Master the board step 3

Master the Boards: USMLE Step 3
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Treat by administering bicarbonate. The proximal tubule is still working;
therefore the patient will still absorb the bicarbonate.
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Proximal RTA (Type II): There is an inability to reabsorb bicarbonate in
the proximal renal tubule. Initially there is an elevated urine pH, but when
the body loses substantial amounts of bicarbonate, the urine pH drops.
Because urine pH is often low, kidney stones do not develop. A low serum
bicarbonate leaches calcium out of the bones, and there is also osteomalacia.
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Test by giving bicarbonate. A normal person with metabolic acidosis will
absorb all of the bicarbonate, and there should still be a low urine pH in a
normal patient. In proximal RTA, the patient cannot absorb the bicarbonate
and the urine pH rises from the bicarbonate malabsorption.
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Treat by giving a thiazide diuretic, which results in a volume contraction.
The contracted blood volume raises the concentration of serum bicarbonate.
Large quantities of serum bicarbonate are also given (bicarbonate is generally
ineffective and that is why they must be used in such high amounts).
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Hyporeninemic hypoaldosteronism (Type IV): There is decreased aldosterone
production or effect. Look for a diabetic patient with a normal anion
gap metabolic acidosis. This is the only RTA with an elevated potassium level.
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Treat with aldosterone administration in the form of fludrocortisone,
which is the steroid with the highest mineralocorticoid content.
The following table compares Types I, II, and IV RTA.
Distal RTA (Type I) Proximal RTA (Type II) Type IV (Diabetes)
Urine pH High Low Low
Serum Potassium Low Low High
Stones Yes No No
Test Give acid Give bicarbonate Urine sodium loss
Treatment Bicarbonate Thiazide diuretic
High dose bicarbonate
Fludrocortisone
Urine Anion Gap (UAG)
The UAG is the way to distinguish between diarrhea and RTA as the cause of
the normal anion gap metabolic acidosis:
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UAG = Urine Na + – Urine Cl –
When acid is excreted from the kidney, it goes out as NH 4 Cl. Acid excretion
from the kidney goes out with chloride.
If you can excrete acid from the kidney, the urine chloride goes up. If the urine
chloride is up, then the number (UAG) is negative. Diarrhea causes a negative
UAG, because the kidney can excrete acid and the net UAG is negative. In
metabolic acidosis, a negative UAG means the kidney works.
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