What is the TTKG formula?

TTKG = (UK / PK) / (Uosm / Posm), where UK and PK are urine and serum potassium (mmol/L) and Uosm and Posm are urine and serum osmolality (mOsm/kg). The osmolality ratio corrects urine potassium for water reabsorbed in the collecting duct.

How do I interpret TTKG in hyperkalaemia?

With hyperkalaemia an intact kidney should excrete potassium briskly, giving a TTKG above 7–8. A TTKG below 7 is inappropriately low and points to hypoaldosteronism or aldosterone resistance such as type 4 renal tubular acidosis.

How do I interpret TTKG in hypokalaemia?

With hypokalaemia the kidney should conserve potassium, giving a TTKG below 3. A TTKG above 3–4 is inappropriately high and indicates renal potassium wasting, for example from diuretics, hyperaldosteronism, or Bartter and Gitelman syndromes.

When is TTKG invalid?

TTKG requires urine osmolality greater than serum osmolality and urine sodium above roughly 25 mmol/L. If the urine is dilute or sodium-poor the underlying assumptions break down and the value should not be used.

Is TTKG still recommended?

Its validity has been questioned because medullary urea recycling complicates the osmolality assumption, and many nephrologists now prefer the urine potassium-to-creatinine ratio. Use TTKG as one input alongside the full clinical picture rather than in isolation.

Transtubular Potassium Gradient (TTKG) Calculator

The transtubular potassium gradient (TTKG) estimates the potassium concentration gradient across the cortical collecting duct — in effect, how hard the kidney is trying to excrete or conserve potassium. It helps nephrologists decide whether a high or low serum potassium is the kidney’s fault or its appropriate response to something else.

How it works

The calculator implements the standard formula:

TTKG = (UK / PK) / (Uosm / Posm)

The first ratio is the urine-to-serum potassium concentration. Dividing by the urine-to-serum osmolality ratio corrects for the water that is reabsorbed in the medullary collecting duct, which would otherwise concentrate potassium and exaggerate the gradient. The result approximates the potassium concentration in the lumen of the cortical collecting duct, where aldosterone acts.

Interpretation

Hyperkalaemia: a healthy aldosterone response should drive a TTKG > 7–8. A lower value is inappropriate and suggests hypoaldosteronism or aldosterone resistance (e.g. type 4 renal tubular acidosis, certain drugs).
Hypokalaemia: the kidney should conserve potassium, giving TTKG < 3. A higher value indicates inappropriate renal potassium wasting — diuretics, hyperaldosteronism, Bartter or Gitelman syndrome.

Validity and caveats

TTKG is only valid when urine osmolality exceeds serum osmolality and urine sodium is above roughly 25 mmol/L (adequate distal sodium delivery). The tool warns you if the osmolality condition fails. The test has been criticised because urea recycling in the medulla complicates the osmolality correction, and many nephrologists now favour the urine potassium-to-creatinine ratio. Use TTKG as a supporting clue, not a standalone diagnosis. All calculation runs locally in your browser.