The Transtubular Potassium Gradient (TTKG) is a nephrology calculation used to deduce the physiological driving forces behind potassium abnormalities.
Renal Potassium Handling
The kidneys are entirely responsible for regulating the body's potassium levels. If you eat a meal rich in potassium, your serum potassium will rise. The hormone Aldosterone is immediately released, traveling to the kidneys and commanding the distal tubules to excrete the excess potassium into the urine.
Diagnosing the Defect
- In Hyperkalemia: If a patient has high blood potassium, a healthy kidney should be actively dumping potassium into the urine, resulting in a high TTKG (> 10). If the TTKG is low (< 7), the kidneys are failing to respond, indicating aldosterone deficiency (e.g., Addison's disease) or aldosterone-blocking medications.
- In Hypokalemia: If a patient has dangerously low blood potassium, a healthy kidney should shut down potassium excretion completely, resulting in a low TTKG (< 3). If the TTKG is high (> 4), the kidneys are inappropriately wasting potassium (e.g., diuretic abuse or hyperaldosteronism).
TTKG = (Urine K / Serum K) / (Urine Osm / Serum Osm)