How are transtubular potassium gradient (TTKG) levels interpreted in the workup of type 4 renal tubular acidosis (RTA)?

Updated: Oct 10, 2018
  • Author: Christie P Thomas, MBBS, FRCP, FASN, FAHA; Chief Editor: Vecihi Batuman, MD, FASN  more...
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Answer

A TTKG greater than 8 indicates that aldosterone is present and the collecting duct is responsive to it. A TTKG less than 5 in the presence of hyperkalemia indicates aldosterone deficiency or resistance. For the test to be interpretable, the urine Na+ level should be greater than 10 mEq/L and the urine osmolality should be greater than or equal to serum osmolality.

The hyperkalemia suppresses renal ammoniagenesis, leading to a lack of urinary buffers to excrete the total H+ load. The urine AG will be positive. Note that patients with hyperkalemic type 1 RTA have a urine pH greater than 5.5 and a low urine Na+.


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