TTKG Calculator (transtubular potassium gradient)

Understanding the TTKG calculator — concept and principle

The transtubular potassium gradient (TTKG) estimates how strongly the kidney's collecting duct is secreting potassium. Clinicians use it when potassium is too high or too low to judge whether the kidney is responding appropriately, for example to aldosterone.

Potassium ratio ÷ osmo ratio: with the default inputs the result is TTKG 4 (serum K 5.8 (high)).

How it is calculated

TTKG = (urine K ÷ serum K) ÷ (urine osmolality ÷ serum osmolality)

Valid only when urine osmolality is above serum osmolality and urine sodium is above about 25 mmol/L.

The principle

Potassium is secreted into the urine mainly in the cortical collecting duct, under the control of aldosterone. Further down the nephron water is removed, which concentrates the urine potassium without any extra secretion. Dividing the urine/plasma potassium ratio by the urine/plasma osmolality ratio removes that concentration effect, leaving an estimate of the gradient at the collecting duct. Enough sodium must reach that segment for secretion to occur, hence the sodium condition.

Worked example

Default inputs: urine K 40, serum K 5.8 mmol/L, urine osmolality 500, serum osmolality 290, urine sodium 60.

Urine/plasma K = 40 ÷ 5.8 = 6.9.
Urine/plasma osmolality = 500 ÷ 290 = 1.72.
TTKG = 6.897 ÷ 1.724 = 4.
Serum K 5.8 → High (above 5.0); conditions met → Yes.

Each point is the calculator re-run with only “Serum potassium (mmol/L)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Serum potassium (mmol/L)	TTKG
1.45	16
2.9	8
4.35	5.3
5.8	4
7.25	3.2
8.7	2.7
11.6	2

How to read the result

With high potassium, a TTKG below about 5–7 suggests too little aldosterone effect (for example hypoaldosteronism or certain medicines), while above about 7–10 suggests the kidney is responding. With low potassium, a TTKG under 3 points to losses outside the kidney and above 3 to renal potassium loss. Because urea recycling breaks one assumption, the authors who popularised the TTKG later advised relying more on the urine potassium/creatinine ratio.

Common mistakes

Using it when the urine is dilute (urine osmolality below serum).
Ignoring low urine sodium.
Treating the cut-offs as exact.

Keep in mind: This is a decision aid for clinicians, not a diagnosis. Abnormal potassium levels can be dangerous and need medical care.

Frequently asked questions

What is a normal TTKG?

It depends on potassium intake and level; interpretation is always in relation to serum potassium.

Why is TTKG less used now?

Its assumptions about urea and water handling are not always true; the urine K/creatinine ratio is often preferred.

What causes a low TTKG with high potassium?

Low aldosterone or medicines that block it, such as some diuretics.

Sources: West ML, Marsden PA, Richardson RM, Zettle RM, Halperin ML, "New clinical approach to evaluate disorders of potassium excretion", Miner Electrolyte Metab 1986;12(4):234–238 · Kamel KS, Halperin ML, "Intrarenal urea recycling leads to a higher rate of renal excretion of potassium: an hypothesis with clinical implications", Curr Opin Nephrol Hypertens 2011;20(5):547–554

Checked 9 Nov 2026. TTKG formula and interpretation thresholds as standard values (West ML et al., 1986; Ethier JH et al., Am J Kidney Dis 1990;15:309–315); limitations as later noted by Kamel KS and Halperin ML, Curr Opin Nephrol Hypertens 2011.

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