Transtubular potassium gradient (TTKG)
Index of distal nephron potassium secretion from urine and plasma K and osmolality
Sources last checked: 2026-11-03
Transtubular potassium gradient (TTKG)
Index of distal nephron potassium secretion from urine and plasma K and osmolality
Its own originators later questioned the TTKG: Kamel and Halperin (2011) showed that substantial urea reabsorption in the inner medullary collecting duct invalidates the assumption that no osmoles are reabsorbed downstream of the cortical collecting duct. Many nephrologists now prefer the urine K/creatinine ratio. Not valid with hypotonic urine or urine Na ≤ 25 mmol/L.
What is the Transtubular potassium gradient (TTKG)?
The transtubular potassium gradient estimates the potassium concentration gradient across the cortical collecting duct, as a semiquantitative marker of aldosterone-driven potassium secretion. West, Halperin and colleagues introduced it in 1986; it corrects the urine-to-plasma potassium ratio for water reabsorption in the medullary collecting duct.
When to use it
- Hypokalaemia: distinguishing renal from extrarenal potassium loss
- Hyperkalaemia: asking whether the renal response to aldosterone is appropriate
- Teaching renal potassium physiology
When not to use it
- Hypotonic urine (urine osmolality not above plasma osmolality)
- Urine sodium ≤ 25 mmol/L, when distal sodium delivery limits potassium secretion
- As the only test — the urine potassium/creatinine ratio is often preferred
How it is calculated
TTKG = (urine K / plasma K) / (urine osmolality / plasma osmolality)
Clinical pearls and limitations
- Its own originators later questioned the TTKG: Kamel and Halperin (2011) showed that substantial urea reabsorption in the inner medullary collecting duct invalidates the assumption that no osmoles are reabsorbed downstream of the cortical collecting duct. Many nephrologists now prefer the urine K/creatinine ratio. Not valid with hypotonic urine or urine Na ≤ 25 mmol/L.
- Ethier et al. (1990) measured TTKG about 0.9 in hypokalaemia of non-renal origin and 6.7 in hyperaldosteronism; after an oral potassium load healthy subjects reached about 13.
- Expected values depend on the plasma potassium, so there is no single normal range.
- Kamel and Halperin (2011) withdrew support for the core assumption: urea recycling means osmoles are reabsorbed downstream of the cortical collecting duct, so TTKG can mislead.
- Urine and plasma samples should be taken at about the same time.
Frequently asked questions
How do you calculate TTKG?
(Urine K / plasma K) divided by (urine osmolality / plasma osmolality).
Is TTKG still used?
Less than before: its originators questioned its validity in 2011, and many nephrologists now use the urine K/creatinine ratio or 24-hour urine potassium.
What TTKG suggests renal potassium loss in hypokalaemia?
A low TTKG (around 1 in the Ethier study) points to extrarenal loss; a clearly higher value during hypokalaemia suggests mineralocorticoid-driven renal loss.
Evidence and guidelines
- Primary source: 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. PMID 3762510.
- Ethier JH, Kamel KS, Magner PO, Lemann J Jr, Halperin ML. The transtubular potassium concentration in patients with hypokalemia and hyperkalemia. Am J Kidney Dis 1990;15(4):309-315.
- 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.
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