About the TTKG Calculator - Transtubular Potassium Gradient
The transtubular potassium gradient (TTKG) is a bedside estimate of the potassium concentration gradient across the cortical collecting duct. It combines a paired urine and plasma potassium with a paired urine and plasma osmolality to gauge whether the kidney's aldosterone-driven potassium secretion looks appropriate given the plasma potassium level. It is calculated, not measured directly, and is best used alongside the rest of the clinical picture.
The formula
TTKG = (Urine potassium ÷ Plasma potassium) ÷ (Urine osmolality ÷ Plasma osmolality). The first ratio (urine-to-plasma potassium) reflects how concentrated potassium is in the urine relative to plasma; the second ratio (urine-to-plasma osmolality) corrects for how much water has been reabsorbed by the time urine reaches the cortical collecting duct. Dividing the potassium ratio by the osmolality ratio removes the effect of water reabsorption, leaving an estimate of the potassium gradient that aldosterone and the collecting duct actually generated.
Why urine osmolality has to exceed plasma osmolality
The formula assumes that once urine leaves the cortical collecting duct, no further water is reabsorbed — an assumption that only holds when antidiuretic hormone is active and urine is more concentrated than plasma. If urine osmolality is at or below plasma osmolality, the kidney is producing dilute urine and the calculation no longer reflects the cortical collecting duct gradient. In that situation the result is not interpretable and should not be used to guide reasoning about potassium handling.
Reading the number
TTKG is interpreted differently depending on whether plasma potassium is high or low. In hyperkalemia, a low TTKG (commonly cited as below about 7) suggests the kidney is not appropriately excreting potassium — consistent with hypoaldosteronism or a related defect in the aldosterone pathway — while a higher TTKG suggests the kidney's response is intact and the cause of the hyperkalemia lies elsewhere (increased intake, cell breakdown, or a transcellular shift). In hypokalemia, the logic flips: a high TTKG (commonly cited as above about 4) suggests the kidney is inappropriately wasting potassium despite low plasma levels, while a low TTKG (at or below about 2) suggests the kidney is appropriately conserving potassium and the loss is extrarenal — for example gastrointestinal losses or a shift of potassium into cells. On a normal diet with normal plasma potassium, TTKG commonly falls in a wide reference band, often cited as roughly 6 to 12, though this range varies with potassium and sodium intake.
When to consult a professional
This calculator performs the standard TTKG arithmetic; it does not diagnose a cause of hyperkalemia or hypokalemia and does not direct treatment. TTKG also has recognized physiologic limitations — it assumes potassium secretion happens only in the cortical collecting duct, which is not strictly true — so many clinicians treat it as one supporting data point rather than a definitive test. Any decision about diagnosis or treatment of a potassium disorder should be made by a licensed healthcare provider using the full clinical picture.