2025 – PAGE 368 – ACID-BASE DISORDERS
RENAL TUBULAR ACIDOSIS (RTA)
All types of renal tubular acidosis (RTA) have a NORMAL ANION GAP metabolic acidosis.
PEARLS: You don’t need to know about RTA IV for the pediatric certification exam (it’s not really seen in children).
RENAL TUBULAR ACIDOSIS TYPE I (RTA I, AKA CLASSIC DISTAL RTA)
The collecting duct is the main site of hydrogen ion secretion. In renal tubular acidosis type I (RTA I, AKA classic distal RTA), the serum acidosis occurs due to hydrogen ions (H+) not being excreted as NH4Cl. Therefore, the urine is more ALKALINE than it should be because the body is holding on to too much acid. Look for a HIGH (alkaline) urine pH of > 5.5 and LOW urine NH4Cl. Since chloride is being kept in the body, there will be a HIGH serum chloride level (hyperchloremic non-gap metabolic acidosis). Also look for HYPOKALEMIA.
PEARL: If you see a NON-ANION GAP + a urine pH of > 5.5, PICK THIS! If you’re only given the elevated urine pH, ASSUME there’s a serum acidosis! RTA I can be found in Sjogren’s, Rheumatoid Arthritis, and Lupus (SLE). It can also lead to stones and therefore urinary tract obstruction! Keep all of these in mind as a possible answer choice. Lastly, keep this very high on your differential for a child who is having FTT.
MNEMONIC: In case you get these mixed up, think of it this way: RTA I was the first one to be discovered, so it works pretty much as the scientists who discovered it thought it would. The renal tubules are dysfunctional and not excreting acid. Therefore, the serum is ACIDOTIC while the urine is ALKALINE. Classic!
MNEMONIC: RTA I = 1+ = H1+ = an H1+ secretion defect.
RENAL TUBULAR ACIDOSIS TYPE II (RTA II, AKA PROXIMAL RTA)
Renal tubular acidosis type II (RTA II, AKA proximal RTA) is caused by a partial BICARBONATE reabsorption defect. That means some of the bicarb that is supposed to be kept is actually lost in the urine. Bicarb is a base, so the loss of this base results in a serum acidosis. Here’s the tricky part: the urine is not alkaline as you would expect. Since NH4Cl (an acid) secretion is normal, and some of the base is still being reabsorbed, the urine pH ends up being LOW. So, look for a urine pH of < 5.5. In other words, enough bicarb is lost to create a serum acidosis, but enough NH4CL/ACID waste is being normally excreted to create an acidic urine with pH < 5.5. Like RTA I, this one also has a HYPOKALEMIA. RTA II can be seen in multiple myeloma and paroxysmal nocturnal hemoglobinuria. (These last two facts are likely low-yield.)
MNEMONIC: RTA II = 2 = Bi = BIcarb problem (partial)
PEARL: Acetazolamide is a carbonic anhydrase Inhibitor that inhibits bicarb reabsorption. It also creates a metabolic acidosis (“ACID-azolamide”), but it alkalinizes the urine.