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Renal Tubular Acidosis

By

L. Aimee Hechanova

, MD, Texas Tech University Health Sciences Center, El Paso

Last full review/revision Jul 2020| Content last modified Jul 2020
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Renal tubular acidosis (RTA) is acidosis and electrolyte disturbances due to impaired renal hydrogen ion excretion (type 1), impaired bicarbonate resorption (type 2), or abnormal aldosterone production or response (type 4). (Type 3 is extremely rare and is not discussed.) Patients may be asymptomatic, display symptoms and signs of electrolyte derangements, or progress to chronic kidney disease. Diagnosis is based on characteristic changes in urine pH and electrolytes in response to provocative testing. Treatment corrects pH and electrolyte imbalances using alkaline agents, electrolytes, and, rarely, drugs.

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Type 1 (distal) RTA

Type 1 is impairment in hydrogen ion secretion in the distal tubule, resulting in a persistently high urine pH (>5.5) and systemic acidosis. Plasma bicarbonate is frequently <15 mEq/L (15 mmol/L), and hypokalemia Hypokalemia Hypokalemia is serum potassium concentration 3.5 mEq/L ( 3.5 mmol/L) caused by a deficit in total body potassium stores or abnormal movement of potassium into cells. The most common cause is... read more , hypercalciuria, and decreased citrate excretion are often present. Hypercalciuria is the primary abnormality in some familial cases, with calcium-induced tubulointerstitial damage causing distal RTA. Nephrocalcinosis and nephrolithiasis Urinary Calculi Urinary calculi are solid particles in the urinary system. They may cause pain, nausea, vomiting, hematuria, and, possibly, chills and fever due to secondary infection. Diagnosis is based on... read more are possible complications of hypercalciuria and hypocitraturia if urine is relatively alkaline.

This syndrome is rare. Sporadic cases occur most often in adults and may be primary (nearly always in women) or secondary. Familial cases usually first manifest in childhood and are most often autosomal dominant. Secondary type 1 RTA may result from drugs, kidney transplantation Kidney Transplantation Kidney transplantation is the most common type of solid organ transplantation. (See also Overview of Transplantation.) The primary indication for kidney transplantation is End-stage renal failure... read more , or various disorders:

Potassium level may be high in patients with chronic obstructive uropathy or sickle cell anemia.

Type 2 (proximal) RTA

Type 2 is impairment in bicarbonate resorption in the proximal tubules, producing a urine pH > 7 if plasma bicarbonate concentration is normal, and a urine pH < 5.5 if plasma bicarbonate concentration is already depleted as a result of ongoing losses.

This syndrome may occur as part of a generalized dysfunction of proximal tubules and patients can have increased urinary excretion of glucose, uric acid, phosphate, amino acids, citrate, calcium, potassium, and protein. Osteomalacia or osteopenia (including rickets in children) may develop. Mechanisms may include hypercalciuria, hyperphosphaturia, alterations in vitamin D metabolism, and secondary hyperparathyroidism.

Type 2 RTA is very rare and most often occurs in patients who have one of the following:

Type 4 (generalized) RTA

Type 4 results from aldosterone deficiency or unresponsiveness of the distal tubule to aldosterone. Because aldosterone triggers sodium resorption in exchange for potassium and hydrogen, there is reduced potassium excretion, causing hyperkalemia Hyperkalemia Hyperkalemia is a serum potassium concentration > 5.5 mEq/L (> 5.5 mmol/L), usually resulting from decreased renal potassium excretion or abnormal movement of potassium out of cells. There are... read more and reduced acid excretion. Hyperkalemia may decrease ammonia excretion, contributing to metabolic acidosis Metabolic Acidosis Metabolic acidosis is primary reduction in bicarbonate (HCO3−), typically with compensatory reduction in carbon dioxide partial pressure (Pco2); pH may be markedly low or slightly subnormal... read more . Urine pH is usually appropriate for serum pH (usually < 5.5 when there is serum acidosis). Plasma bicarbonate is usually > 17 mEq/L (17 mmol/L). This disorder is the most common type of RTA. It typically occurs sporadically secondary to impairment in the renin-aldosterone-renal tubule axis (hyporeninemic hypoaldosteronism), which occurs in patients with the following:

Other factors that can contribute to type 4 RTA include the following:

Symptoms and Signs of Renal Tubular Acidosis

RTA is usually asymptomatic. Severe electrolyte disturbances are rare but can be life threatening.

