TTKG Calculator (Transtubular Potassium Gradient)
Transtubular potassium gradient from urine and serum potassium and osmolality — assess renal potassium handling in hyperkalaemia (expect > 7) and hypokalaemia (expect < 3) and detect hypoaldosteronism.
How it works
TTKG = (urine K ÷ plasma K) ÷ (urine osmolality ÷ plasma osmolality); valid when urine osm > plasma osm and urine Na > 25 mmol/L
Potassium is secreted into the urine in the cortical collecting duct under the control of aldosterone, and then concentrated as water is reabsorbed further down the nephron. Dividing the urine potassium by the osmolality ratio undoes that concentration step and estimates the potassium gradient the collecting duct actually generated. In hyperkalaemia a healthy aldosterone response drives the gradient above 7; a value under 5 means the kidney is not secreting potassium as it should — the signature of hypoaldosteronism or of drugs that block it. In hypokalaemia the reverse holds: a gradient below 3 shows the kidney is conserving potassium, pointing to gut losses, while a higher value implicates renal wasting. The formula assumes ADH is acting (concentrated urine) and enough sodium reaches the distal nephron; some authors now prefer the urine potassium-to-creatinine ratio.
Worked example
Urine potassium 25, serum potassium 6.1, urine osmolality 600, serum osmolality 290: TTKG = (25/6.1) / (600/290) = 2.0 — inappropriately low for hyperkalaemia; suspect hypoaldosteronism or a potassium-sparing drug.
Frequently asked questions
Why has TTKG fallen out of favour?
Its derivation assumed no urea reabsorption in the medullary collecting duct, which is incorrect; many nephrologists now use the spot urine K/creatinine ratio (> 13 mmol/g or > 1.5 mmol/mmol suggests renal wasting in hypokalaemia).
What is a fludrocortisone challenge?
Giving 0.05–0.1 mg of fludrocortisone and repeating the TTKG four hours later — a rise above 7 confirms aldosterone deficiency, no rise suggests resistance.
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Clinical decision tool for education and for use by trained clinicians alongside judgement — not a diagnosis and not a substitute for medical assessment. Scores validated in specific populations may not apply to every patient.