TTKG Calculator - Transtubular Potassium Gradient

The TTKG calculation estimates the ratio of potassium concentration in the lumen of the cortical collecting duct to that in the peritubular capillary blood. The standard mathematical expression is: TTKG = ([Urine K] / [Serum K]) / ([Urine Osmolality] / [Serum Osmolality]). This dimensionless index accounts for medullary water reabsorption along the nephron, offering a clearer picture of distal potassium secretion than a simple spot urine potassium test.

Consider a clinical scenario where a patient has a Serum Potassium level of 4.5 mEq/L and a Serum Osmolality of 290 mOsm/kg. Laboratory tests on a spot urine sample return a Urine Potassium level of 45 mEq/L and a Urine Osmolality of 580 mOsm/kg. First, calculate the urine-to-serum potassium ratio: 45 / 4.5 = 10. Next, calculate the urine-to-serum osmolality ratio: 580 / 290 = 2. Finally, divide the potassium ratio by the osmolality ratio: 10 / 2 = 5. The resulting TTKG value is 5, indicating normal distal potassium secretory activity under these conditions.

Ensure that urine and serum samples are drawn concurrently to maintain clinical accuracy. Avoid using the TTKG if the urine osmolality is less than the serum osmolality (i.e., when urine is significantly hypotonic), as medullary urea recycling and water movement can invalidate the assumptions of the formula. Always interpret TTKG values in the broader context of the patient's volume status, acid-base balance, and concurrent medications.