Renal Tubular Acidosis

A clinical syndrome in which impaired renal acidification is manifested by a hyperchloremic metabolic acidosis. (non-gap acidosis)

Proximal RTA (type II)

• a defect in the Na/H exchange mechanism in the proximal tubule (which is where the majority of bicarbonate is reabsorbed by coupling it to Na reabsorption)
• bicarbonate excretion threshold: when plasma bicarb is above threshold, bicarb is excreted; if below threshold, bicarb is retained in plasma.
• have bicarbonate wasting at normal plasma bicarb concentrations, because the threshold is lower than normal, and bicarb is excreted more readily i.e. "a smaller cup to hold bicarb"
• the threshold for bicarb excretion in the urine is lowered from 24-26 to 15-18meq/L
• so when the plasma bicarb falls below 15-18, plasma bicarb is retained (doesn't spill over) and the urine pH becomes acid in patients with proximal RTA
• suggesting that the acidification process in the distal nephron is intact
• characterized by hypokalemia, an alkaline urine pH, and severe bicarbonaturia at a nearly normal plasma bicarb concentration, and an acid urine during spontaneous acidosis.
• also see Cystinosis

Distal RTA (type I)

• an inability to acidify the urine appropriately with spontaneous or chemically induced metabolic acidosis; urine pH is above 5.5
• "can't dump acid into the urine, so stays in the body"... potassium and calcium get dumped instead
• often have hypokalemia and hypercalciuria; This leads to urinary calcium precipitation, with possible development of both nephrocalcinosis and nephrolithiasis (do KUB to screen)

Type IV RTA: - a disorder of the distal nephron usually associated with hypoaldosteronism or pseudohypoaldosteronism, and hyperkalemia.

CHLA Board Review 2005