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Renal tubular acidosis (RTA) is a medical condition that involves an accumulation of acid in the body due to a failure of the kidneys to appropriately acidify the urine.[1] When blood is filtered by the kidney, the filtrate passes through the tubules of the nephron, allowing for exchange of salts, acid equivalents, and other solutes before it drains into the bladder as urine. The metabolic acidosis that results from RTA may be caused either by failure to recover sufficient (alkaline) bicarbonate ions from the filtrate in the early portion of the nephron (proximal tubule) or by insufficient secretion of (acid) hydrogen ions into the latter portions of the nephron (distal tubule). Although a metabolic acidosis also occurs in those with renal insufficiency, the term RTA is reserved for individuals with poor urinary acidification in otherwise well-functioning kidneys. Several different types of RTA exist, which all have different syndromes and different causes.
The word acidosis refers to the tendency for RTA to lower the blood’s pH. When the blood pH is below normal (7.35), this is called acidemia. The metabolic acidosis caused by RTA is a normal anion gap acidosis.
Renal tubular acidosis was first described by Lightwood[2] and Butler et al.[3] in children and by Baines et al.[4] in adults.
Distal RTA (dRTA) is the classical form of RTA, being the first described. Distal RTA is characterized by a failure of acid secretion by the alpha intercalated cells of the cortical collecting duct of the distal nephron. This failure of acid secretion may be due to a number of causes, and it leads to an inability to acidify the urine to a pH of less than 5.3. Because renal excretion is the primary means of eliminating acid from the body, there is consequently a tendency towards acidemia. This leads to the clinical features of dRTA;[1]
The acidosis is variable, and one may have dRTA with alpha intercalated cell failure without necessarily being acidemic, this is termed incomplete dRTA. The diagnosis of dRTA can be made by the observation of a urinary pH of greater than 5.3 in the face of a systemic acidemia (usually taken to be a serum bicarbonate of 20 mmol/l or less). In the case of an incomplete dRTA, failure to acidify the urine following an oral acid loading challenge is often used as a test. The test usually performed is the short ammonium chloride test,[6] in which ammonium chloride capsules are used as the acid load. More recently, an alternative test using furosemide and fludrocortisone has been described.[7]
The symptoms and sequelae of dRTA are variable and ranging from being completely asymptomatic, through loin pain and hematuria from kidney stones to failure to thrive and severe rickets in childhood forms as well as possible renal failure and even death.
Interestingly, dRTA has been proposed as a possible diagnosis for the unknown malady plaguing Tiny Tim in Charles Dickens’ A Christmas Carol.[8][9]
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