Renal Tubular Acidosis (RTA)

The so-called “Type 2”, or “proximal” Renal Tubular Acidosis (RTA), accounts for greater than 95 percent of all of the types of RTA. It presents in early infancy and is expected to resolve with time (by age 2-6 years) and renal maturation.

The work-up of RTA includes measurement of electrolytes to document a low serum bicarbonate (HCO3 < 20meq/L) level in the presence of no anion gap, and a simultaneous urine pH above 5.5

[NOTE: In proximal (type 2) RTA, where the kidney has a lower threshold at which to retain HCO3, if there is cause for the serum HCO3 to fall below that threshold, the kidney will acidify the urine, and the urine pH can drop anywhere below 6.5. Whereas in Type 1, or distal RTA, the urine can never acidify, i.e. fall below 6.5].

Borderline levels of serum HCO3 (e.g. 17-19mEq/L) may contribute to an over-diagnosis of the presence of true acidosis. In such cases it is recommended that one obtain a venous blood gas (VBG) to verify acidosis. Interpretation of the presence of acidosis can be confounding in a crying child, however, secondary to the associated mild, acute respiratory alkalosis in this setting.

Early childhood RTA Type 2 in the typical case is an isolated renal abnormality. It is the only thing that is (temporarily) wrong with the kidney, so serum creatinine and phosphorus should be demonstrated to be normal (e.g. if serum phosphorus is low then the renal tubule is wasting phosphorus as well as bicarbonate, suggesting a more extensive abnormality of the renal tubule (e.g. renal Fanconi Syndrome). A renal ultrasound is generally recommended to rule out the possibility of significant hydronephrosis, since RTA can be caused by urinary tract obstruction.  Admittedly the yield is very low in children with no additional history or findings suggesting urinary tract anomalies (e.g. urinary tract infection).

Because Type 2 is by far the most common type of RTA observed in the very young, it is rarely necessary to utilize the “classic” acid loading test (to see if the kidney can eventually create an acid urine) to separate it from Type I. The mere fact that the “bicarbonate wasting” aspect of proximal RTA necessitates much larger amounts of extra buffer, i.e. citrate, to correct the serum HCO3 level, is a more practical, less hazardous way, to obtain the clinical data in support of the more common diagnosis of proximal RTA, with its favorable prognosis.

As stated, most young children with RTA will have Type 2 RTA. The bicarbonate wasting results from an immaturity of the kidney resulting in the sole temporary defect of a lower bicarbonate threshold. It is expected to normalize anywhere from age 2 to 6 years. The other types are rare. The large amount (5-20meq/kg) of supplemental buffer (generally in the form of citrate) required to achieve an acceptable serum level (20meq/L) of bicarbonate (HCO3) supports bicarbonate wasting (Type 2). The total daily requirement is divided at least tid in order to try to maintain a more constant level in the blood, since there is continual loss via the kidneys.

The taste of Bicitra (1meq citrate/ml) or Polycitra (2meq/ml) is somewhat citrus in nature, but can be improved by adding cherry flavoring or putting the dose in lemonade. Care should be taken not to add the medication to so much volume   that the child cannot consume a complete dose. Unfortunately, there are no other medication choices currently available on the market. A reasonable initial dose is 3-5meq/kg/day divided tid [Note: achieving serum HCO3 of 20meq/L with lower doses suggests the possibility of Type 1 RTA].  After a week of initial dosing, assuming the child is taking the medication faithfully, a serum bicarbonate level should be checked to assure the goal of 20meq/L has been reached.  If not, the individual dose is increased by 3-5ml, and the blood level checked each week anytime the dose is changed.

Once the level of 20meq/L is achieved the level need only be checked every 3 months while documenting growth [Note: this assumes the pursuit of the diagnosis was prompted by failure to thrive—which is a common association—an improvement in height is generally more impressive than weight gain]. Blood HCO3 levels should always be checked at the same time relative to a dose, preferably a trough level, (i.e. before a given dose). When a subsequent HCO3 level is higher than expected (no dose increase having been made), the RTA has resolved, and treatment/follow-up for RTA may cease

Since Type 2 RTA is expected to resolve with time, and in the absence of any other renal concerns, follow-up can be through the primary care provider’s office, but referral is always acceptable for problem cases and parental reassurance. Any other Type of RTA (rare) should be referred to Nephrology.