Diagnosis of Renal Tubular Acidosis

 

Types of renal tubular acidosis:

(1) type I (distal RTA, either complete or incomplete, involving one or more defects in the secretion of hydrogen ion at the distal tubule or collecting duct)

(2) type II (proximal RTA)

(3) type III (mixture of types I and II in infants)

(4) type IV (hypoaldosteronism, "distal like" with hyperkalemia)

 

Features of renal tubular acidosis in a child or adult (not Type III):

(1) hyperchloremic metabolic acidosis with arterial pH < 7.35 (if "No", consider low serum bicarbonate from artifact)

(2) serum anion gap < 16 (if > 16, consider an inborn error of metabolism)

(3) urinary anion gap positive (if positive, consider either (a) an extrarenal source for the acidosis or (b) proximal RTA with serum bicarbonate below the threshold)

 

Determining the type of RTA

Finding	Secretory dRTA	Rate Dependent dRTA	Type II (proximal RTA)	Type IV
urine pH	> 6.0	< 5.5	< 5.5	< 5.5
serum potassium		not elevated	not elevated	elevated
fractional excretion of bicarbonate		<= 15%	> 15%
U-B pCO2 gradient	< 20 mm Hg	< 20 mm Hg

 

where:

• "U-B": urine minus blood

 

Additional testing and comments:

(1) Confirmation of secretory dRTA can also be done using an acid load.

(2) A child with proximal RTA should be tested to exclude Fanconi's syndrome.

(3) Type III RTA is not identified by this algorithm.