How is the transtubular potassium gradient (TTKG) measured in patients with hypokalemia (low potassium level)?

Updated: Dec 06, 2018
  • Author: Eleanor Lederer, MD, FASN; Chief Editor: Vecihi Batuman, MD, FASN  more...
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Answer

This test is performed using the following equation:

TTKG = Urine potassium x serum osmolality/Serum potassium x urine osmolality

A TTKG value of less than 3 suggests that the kidney is not wasting excessive potassium, while a value of greater than 7 suggests a significant renal loss. This test cannot be applied when the urine osmolality is less than the serum osmolality. Potassium excretion at the distal nephron is highly dependent on sodium delivery to that site. Therefore, low urine potassium in the presence of very low urine sodium (< 25 mEq/L) does not allow the clinician to exclude the possibility of a potassium-wasting syndrome.

Measurement of the TTKG was initially considered superior to measurement of urine potassium alone for assessing the contribution of renal excretion to potassium levels. However, it is important to recognize that the TTKG is valid for this purpose only if (1) the urine osmolality is greater than the serum osmolality—that is, the urine is concentrated relative to the serum—and (2) the urine sodium is greater than 20 mEq/L—that is, distal delivery of sodium is adequate for potassium excretion.

Furthermore, recent evidence suggesting that urea recycling may influence potassium secretion has cast some doubt on the utility of the TTKG. [50] One assumption inherent in the calculation of the TTKG is that the absorption of osmoles distal to the cortical collecting duct is negligible. If further studies suggest that urea transport can influence potassium handling, this test may have to be abandoned.


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