TTKG Calculator — Transtubular Potassium Gradient

Formula

How this is calculated

In the cortical collecting duct, aldosterone stimulates principal cells to secrete potassium into the tubular lumen. The TTKG corrects the urine-to-serum potassium ratio for water reabsorption that occurs downstream (in the medullary collecting duct), giving an estimate of the potassium concentration ratio specifically at the cortical collecting duct. Dividing the urine-to-serum K⁺ ratio by the urine-to-serum osmolality ratio achieves this correction.

Interpretation depends on the clinical context. In hyperkalemia (serum K⁺ > 5.5 mEq/L), a TTKG below 7 suggests inadequate aldosterone effect — consistent with hypoaldosteronism or aldosterone resistance (type IV RTA, drugs like ACE inhibitors or spironolactone) — while a TTKG above 7 indicates that the kidney is attempting to excrete potassium appropriately. In hypokalemia (serum K⁺ < 3.5 mEq/L), a TTKG above 3 implies inappropriate renal potassium wasting (primary hyperaldosteronism, Bartter/Gitelman syndrome, diuretics), whereas a TTKG below 2 suggests the kidney is conserving potassium correctly and the loss is extrarenal (GI losses, low intake).

Important limitations: TTKG is only valid when the urine osmolality equals or exceeds the serum osmolality; if the urine is hypotonic (ADH not acting), the downstream concentration effect is reversed and the formula becomes meaningless. The concept has been criticised for assuming a constant osmolality correction and for not accounting for non-aldosterone regulators of potassium secretion. Many centres now prefer a fractional excretion of potassium (FEK) approach. Use TTKG as one piece of the clinical picture alongside aldosterone levels, renin, blood pressure, medications and acid-base status.