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Transtubular Potassium Gradient (TTKG)

TTKG = (urine K ÷ plasma K) × (plasma osm ÷ urine osm), an index of distal potassium secretion. In hyperkalaemia a TTKG <7 suggests inadequate aldosterone effect; in hypokalaemia a TTKG <3 suggests extrarenal loss and >7 renal loss. Valid only if urine osm exceeds plasma osm.

  • Urine potassium (mmol/L or mEq/L)
  • Plasma potassium (mmol/L or mEq/L)
  • Urine osmolality
  • Plasma osmolality
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Interpretation

ResultMeaning
Hyperkalaemia, TTKG < 7Low aldosterone effect
Hyperkalaemia, TTKG ≥ 7Appropriate secretion
Hypokalaemia, TTKG < 3Extrarenal loss
Hypokalaemia, TTKG > 7Renal wasting

Formula

TTKG = (U_K ÷ P_K) × (P_osm ÷ U_osm). Valid when U_osm > P_osm and urine Na > 25 mmol/L.

Clinical pearls

  • Its originators (Kamel and Halperin) no longer recommend TTKG because urea recycling invalidates its assumptions; urine K:creatinine ratio is an alternative.
  • Requires adequate distal Na delivery (urine Na >25 mmol/L) and concentrated urine.

Examples

ttkg urine k 40 serum k 6 urine osm 580 serum osm 290transtubular potassium gradient k 2.8 uk 30 uosm 600 posm 285

FAQ

What is a normal TTKG?

About 8–9 on a normal diet; it should rise above 7–10 in hyperkalaemia and fall below 2–3 in hypokalaemia of extrarenal origin.

Is the TTKG still used?

Less often; the authors who introduced it advised against it in 2011, and urine K:creatinine ratio is commonly used instead.

Evidence

  1. Ethier JH et al. The transtubular potassium concentration in patients with hypokalemia and hyperkalemia. Am J Kidney Dis 1990;15:309–15.

Nephrology & Electrolytes · Reviewed 2026-10-14 · Methodology