A 9-month-old male presents with failure to thrive

distal renal tubular acidosis

Looking at his labs, you can see that this patient has a metabolic acidosis with a normal anion gap (14). In the United States, the anion gap is calculated with the equation AG = [Na+] – [Cl-] – [HCO3-], which in this case is 140 – 114 – 12 = 14. Although the serum is acidotic, the urine is alkalotic. Acidification of the urine occurs in two steps. First, in the proximal tubule, bicarbonate is exchanged for hydrogen ion -- bicarbonate is pulled back into the tubule, while hydrogen is secreted into the lumen. 85% of bicarbonate is filtered here. The other 15% of bicarbonate is reabsorbed in the distal tubule, where, in a normal kidney, all the bicarbonate is reabsorbed. Also, the distal tube acidifies the urine by the excretion of both hydrogen ion and ammonia.

In distal renal tubular acidosis (RTA), hydrogen ion is not excreted as it should be. Sodium bicarbonate is lost in the urine, causing the hyperchloremia and hypokalemia seen in our patient (disabled exchange mechanisms).

One clue that this patient has distal RTA rather than proximal is that the urine chloride is less than the sum of the urine sodium and urine potassium.

Proximal renal tubular acidosis tends to have a more acute and severe onset, whereas its distal counterpart can present much like this patient, with a delay in diagnosis.

Reference:

Marx: Rosen's Emergency Medicine, 7th ed.: Mosby; 2009. CHAPTER 172 - Genitourinary and Renal Tract Disorders