The transtubular potassium gradient (TTKG) estimates the potassium concentration gradient across the cortical collecting duct — in effect, how hard the kidney is trying to excrete or conserve potassium. It helps nephrologists decide whether a high or low serum potassium is the kidney’s fault or its appropriate response to something else.
How it works
The calculator implements the standard formula:
TTKG = (UK / PK) / (Uosm / Posm)
The first ratio is the urine-to-serum potassium concentration. Dividing by the urine-to-serum osmolality ratio corrects for the water that is reabsorbed in the medullary collecting duct, which would otherwise concentrate potassium and exaggerate the gradient. The result approximates the potassium concentration in the lumen of the cortical collecting duct, where aldosterone acts.
Interpretation
- Hyperkalaemia: a healthy aldosterone response should drive a TTKG > 7–8. A lower value is inappropriate and suggests hypoaldosteronism or aldosterone resistance (e.g. type 4 renal tubular acidosis, certain drugs).
- Hypokalaemia: the kidney should conserve potassium, giving TTKG < 3. A higher value indicates inappropriate renal potassium wasting — diuretics, hyperaldosteronism, Bartter or Gitelman syndrome.
Sample values and what they suggest
| TTKG | Context | Likely interpretation |
|---|---|---|
| 2 | Hypokalaemia | Appropriate renal conservation — look elsewhere (GI losses, redistribution) |
| 6 | Hypokalaemia | Renal wasting — consider diuretics, hyperaldosteronism, Bartter/Gitelman |
| 5 | Hyperkalaemia | Appropriate? No — too low; suggests aldosterone deficiency or resistance |
| 10 | Hyperkalaemia | Appropriate excretion — kidney responding correctly; non-renal cause likely |
These are illustrative ranges. Always interpret alongside the full clinical picture, urinary sodium, and medication history.
Validity conditions and caveats
TTKG is only valid when urine osmolality exceeds serum osmolality and urine
sodium is above roughly 25 mmol/L (adequate distal sodium delivery). The tool
warns you if the osmolality condition fails.
Two situations frequently invalidate the calculation and should prompt caution:
- Concentrated urine below serum osmolality — the osmolality correction reverses in sign and the TTKG becomes meaningless. This often happens in volume-depleted patients.
- Very low urine sodium — if distal sodium delivery is minimal, potassium secretion cannot occur regardless of aldosterone status, so a low TTKG may reflect sodium restriction rather than aldosterone deficiency.
The test has been criticised because urea recycling in the medulla complicates the osmolality correction. Many nephrologists now favour the urine potassium-to-creatinine ratio as a more reliable alternative that sidesteps the osmolality assumption entirely. Use TTKG as a supporting clue alongside the full clinical picture, not as a standalone diagnostic test.
When to use the urine potassium-to-creatinine ratio instead
The urine potassium-to-creatinine ratio (uK:Cr) avoids the osmolality assumption entirely. A ratio above approximately 13–15 mmol/mmol suggests renal wasting in a hypokalaemic patient. Many nephrologists now use this as a first-line test and reserve TTKG for situations where uK:Cr is borderline or the clinical picture is unusual. Both values can be entered into this calculator’s companion tools for a side-by-side comparison.
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