Chapter

The renal tubular acidoses

Fiona E. Karet

in Oxford Textbook of Medicine

Published on behalf of Oxford University Press

Published in print May 2010 | ISBN: 9780199204854
Published online May 2011 | e-ISBN: 9780199570973 | DOI: http://dx.doi.org/10.1093/med/9780199204854.003.2115_update_001
The renal tubular acidoses

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Renal tubular acidosis (RTA) arises when the kidneys either fail to excrete sufficient acid, or are unable to conserve bicarbonate, 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. The condition may be secondary (e.g. associated with drugs, autoimmune disease, or diabetes mellitus) or inherited, and there may be renal tract calcification and—in chronic cases—metabolic bone disease.

Proximal RTA

Aetiology and diagnosis – the condition may be (1) secondary to generalized proximal tubular dysfunction (part of the renal Fanconi syndrome), or rarely (2) due to 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 (as bicarbonate or citrate), with (in most cases) potassium supplements to prevent severe hypokalaemia. Associated phosphate and vitamin D deficiencies may also require treatment. Precipitating drugs should be stopped if possible.

Distal RTA

Aetiology—two main classes are differentiated by whether (1) the acid-handling cells (α‎-intercalated 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, which is most often secondary to hyporeninaemic hypoaldosteronism (e.g. in diabetes mellitus or critical illness) but may also be reversibly precipitated by drugs such as trimethoprim or ciclosporin.

Diagnosis—the combination of normal anion gap acidosis with a urine pH higher than 5.5 suggests classic distal RTA, especially if renal tract calcification is present or there is coexistent autoimmune disease. Diagnosis may require an oral urine acidification test if the metabolic abnormalities are partially masked by compensatory mechanisms, when inability to achieve a urine pH less than 5.5 clinches the diagnosis of classic distal RTA. By contrast, urine-acidification capacity is normal in hyperkalaemic distal RTA.

Management—(1) Classic distal RTA—1 to 3 mg/kg per day of oral alkali (additional supplements should not be required); (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.

Chapter.  4850 words.  Illustrated.

Subjects: Nephrology

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