The normal renal response when hypokalemia is due to non-renal causes is a TTKG <2, where a TTKG >5 is indicative of increased secretion of K+ in the cortical collecting ducts. Thus a transtubular potassium concentration gradient (TTKG) of greater than 3.0 indicates hypokalemia of renal origin, while a value below 2.0 indicates intracellular shift of K+, as found in ion channelopathy hypokalemic periodic paralysis.
The transtubular K+ concentration ([K+]) gradient (TTKG) is calculated using the following formula:
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