Signs of ECF volume depletion may develop from urinary water loss accompanying electrolyte excretion in type 2 RTA.

People with type 1 or type 2 RTA may show symptoms and signs of hypokalemia Hypokalemia Hypokalemia is serum potassium concentration 3.5 mEq/L ( 3.5 mmol/L) caused by a deficit in total body potassium stores or abnormal movement of potassium into cells. The most common cause is... read more , including muscle weakness, hyporeflexia, and paralysis. Bony involvement (eg, bone pain and osteomalacia in adults and rickets in children) may occur in type 2 and sometimes in type 1 RTA.

Diagnosis of Renal Tubular Acidosis

  • Suspected in patients with metabolic acidosis with normal anion gap or with unexplained hyperkalemia

  • Serum and urine pH, electrolyte levels, and osmolalities

  • Often, testing after stimulation (eg, with ammonium chloride, bicarbonate, or a loop diuretic)

RTA is suspected in any patient with unexplained metabolic acidosis Metabolic Acidosis Metabolic acidosis is primary reduction in bicarbonate (HCO3−), typically with compensatory reduction in carbon dioxide partial pressure (Pco2); pH may be markedly low or slightly subnormal... read more (low plasma bicarbonate and low blood pH) with normal anion gap. Type 4 RTA should be suspected in patients who have persistent hyperkalemia with no obvious cause, such as potassium supplements, potassium-sparing diuretics, or chronic kidney disease Chronic Kidney Disease Chronic kidney disease (CKD) is long-standing, progressive deterioration of renal function. Symptoms develop slowly and in advanced stages include anorexia, nausea, vomiting, stomatitis, dysgeusia... read more Chronic Kidney Disease . ABG sampling is done to help confirm RTA and to exclude respiratory alkalosis as a cause of compensatory metabolic acidosis. Serum electrolytes, BUN, creatinine, and urine pH are measured in all patients. Further tests and sometimes provocative tests are done, depending on which type of RTA is suspected:

  • Type 1 RTA is confirmed by a urine pH that remains > 5.5 during systemic acidosis. The acidosis may occur spontaneously or be induced by an acid load test (administration of ammonium chloride 100 mg/kg po). Normal kidneys reduce urine pH to < 5.2 within 6 h of acidosis.

  • Type 2 RTA is diagnosed by measurement of the urine pH and fractional bicarbonate excretion during a bicarbonate infusion (sodium bicarbonate 0.5 to 1.0 mEq/kg/h [0.5 to 1.0 mmol/L] IV). In type 2, urine pH rises above 7.5, and the fractional excretion of bicarbonate is > 15%. Because IV bicarbonate can contribute to hypokalemia, potassium supplements should be given in adequate amounts before infusion.

  • Type 4 RTA is confirmed by a history of a condition that could be associated with type 4 RTA, chronically elevated potassium, and normal or mildly decreased bicarbonate. In most cases plasma renin activity is low, aldosterone concentration is low, and cortisol is normal.

Treatment of Renal Tubular Acidosis

  • Varies by type

  • Often alkali therapy

  • Treatment of concomitant abnormalities related to potassium, calcium, and phosphate metabolism

Treatment consists of correction of pH and electrolyte balance with alkali therapy. Failure to treat RTA in children slows growth.

