TTKG Calculator — Transtubular Potassium Gradient

The transtubular potassium gradient (TTKG) is a clinical index used to estimate the activity of aldosterone in the cortical collecting duct of the kidney. It provides insight into whether abnormal serum potassium levels — either too high (hyperkalemia) or too low (hypokalemia) — stem from the kidneys excreting too much or too little potassium. Since the kidney is the primary regulator of long-term potassium balance, distinguishing renal from non-renal causes has major therapeutic implications. The formula accounts for the fact that water is reabsorbed in the medullary collecting duct after the cortical collecting duct, which would artificially concentrate the potassium remaining in the tubule. By multiplying urine potassium by plasma osmolality and dividing by the product of plasma potassium and urine osmolality, the calculation corrects for this water reabsorption step, giving an index that reflects conditions specifically in the aldosterone-sensitive cortical collecting duct where potassium secretion is regulated. In healthy individuals who are normokalemic, TTKG typically ranges from 5 to 10. The test is most diagnostically useful when potassium is clearly abnormal. In hypokalemia (serum K+ below 3.5 mEq/L), a TTKG below 2 suggests the kidneys are appropriately conserving potassium and the problem lies outside the kidney — such as prolonged vomiting or diarrhea, inadequate dietary potassium, or transcellular shifts. A TTKG above 2 in a hypokalemic patient indicates that the kidneys are inappropriately continuing to excrete potassium, pointing toward causes such as primary hyperaldosteronism, Bartter syndrome, Gitelman syndrome, or diuretic use. In hyperkalemia (serum K+ above 5.0 mEq/L), a TTKG above 10 suggests the kidneys are responding appropriately by excreting the excess potassium, implying the cause of hyperkalemia is extracellular (such as a transcellular shift from acidosis or cell lysis). A TTKG below 5 in a hyperkalemic patient implies the kidney is failing to excrete enough potassium, raising concern for conditions like type IV renal tubular acidosis, adrenal insufficiency (hypoaldosteronism), or potassium-sparing diuretic use. It is important to note that TTKG has recognized limitations. It requires that the urine osmolality be higher than plasma osmolality to ensure adequate ADH activity in the collecting duct; if urine osmolality is lower than plasma osmolality, the TTKG value may be unreliable. Some experts have raised concerns about the assumptions underlying the formula, and in clinical practice TTKG is used as one piece of information alongside full electrolyte panels, blood gas analysis, aldosterone and renin levels, and imaging. This calculator is intended for educational and clinical reference purposes only, and results should be interpreted by a qualified healthcare provider.