TTKG(Transtubular K gradient)は、腎臓における主たるK排泄のセグメントである皮質集合管におけるK分泌の指標であり、臨床において、低K血症、高K血症の鑑別診断に有用とされてきた。


Kamel KS, Halperin ML. Intrarenal urea recycling leads to a higher rate of renal excretion of potassium: an hypothesis with clinical implications. Curr Opin Nephrol Hypertens. 2011;20(5):547-54.

As a large quantity of urea is reabsorbed daily in the inner medullary collecting duct, the assumption made in the calculation of the transtubular K concentration gradient that there is no appreciable reabsorption of osmoles downstream CCD is not valid.


Transtubular potassium gradient — In a later publication, the authors of the original studies found that the assumptions underlying the TTKG were not valid. It was concluded that the TTKG was not a reliable test for the diagnosis of hyperkalemia. We recommend not using the TTKG to evaluate patients with hyperkalemia.(UpToDate, Causes and evaluation of hyperkalemia in adults)



 Assessment of urinary potassium excretion — The best method for assessing renal potassium excretion is a 24-hour urine collection. However, the potassium concentration or, preferably, potassium-to-creatinine ratio on a spot urine are alternatives.(UpToDate, Evaluation of the patient with hypokalemia)

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