Trans-tubular potassium gradient

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The trans-tubular potassium gradient (TTKG) is an index reflecting the conservation of potassium in the cortical collecting ducts of the kidneys. It is useful in diagnosing the cause of hyperkalemia.

The following is the formula for calculating the TTKG:

$TTKG = {\frac{urine_K}{plasma_K}} \div {\frac{urine_{osm}}{plasma_{osm}}}$

Note that this formula is valid only when U_osm >300 and U_Na >25

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.

During potassium depletion or hypokalemia, the TTKG should fall to less than 3, indicating appropriately reduced urinary excretion of potassium.