Alkaline agents such as sodium bicarbonate, potassium bicarbonate, or sodium citrate help achieve a relatively normal plasma bicarbonate concentration (22 to 24 mEq/L [22 to 24 mmol/L]). Potassium citrate can be substituted when persistent hypokalemia Hypokalemia Hypokalemia is serum potassium concentration 3.5 mEq/L ( 3.5 mmol/L) caused by a deficit in total body potassium stores or abnormal movement of potassium into cells. The most common cause is... read more is present or, because sodium increases calcium excretion, when calcium calculi are present.

Vitamin D (eg, ergocalciferol 800 IU po once/day) and oral calcium supplements (elemental calcium 500 mg po tid, eg, as calcium carbonate, 1250 mg po tid) may also be needed to help reduce skeletal deformities resulting from osteomalacia or rickets Hypophosphatemic Rickets Hypophosphatemic rickets is a disorder characterized by hypophosphatemia, defective intestinal absorption of calcium, and rickets or osteomalacia unresponsive to vitamin D. It is usually hereditary... read more .

Type 1 RTA

Adults are given sodium bicarbonate or sodium citrate 0.25 to 0.5 mEq/kg (0.25 to 0.5 mmol/L) po q 6 h. In children, the total daily dose may need to be as much as 2 mEq/kg (2 mmol/L) q 8 h; this dose can be adjusted as the child grows. Potassium supplementation is usually not required when the dehydration and secondary aldosteronism are corrected with bicarbonate therapy.

Type 2 RTA

Plasma bicarbonate cannot be restored to the normal range, but bicarbonate replacement should exceed the acid load of the diet (eg, sodium bicarbonate 1 mEq/kg [1 mmol/L] po q 6 h in adults or 2 to 4 mEq/kg [2 to 4 mmol/L] q 6 h in children) to maintain serum bicarbonate at about 22 to 24 mEq/L (22 to 24 mmol/L) because lower levels risk growth disturbance. However, excess bicarbonate replacement increases potassium bicarbonate losses in the urine. Thus, citrate salts can be substituted for sodium bicarbonate and may be better tolerated.

Potassium supplements or potassium citrate may be required in patients who become hypokalemic when given sodium bicarbonate but is not recommended in patients with normal or high serum potassium levels. In difficult cases, treatment with low-dose hydrochlorothiazide 25 mg po bid may stimulate proximal tubule transport functions. In cases of generalized proximal tubule disorder, hypophosphatemia and bone disorders are treated with phosphate and vitamin D supplementation to normalize the plasma phosphate concentration.

Type 4 RTA

Hyperkalemia is treated with volume expansion, dietary potassium restriction, and potassium-wasting diuretics (eg, furosemide 20 to 40 mg po once/day or bid titrated to effect). Alkalinization is often unnecessary. A few patients need mineralocorticoid replacement therapy (fludrocortisone 0.1 to 0.2 mg po once/day, often higher in hyporeninemic hypoaldosteronism); mineralocorticoid replacement should be used with caution because it may exacerbate underlying hypertension, heart failure, or edema.

Key Points

  • Renal tubular acidosis is a class of disorders in which excretion of hydrogen ions or reabsorption of filtered bicarbonate is impaired, leading to a chronic metabolic acidosis with a normal anion gap.

  • RTA is usually due to abnormal aldosterone production or response (type 4), or less often, due to impaired hydrogen ion excretion (type 1) or impaired bicarbonate resorption (type 2).

  • Consider RTA if patients have metabolic acidosis with a normal anion gap or unexplained hyperkalemia

  • Check ABG and serum electrolytes, BUN, and creatinine, and urine pH.

  • Do other testing to confirm type of RTA (eg, acid load test for type 1, bicarbonate infusion for type 2).

  • Treat using alkali therapy and measures to correct low serum potassium in type 2 and sometimes type 1 RTA , and using potassium restriction or potassium-wasting diuretics in type 4 RTA; give other electrolytes as needed.

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