The transtubular potassium gradient (TTKG) is a bedside index clinicians have used to estimate whether the kidney's potassium-secreting response to hyperkalemia or hypokalemia is appropriate. This article walks through the formula, what each input means, and the validity limitations you need to know before relying on the result.
How the TTKG calculator works
TTKG corrects urine potassium for how much water was reabsorbed as urine passed through the medullary collecting duct, using the ratio of urine to serum osmolality. The formula is TTKG = (Urine K x Serum Osm) / (Serum K x Urine Osm), first described by Ethier and colleagues in 1990. A typical TTKG on a normal diet is roughly 8-9.
Inputs and what they mean
The calculator needs four values from labs drawn close together in time: urine potassium and serum potassium (both in mEq/L), and urine osmolality and serum osmolality (both in mOsm/kg). Serum potassium and urine osmolality both sit in the denominator, so a serum potassium near zero or a very dilute urine sample will produce an unreliable or invalid result.
Applying the right threshold
The threshold you compare TTKG against depends on the clinical question. In hyperkalemia, an appropriate kidney response drives TTKG above 7; a lower value points to hypoaldosteronism. In hypokalemia, an appropriate kidney response keeps TTKG below 3; a higher value points to renal potassium wasting. Using the wrong threshold for the clinical context will give a misleading interpretation.
Limits and edge cases
TTKG is only interpretable when urine osmolality exceeds serum osmolality, and traditionally when urine sodium exceeds roughly 25 mmol/L. More fundamentally, modern nephrology literature questions TTKG's core assumption because intrarenal urea recycling means solute is still reabsorbed downstream of the cortical collecting duct. Treat TTKG as one adjunct data point alongside the full clinical picture, not a standalone diagnostic test.