What this calculator does
Transtubular potassium gradient (TTKG) works out estimates how well the kidney is handling potassium from paired urine and serum readings. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
The inputs explained
| Field | What to enter |
|---|---|
| Urine potassium (mEq/L) | A number, measured in mEq/L. Starts at 30. |
| Serum potassium (mEq/L) | A number, measured in mEq/L. Starts at 4. |
| Urine osmolality (mOsm/kg) | A number, measured in mOsm/kg. Starts at 400. |
| Serum osmolality (mOsm/kg) | A number, measured in mOsm/kg. Starts at 290. |
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 the answer changes with urine potassium
Every other input is held at the calculator’s starting values while urine potassium varies. Select any row to load that scenario into the calculator.
| Urine potassium (mEq/L) | TTKG | Rough reference | Note |
|---|---|---|---|
| 15 | 2.7 | about 8-9 with normal potassium; higher expected if potassium is high, lower if low | Only valid when urine osmolality is at least 300 mOsm/kg; some nephrologists consider the formula’s underlying assumptions outdated, so treat it as one data point among several. |
| 22.5 | 4.1 | about 8-9 with normal potassium; higher expected if potassium is high, lower if low | Only valid when urine osmolality is at least 300 mOsm/kg; some nephrologists consider the formula’s underlying assumptions outdated, so treat it as one data point among several. |
| 30 | 5.4 | about 8-9 with normal potassium; higher expected if potassium is high, lower if low | Only valid when urine osmolality is at least 300 mOsm/kg; some nephrologists consider the formula’s underlying assumptions outdated, so treat it as one data point among several. |
| 45 | 8.2 | about 8-9 with normal potassium; higher expected if potassium is high, lower if low | Only valid when urine osmolality is at least 300 mOsm/kg; some nephrologists consider the formula’s underlying assumptions outdated, so treat it as one data point among several. |
| 60 | 10.9 | about 8-9 with normal potassium; higher expected if potassium is high, lower if low | Only valid when urine osmolality is at least 300 mOsm/kg; some nephrologists consider the formula’s underlying assumptions outdated, so treat it as one data point among several. |
| 90 | 16.3 | about 8-9 with normal potassium; higher expected if potassium is high, lower if low | Only valid when urine osmolality is at least 300 mOsm/kg; some nephrologists consider the formula’s underlying assumptions outdated, so treat it as one data point among several. |