*Distal (Type 1) Renal Tubular Acidosis

Definition and EtiologyTop

Distal renal tubular acidosis (dRTA), also known as type 1 renal tubular acidosis, results from reduction of H+ secretion in the distal nephron, which prevents urinary acidification and thereby minimizes urinary ammonium excretion with mild urinary bicarbonate losses (fractional excretion of bicarbonate [FEHCO3-] <3%-5%). A constant feature of dRTA is the inability to acidify urine to pH <5.5 in the presence of nonrespiratory acidosis. Impaired H+ secretion results in urinary sodium loss, secondary activation of the renin-angiotensin-aldosterone system, and increased urinary potassium loss. Hypercalciuria and hypocitraturia (causative factors for nephrocalcinosis and urolithiasis) frequently occur.

dRTA can be caused by a number of illnesses (Table 1). Rare autosomal dominant and autosomal recessive channelopathies will not be discussed, as they are beyond the scope of this chapter.

DiagnosisTop

In patients with dRTA urinary pH is persistently high, independently of acidifying challenge testing. To make a diagnosis of dRTA in a patient with hyperchloremic metabolic acidosis, urinary pH should be ≥5.5 regardless of serum bicarbonate levels with a positive urinary anion gap (UAG).

TreatmentTop

If renal tubular acidosis (RTA) persists following treatment of the underlying cause, it can be managed with supplemental doses of alkali therapy.

Treatment of patients with dRTA requires relatively lower doses of supplemental bicarbonate therapy (1-2 mEq/kg/d) compared with proximal RTA. Children have higher requirements than adults because of their overall net acid production being higher on a daily basis. Sodium bicarbonate or sodium citrate is usually adequate to raise serum bicarbonate concentration back to the normal range. Potassium-based citrate, alone or with sodium citrate, can be used if hypokalemia persists.

TablesTop