What this calculator does
When potassium is abnormal, the question is whether the kidney is handling it appropriately. The transtubular potassium gradient attempts to estimate potassium secretion in the collecting duct from a paired urine and blood sample.
It should be read with caution. The osmolality correction rests on assumptions about water reabsorption that several nephrologists now consider unsound, so it is best treated as one data point among several rather than a decisive figure.
The formula
The urine to serum potassium ratio is divided by the urine to serum osmolality ratio. The osmolality term is intended to correct for water reabsorbed downstream of where potassium is secreted.
| Term | Meaning |
|---|---|
| TTKG | Transtubular potassium gradient, a dimensionless estimate of collecting duct secretion. |
| Osmolality correction | The term intended to account for water reabsorption, and the disputed part of the formula. |
| Validity condition | Urine osmolality must exceed serum osmolality, generally at least 300 mOsm/kg. |
The inputs explained
| Field | What to enter |
|---|---|
| Urine potassium (mEq/L) | Urine potassium in mEq/L. |
| Serum potassium (mEq/L) | Serum potassium in mEq/L. |
| Urine osmolality (mOsm/kg) | Urine osmolality in mOsm/kg. The formula is invalid below about 300. |
| Serum osmolality (mOsm/kg) | Serum osmolality in mOsm/kg. |
When to use it
Investigating low potassium
A high gradient suggests renal potassium wasting rather than a dietary or gut cause.
Investigating high potassium
A low gradient suggests impaired secretion, which narrows the causes.
Understanding a nephrology workup
The gradient appears in teaching and in some protocols despite its limitations.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does urine potassium move the gradient?
Three urine potassium levels at the same osmolalities.
| Urine potassium | TTKG | Rough reference |
|---|---|---|
| 20 mEq/L | 3.6 | about 8-9 with normal potassium; higher expected if potassium is high, lower if low |
| 30 mEq/L | 5.4 | about 8-9 with normal potassium; higher expected if potassium is high, lower if low |
| 60 mEq/L | 10.9 | about 8-9 with normal potassium; higher expected if potassium is high, lower if low |
Questions
Why is the formula disputed?
Because it assumes no significant potassium reabsorption in the medullary collecting duct, and evidence suggests that assumption does not hold. Several of its original proponents have since published reservations about using it.
When is the result invalid?
When urine osmolality is below serum osmolality, or below roughly 300 mOsm/kg, and when urine sodium is low. Under those conditions the arithmetic still produces a number but it does not mean anything.
What is used instead?
The urine potassium-to-creatinine ratio is increasingly preferred, along with 24 hour urine potassium where precision matters. Both avoid the osmolality assumption entirely.
How should the number be read?
Against the serum potassium rather than against a fixed range. A gradient of 5 is low if serum potassium is high, because the kidney should be excreting more, and it is appropriate if serum potassium is low.
For the urinary gap in acidosis, see the urine anion gap calculator. For sodium handling, see the FeNa calculator.