What is a normal TTKG value?

In patients with normal potassium, TTKG is typically 6–12. In hyperkalemia, a normal kidney should raise TTKG above 10–11 (often >7 is considered appropriate). In hypokalemia, the kidney should conserve potassium, so TTKG should be below 3 (values above 3 suggest renal potassium wasting).

What does a high TTKG in hyperkalemia mean?

A TTKG above 10 in the setting of hyperkalemia suggests the kidney is appropriately excreting potassium, pointing to a non-renal (extrarenal) cause such as excessive intake, cell lysis, or metabolic acidosis.

What does a low TTKG in hyperkalemia mean?

A TTKG below 6–7 in the context of hyperkalemia suggests the kidney is not excreting potassium appropriately. This points to renal causes such as hypoaldosteronism, type 4 RTA, or aldosterone resistance (pseudohypoaldosteronism).

What does a high TTKG in hypokalemia mean?

A TTKG above 3–4 in the setting of hypokalemia indicates inappropriate renal potassium wasting. Causes include primary or secondary hyperaldosteronism, Bartter syndrome, Gitelman syndrome, or diuretic use.

What are the limitations of TTKG?

TTKG requires urine osmolality to be greater than serum osmolality (collecting duct must be concentrating urine). It is unreliable if urine osmolality is low, in advanced renal failure, or with severe volume depletion. It has largely been supplanted by spot urine K/Cr ratio in many guidelines, but remains a useful teaching concept.

What units should I use?

Urine and serum potassium should both be in the same units (mEq/L or mmol/L — they are numerically equivalent). Osmolality values should both be in mOsm/kg. Mixing units will produce an incorrect result.

Transtubular Potassium Gradient (TTKG) Calculator

Enter urine potassium, serum potassium, urine osmolality, and serum osmolality to calculate TTKG and interpret renal potassium excretion.

Enter Lab Values

Use concurrent urine and serum samples. Urine osmolality must exceed serum osmolality for a valid result.

Urine Potassium (K⁺) (mEq/L)

Serum Potassium (K⁺) (mEq/L)

Urine Osmolality (mOsm/kg)

Serum Osmolality (mOsm/kg)

Clinical Context (optional — improves interpretation)

Result

Enter values and click Calculate.

Reference Ranges

Normal K TTKG 6–12

Hyperkalemia > 10 = appropriate renal excretion (extrarenal cause likely)
< 6–7 = inadequate excretion (renal/aldosterone deficiency)

Hypokalemia < 3 = appropriate conservation (extrarenal loss)
> 3–4 = renal K wasting (diuretics, hyperaldosteronism)

Valid only when urine osmolality > serum osmolality. Unreliable in severe renal failure or hypotonic urine.

Summary

The Transtubular Potassium Gradient (TTKG) estimates potassium concentration in the cortical collecting duct relative to plasma, corrected for water reabsorption. The formula is TTKG = (Urine K / Serum K) ÷ (Urine Osmolality / Serum Osmolality). It is used to evaluate whether the kidney is appropriately responding to hyperkalemia or hypokalemia.

How it works

Enter the urine potassium concentration (mEq/L or mmol/L).
Enter the serum (plasma) potassium concentration (mEq/L).
Enter the urine osmolality (mOsm/kg).
Enter the serum osmolality (mOsm/kg).
The calculator applies the formula: TTKG = (Urine K / Serum K) / (Urine Osm / Serum Osm).
Interpret the result in the context of the clinical potassium disorder.

Use cases

Workup of unexplained hyperkalemia to assess aldosterone response.
Workup of hypokalemia to determine if the kidney is wasting potassium.
Differentiating renal from extrarenal causes of potassium disorders.
Evaluating effectiveness of mineralocorticoid activity in the collecting duct.
Monitoring patients on aldosterone antagonists or ACE inhibitors.
Nephrology teaching and clinical decision support.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu