Transtubular Potassium Gradient
Input urine and serum potassium and osmolality values. TTKG helps differentiate renal versus extrarenal potassium abnormalities.
How to Use This Calculator
Ensure proper sampling conditions
Collect urine and serum simultaneously. TTKG is valid only when antidiuretic hormone is present (urine osmolality > serum osmolality).
Enter electrolyte and osmolality data
Use mmol/L for potassium and mOsm/kg for osmolality. The calculator validates urine osmolality before computing TTKG.
Interpret relative to serum potassium
In hypokalaemia, TTKG should be low (<2); in hyperkalaemia, it should be high (>7). Deviations suggest renal tubular disorders or hormonal abnormalities.
Formula
TTKG = (Urine K⁺ ÷ Serum K⁺) × (Serum Osmolality ÷ Urine Osmolality)
Valid only when urine osmolality > serum osmolality and urine sodium >25 mmol/L (sodium check assumed).
Full Description
The transtubular potassium gradient estimates distal nephron potassium secretion. It is particularly useful in distinguishing renal from extrarenal potassium disorders. In hypokalaemia, low TTKG indicates appropriate renal conservation; high TTKG signals renal potassium wasting. Conversely, in hyperkalaemia, an inadequate TTKG suggests impaired aldosterone effect or distal tubular dysfunction. Always interpret TTKG alongside clinical context, acid–base status, renin-aldosterone measurements, and medication history.
Frequently Asked Questions
Why must urine osmolality exceed serum osmolality?
TTKG assumes water reabsorption in the collecting duct. If urine is dilute (osmolality ≤ serum), the model breaks down and TTKG is invalid.
Do diuretics affect TTKG?
Loop and thiazide diuretics increase distal sodium delivery, potentially elevating TTKG. Interpret results alongside medication history.
Can TTKG diagnose adrenal insufficiency?
Low TTKG in hyperkalaemia suggests hypoaldosteronism or adrenal insufficiency but requires confirmatory hormonal testing.
What is the role of urine sodium?
Urine sodium >25 mmol/L indicates adequate distal sodium delivery. If sodium is low, TTKG may appear low despite appropriate aldosterone action.