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| The '''trans-tubular potassium gradient''' ('''TTKG''') is an index reflecting the conservation of [[potassium]] in the [[cortical collecting duct]]s (CCD) of the [[kidney]]s. It is useful in diagnosing the causes of [[hyperkalemia]] or [[hypokalemia]].<ref>{{Cite journal|author=Ethier JH, Kamel KS, Magner PO, Lemann J, Halperin ML |title=The transtubular potassium concentration in patients with hypokalemia and hyperkalemia |journal=Am. J. Kidney Dis. |volume=15 |issue=4 |pages=309–15 |date=April 1990 |pmid=2321642 |doi= |url=}}</ref><ref>{{Cite journal|author=Choi MJ, Ziyadeh FN |title=The utility of the transtubular potassium gradient in the evaluation of hyperkalemia |journal=J. Am. Soc. Nephrol. |volume=19 |issue=3 |pages=424–6 |date=March 2008 |pmid=18216310 |doi=10.1681/ASN.2007091017 |url=}}</ref> The TTKG estimates the ratio of potassium in the lumen of the CCD to that in the peritubular capillaries.
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| The following is the formula for calculating the TTKG:
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| <math> TTKG = {\frac{urine_K}{plasma_K}} \div {\frac{urine_{osm}}{plasma_{osm}}} </math>
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| Note that this formula is valid only when U<sub>osm</sub> >300 and U<sub>Na</sub> >25
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| The validity of this measurement falls on three assumptions:
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| *(1) Few solutes are reabsorbed in the medullary collecting duct (MCD)
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| *(2) Potassium is neither secreted nor reabsorbed in the MCD
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| *(3)The osmolality of the fluid in the terminal CCD is known
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| Significant reabsorption or secretion of K in the MCD seldom occurs, except in profound K depletion or excess, respectively.
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| A typical TTKG in a normal person on a normal diet is 8-9. During hyperkalemia or high potassium intake, more potassium should be excreted in the urine and the TTKG should be above 10. Low levels (<7) during hyperkalemia may indicate [[mineralocorticoid]] deficiency, especially if accompanied by [[hyponatremia]] and high urine Na.
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| During potassium depletion or [[hypokalemia]], the TTKG should fall to less than 3, indicating appropriately reduced urinary excretion of potassium.
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| ==See also==
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| *[[Hyperkalemia]]
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| *[[Hypoaldosteronism]]
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| *[[Aldosterone]]
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| *[[Fractional sodium excretion]]
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| ==References==
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| {{Reflist}}
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| [[Category:Nephrology]]
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| {{Med-stub}}
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Latest revision as of 12:18, 25 November 2014
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