Renal tubular acidosis

Every day the body generates acid from protein metabolism, and the kidney disposes of it in two coordinated steps. The proximal tubule reclaims the filtered bicarbonate so that almost none is lost in the urine, and the distal nephron secretes new hydrogen ion, buffered by ammonia, to regenerate the bicarbonate that buffered the day's acid load. Renal tubular acidosis is what happens when one of those two jobs fails. The result is the same biochemistry — a metabolic acidosis with a normal anion gap, because chloride rises to replace the bicarbonate that was lost — but the mechanism, potassium, urine pH and the treatment all depend on which part of the nephron broke. ^{[9] [2]}

The acidosis in RTA looks deceptively mild next to diabetic ketoacidosis, and that is part of the danger. It is chronic, so it leaches mineral from growing bone and silently stops a child growing. It is often found late, after nephrocalcinosis has already appeared on an ultrasound done for failure to thrive. And because the potassium is abnormal in a direction that depends on the type, a child can present with weakness, polyuria, or — in type 4 — an arrhythmia from hyperkalaemia long before anyone thinks of the kidney tubule. This page builds the model around three questions the bedside forces: is the anion gap normal, what does the potassium do, and is the kidney excreting ammonium. ^{[1] [5]}

Overview & Definition

Renal tubular acidosis is defined as a persistent metabolic acidosis with a normal anion gap and a near-normal glomerular filtration rate, caused by a specific defect in renal acid-base handling rather than by kidney failure itself. The anion gap — calculated as serum sodium minus the sum of chloride and bicarbonate — is normal (typically 8 to 12, up to about 16 mmol per litre) because the kidney does not retain unmeasured acids; instead it loses bicarbonate, and chloride rises to preserve electroneutrality. That single distinction, normal gap versus high gap, is the gate through which every metabolic acidosis must pass before RTA is even considered. ^{[9] [2]}

A high anion gap acidosis — diabetic ketoacidosis, lactic acidosis, renal failure, salicylate or methanol toxicity — is never RTA. In those states unmeasured acids accumulate, widening the gap. RTA belongs to the normal anion gap (hyperchloraemic) group, alongside the bicarbonate loss of diarrhoea and the acid load of some nutritional and drug states. Within that group, the next question is whether the kidney is doing its job: is it excreting ammonium to compensate? A kidney that cannot is in renal tubular acidosis; a kidney that can, but is overwhelmed by gastrointestinal bicarbonate loss, is not. ^{[9] [10]}

The three classical types map onto three sites. Type 1 (distal) RTA is a failure of hydrogen ion secretion by the alpha-intercalated cells of the collecting duct. Type 2 (proximal) RTA is a failure of bicarbonate reabsorption by the proximal tubule. Type 4 RTA is a failure of aldosterone action — deficiency or resistance — on the distal nephron, which impairs potassium and hydrogen secretion and ammonium production. The first two cause hypokalaemia; the third causes hyperkalaemia. That potassium direction is the most reliable bedside discriminator of the three, and the reason it is worth committing to memory before anything else. ^{[1] [2]}

References12ShowHide

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