This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.
What the TTKG measures
The transtubular potassium gradient (TTKG) estimates how much potassium the cortical collecting duct of the kidney is secreting, the step that aldosterone controls. It compares the potassium concentration in the urine with that in the plasma and corrects for how much water was removed further down the tubule:
TTKG = (urine K ÷ plasma K) ÷ (urine osmolality ÷ plasma osmolality)
It was proposed by Mitchell Halperin’s group in Toronto in the late 1980s to sort out the causes of low and high potassium.
Conditions for a valid result
- Urine osmolality must be higher than plasma osmolality
- Urine sodium should be at least 25 mmol/L, so that sodium delivery does not limit potassium secretion
If either is missing, the calculator flags the result as not interpretable.
Interpretation
| Situation | TTKG | Meaning |
|---|---|---|
| Low potassium | under 3 | The kidneys conserve potassium: digestive loss, low intake or a shift into cells |
| Low potassium | over 7 | Renal potassium wasting: diuretics, excess aldosterone or other mineralocorticoids, some tubular disorders |
| High potassium | under 7 | Inadequate excretion: low aldosterone, aldosterone resistance or drugs that block it |
| High potassium | 7 or more | The kidneys respond; look for a high intake or release from cells |
On a usual diet, the TTKG is often around 8 to 9.
Example
A patient with plasma potassium of 3.0 mmol/L, urine potassium of 45 mmol/L, urine osmolality of 580 and plasma osmolality of 290 mOsm/kg has a TTKG of (45 ÷ 3) ÷ (580 ÷ 290) = 7.5, pointing to renal loss.
Limits
In 2011 the authors who devised the TTKG advised caution, because urea is reabsorbed in the medullary collecting duct, which breaks one of the assumptions behind the formula. Many nephrologists now prefer a spot urine potassium to creatinine ratio, or a 24-hour urine potassium. Use the TTKG as one clue among others, alongside the blood pressure, acid-base status, magnesium and the medication list.