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Transtubular potassium gradient (TTKG)

The transtubular potassium gradient (TTKG) reflects the relation of urine potassium to serum potassium in consideration of urine concentration. Thus, TTKG is a measure of net-potassium excretion by the distal tubule after correction for water reabsorption in the cortical collecting duct. Thereby, a renally caused hyperkalemia or hypokalemia can be differentiated from an extrarenally caused hyperkalemia or hypokalemia.

A precondition for the reliability of the formula is, that urine osmolality is greater than 300 mosmol/L und greater than serum osmolality (that means urine more concentrated than serum).

Interpretation

TTKG value	Meaning
8 - 9	Normal
Hyperkalemia
> 10	Normal aldosterone action
< 5 - 7	Extrarenal cause of hyperkalemia Impaired potassium secretion in case of aldosterone deficiency or resistance
After administration of 0.05 mg 9-α fludrocortisone (artificial mineralocorticoid)
Initial < 5 - 7, then >10	Hypoaldosteronism likely
Initial < 5 - 7, no change	Potassium-sparing diuretics (spironolactone, triamterene, amiloride) Tubular aldosterone resistance (interstitial nephritis, obstruction, pseudohypoaldosteronism type I, sickle cell disease) Drugs (trimethoprim, pentamidine) Increased distal potassium reabsorption (pseudohypoaldosteronism type II, obstruction)
Hypokalemia
< 2	Extrarenal loss
> 2	Renal loss

Modified according to:
Ethier JH et al, Am J Kidney Dis 1990;15:309
West ML et al, Mineral Electrolyt Metabc1986;12:234

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Serum potassium (mmol/L)
Serum osmolality (mosmol/L)
Urine potassium (mmol/L)
Urine osmolality (mosmol/L)

Transtubular potassium gradient:

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