The renal tubular acidoses

Renal tubular acidosis (RTA) arises when the kidneys either fail to excrete sufficient acid, or are unable to conserve bicarbonate, with both circumstances leading to metabolic acidosis of varying severity with altered serum potassium. Proximal and distal types of RTA can be differentiated according to which nephron segment is malfunctioning. Proximal RTA: aetiology and diagnosis—the condition may be (1) secondary to generalized proximal tubular dysfunction (part of the renal Fanconi’s syndrome), or rarely (2) due to an inherited mutation of a single transporter (NBC1) located at the basolateral surface of the proximal tubular epithelium. The combination of normal anion gap acidosis with other features of proximal tubular dysfunction such as renal phosphate wasting (and hypophosphataemia), renal glycosuria, hypouricaemia (due to uricosuria), aminoaciduria, microalbuminuria, and other low molecular weight proteinuria suggests the diagnosis. Management—this requires large quantities of oral alkali with (in most cases) potassium supplements to prevent severe hypokalaemia. Distal RTA: aetiology/diagnosis—two main classes are differentiated by whether (1) the acid-handling cells in the collecting ducts are themselves functioning inadequately, in which case there is associated hypokalaemia (this is ‘classic’ distal RTA); or (2) the main abnormality is of the salt-handling principal cells in the same nephron segment, in which case hyperkalaemia occurs and the acidosis is a secondary phenomenon—this is hyperkalaemic distal RTA. The combination of normal anion gap acidosis with a urine pH higher than 5.5 suggests classic distal RTA. Management—(1) classic distal RTA—1 to 3 mg/kg per day of oral alkali; (2) hyperkalaemic distal RTA—treatment is with sodium bicarbonate, but fludrocortisone and/or potassium-lowering measures may also be necessary. Precipitating drugs should be stopped.