination is distal (type 1) renal tubular acidosis. My first step is to confirm the anion gap is normal and exclude a high-gap cause, then treat with cautious alkali and potassium, and investigate for the hereditary form with genetics and a hearing assessment. I would correct the acidosis only partially, to a bicarbonate around 12, because over-rapid correction is dangerous." [1][9]

Examiner: "Why is the anion gap normal, and what would a high gap mean?"

"The anion gap is sodium minus chloride plus bicarbonate. In RTA the kidney loses bicarbonate and chloride rises to preserve electroneutrality, so the gap stays normal. A high gap means accumulation of unmeasured acids — lactic acidosis, diabetic ketoacidosis, renal failure, or a toxin like salicylate or methanol — and that is never RTA. Before I commit to the RTA pathway I must confirm the gap is normal. The gap should also be adjusted for albumin, because it falls by about 2.5 mmol per litre for every 10 grams per litre reduction in serum albumin." [9][2]

Examiner: "How do you use the urine anion gap, and where does it fail?"

"The urine anion gap is urine sodium plus potassium minus chloride, and it is a surrogate for urinary ammonium. A negative gap means high ammonium excretion — the kidney is compensating, so the cause is gastrointestinal, like diarrhoea. A positive gap means low ammonium excretion — the kidney is failing, so the cause is RTA or renal failure. It fails when the urine sodium is below 25 mmol per litre, because distal sodium delivery is then insufficient; when unmeasured urinary anions are present, like ketoacids or hippurate in toluene toxicity; and in advanced renal failure. In those settings I use the urine osmolal gap or direct ammonium measurement." [10][9]

Examiner: "Why does this child have nephrocalcinosis, and why is that almost unique to distal RTA?"

"Three mechanisms converge. Chronic acidosis is buffered by bone, releasing calcium and phosphate into the urine, producing hypercalciuria. Intracellular acidosis increases proximal citrate reabsorption, so urinary citrate falls — and citrate is the main inhibitor of calcium crystallisation. And calcium phosphate precipitates in alkaline urine, so the persistently alkaline urine of distal RTA is the perfect medium. Hypocitraturia plus hypercalciuria plus alkaline urine equals nephrocalcinosis, and you do not get it in proximal RTA because the urine acidifies once the patient is acidotic, nor in type 4." [3][9]

Examiner: "Now turn it around — a child with proximal RTA. Why is the alkali dose so different?"

"In proximal RTA the proximal tubule's bicarbonate threshold is lowered, so the kidney keeps spilling bicarbonate above that threshold no matter how much I give — I need 10 to 20 mEq per kg per day. In distal RTA the threshold is normal, so once the plasma bicarbonate is restored the kidney reabsorbs it and I only need 1 to 4 mEq per kg per day as maintenance. The dose is a direct consequence of the physiology, and under-dosing proximal RTA is the common error that leaves the child acidotic and growth-impaired. I must add potassium in proximal RTA because the distal bicarbonate delivery drives potassium wasting." [6][2]

Examiner: "What are the dangers of correcting the acidosis too quickly?"

"Three. First, potassium shifts into cells as the pH rises, worsening the hypokalaemia of types 1 and 2 — sometimes dangerously, and it can cause arrhythmia. Second, ionised calcium falls as the pH rises, because more calcium binds to albumin, precipitating tetany and seizures. Third, the sodium load causes hypernatraemia and fluid overload. So I correct only partially, to a bicarbonate around 12 mmol per litre, and I replace potassium first. The body has adapted to the chronic acidosis, and I must not reverse all the adaptations at once." [9][2]

Examiner: "Final corner — type 4 RTA at the table."

"Type 4 is the aldosterone problem, and the potassium direction names it: hyperkalaemia, not hypokalaemia. Aldosterone deficiency or resistance impairs distal potassium secretion and ammoniagenesis, so the potassium rises and the ammonium excretion falls, producing a normal anion gap acidosis. The urine pH is below 5.5 because the H-ATPase pump is intact — what is missing is the aldosterone-driven ammonium buffer and potassium secretion. In children the causes are often drugs (ACE inhibitors, ARBs, NSAIDs, potassium-sparing diuretics), obstructive uropathy, or adrenal insufficiency. I treat the hyperkalaemia with diet and binders, give sodium bicarbonate for the acidosis, and fludrocortisone where aldosterone is deficient." [2][9]

Closing summary

"In summary: RTA is a normal anion gap acidosis, never a high-gap one. Potassium direction names the type — low in 1 and 2, high in 4. Urine anion gap names the cause — positive is renal, negative is gut. Nephrocalcinosis names distal RTA. The alkali dose names the segment — 1 to 4 for distal, 10 to 20 for proximal. And correct chronic acidosis slowly, replacing potassium first." [1][10]