Proximal Renal Tubular Acidosis

Question 1

RENAL TUBULAR ACIDOSIS

Answer 1

Renal tubular acidosis (RTA) may be suspected when there is a non-AG gap metabolic acidosis, positive urine AG, and urine osmolality gap < 150 mEq/kg.

Question 2

RENAL TUBULAR ACIDOSIS

Answer 2

In chronic metabolic acidosis where there is no concern for toluene ingestion or D-lactic acidosis (from gut bacterial overgrowth seen in short bowel syndrome), calculation of urine osmolal gap is not necessary.

Question 3

Proximal Renal Tubular Acidosis (pRTA):

Impairment of HCO3− Reabsorption

Answer 3

Clinical Presentation of pRTA

Serum [HCO3−] (SHCO3) is typically greater than 15 mEq/L.

Question 4

Proximal Renal Tubular Acidosis

Answer 4

Urine pH varies with serum [HCO3−] (i.e., urine pH > 5.5) if receiving alkalinization therapy, but appropriately low (i.e., pH < 5.5) in the presence of metabolic acidosis.

Question 5

Proximal Renal Tubular Acidosis

Answer 5

Hypokalemia

Etiologies of pRTA
Isolated defects

Question 6

Proximal Renal Tubular Acidosis

Answer 6

Autosomal dominant pRTA (SCL9A3 gene), encoding NHE3 transporter; autosomal recessive pRTA (SLCA4A4 gene), encoding NBC1—associated with mental retardation, ocular abnormalities (glaucoma, cataracts, band keratopathy); sporadic pRTA—nonfamilial, transient type described in infancy, no defect isolated; possibly immaturity of NHE3 function.

Question 7

Proximal Renal Tubular Acidosis

Answer 7

Isolated defects are rare; typically associated with other proximal tubular transport defects (Fanconi syndrome).

Fanconi syndrome: multiple proximal tubular transport defects.

Question 8

Proximal Renal Tubular Acidosis

Diagnosis of pRTA

Answer 8

Diagnosis of pRTA

Fractional excretion of HCO3− (FeHCO3) > 15% and urine pH typically >7.5 following HCO3− load (0.5 to 1.0 mEq/kg body weight/hour to increase serum [HCO3−] concentration > 20 mEq/L).

FeHCO3 = [(UHCO3 × SCr) / (SHCO3 × UCr)] × 100%

Question 9

Proximal Renal Tubular Acidosis

Management of pRTA

Answer 9

Management of pRTA
Bicarbonate replacement 5 to 15 mEq/kg/d (NOTE: this can worsen hypokalemia)

Potassium replacement

Question 10

Proximal Renal Tubular Acidosis

Management of pRTA

Answer 10

Thiazide diuretics → induces volume contraction → enhances proximal reabsorption of bicarbonate (NOTE: this can also worsen hypokalemia)

Vitamin D and phosphate for patients with rickets and hypophosphatemia.