Renal tubular acidosis

What it is

Renal tubular acidosis produces a metabolic acidosis with a normal anion gap, and it is listed in the HARD-ASS group of normal anion gap causes with diarrhea, acetazolamide and Addison disease. Three types are set out, each with its own defect.

Type 1 is distal. Hydrogen secretion into the urine is poor, so new bicarbonate cannot be generated; the intercalated cells of the distal tubule cannot secrete H+, the HCO3 in the blood falls, and the result is a metabolic acidosis.

Type 2 is proximal. It is a bicarbonate leak: the proximal convoluted tubule cells cannot reabsorb HCO3, so bicarbonate is lost in the urine and there is a net excess of H+. H+ secretion from the collecting duct can still acidify the urine but cannot make up for the bicarbonate being lost.

Type 4 is aldosterone resistance at the distal tubule, also called hyporeninemic hypoaldosteronism, with a defect in the H+/K+ cation exchange. Aldosterone deficiency or resistance raises the potassium, which inhibits ammonia synthesis in the proximal convoluted tubules, so urinary ammonium excretion falls and the urine has less buffering capacity.