The family is a young couple who bring their 4-month-old infant boy to see you. While very happy about the birth of their first child, they are a bit worried. They tell you that their son looks smaller than his same-age cousin, who outweighs him by 4 pounds. He throws up a lot, and they are not sure if this is normal. Even though he throws up, he still makes plenty of wet diapers each day. You confirm that he is below the fifth percentile for both length and weight. You wonder if something in the water is causing kidney problems in every infant in your clinic today, but decide that you should obtain some basic lab studies first. The serum chemistry looks something like this: Na 142 mEq/L, K 3.2 mEq/L, Cl 115 mEq/L, and CO2 10 mEq/L.
To make a diagnosis of renal tubular acidosis, you should see all of the following EXCEPT:
A. Low serum bicarbonate (CO2).
B. High serum chloride.
C. Normal serum anion gap.
D. High urine pH.
E. Normal to low serum chloride.
The correct answer is “E.” Renal tubular acidosis is a clinical condition in which the kidneys either waste too much base (bicarbonate) or cannot get rid of enough acid in the urine in order to maintain a normal pH. The key point is that it produces a metabolic acidosis with a normal anion gap (which comes with a high serum chloride). A normal anion gap is 8 to 16. If you see a positive anion gap, then you are dealing with something else. In this case, the anion gap is 14, which is normal: (142 + 3.2) – (115 + 10). The other thing you may see is a high urine pH relative to the low serum CO2, again highlighting that the kidneys are not able to maintain acid-base balance appropriately. Distal renal tubular acidosis is a rare condition that may present as failure to thrive. The other much more common cause of hyperchloremic normal anion gap metabolic acidosis is diarrhea. Ask about diarrhea in a well-grown older child in whom you see this pattern of electrolyte abnormalities. If you find an increased anion gap metabolic acidosis, think about ingestions (especially salicylates, methanol/propylene glycol), diabetic ketoacidosis, and lactic acidosis.