Renal tubular acidosis (RTA)

In response to acidosis, the kidneys do two things:

1. Reabsorb all filtered bicarbonate

2. Excrete excess acid by increasing levels of NH₄⁺ in the urine 

In renal tubular acidosis, one of these processes fails.

Previously RTA was classified under one of the following headings:

• Type I or distal RTA

• Type II or proximal RTA

• Type IV or hyperkalaemic distal RTA

A clearer understanding of the underlying defects allows for a different approach to diagnosis:

RTA with excessive excretion of bicarbonate

• Proximal RTA

• Acetazolamide use

Low excretion of NH₄⁺ (Distal RTA)

• Reduced availability of ammonia in the renal medulla secondary to

Decreased ammonia generation due to low GFR or hyperkalaemia Medullary-interstitial disease

• Reduced secretion of H⁺ in the collecting duct due to

Failure of the H+ pump Back leak of H+ from the tubular lumen into the tubular cell Failure of voltage augmentation of distal H+ secretion

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