π§ͺ TTKG Calculator
Are the kidneys handling potassium properly?
Transtubular potassium gradient
For informational purposes only, not medical advice.
TTKG equals urine potassium divided by serum potassium, multiplied by serum osmolality divided by urine osmolality. With urine potassium 40, serum potassium 5.5, urine osmolality 600, and serum osmolality 290, the gradient is 3.5. Below 5 during hyperkalemia, the kidneys are not excreting potassium properly, pointing at aldosterone deficiency or resistance rather than a potassium load.
The gradient estimates the potassium concentration in the cortical collecting duct, where aldosterone acts, correcting for water reabsorbed downstream. It answers one focused question: faced with high blood potassium, are the kidneys even trying? A low TTKG says no.
How to Use This Calculator
You need four labs, ideally from the same time window: spot urine potassium, serum potassium, urine osmolality, and serum osmolality. Enter all four and the gradient appears with an interpretation.
Enter urine potassium and serum potassium in mEq/L from near-simultaneous samples.
Enter urine and serum osmolality in mOsm/kg. The tool requires urine osmolality above serum osmolality for validity.
Read the TTKG and the interpretation. Under 5 with hyperkalemia suggests a renal handling defect worth working up.
The Formula
Potassium is secreted in the collecting duct under aldosterone's control, then water is reabsorbed further down, concentrating everything left behind. Raw urine potassium therefore overstates ductal secretion whenever urine is concentrated. Dividing by the osmolality ratio strips out the water effect, leaving an estimate of the gradient aldosterone actually produced.
The validity condition matters: if urine is dilute, ADH is off and the collecting duct dynamics the formula assumes do not hold. The calculator enforces this and refuses dilute-urine inputs rather than returning a misleading number.
Worked Example
Urine potassium 40 mEq/L, serum potassium 5.5 mEq/L, urine osmolality 600, serum osmolality 290. The potassium ratio is 40/5.5 = 7.27. The osmolality correction is 290/600 = 0.48. Multiply: 7.27 times 0.48 gives a TTKG of 3.5.
At 3.5 with hyperkalemia, the kidneys are underperforming: a proper aldosterone response should push the gradient above 7. The differential tilts toward hypoaldosteronism, medications like ACE inhibitors or trimethoprim, or tubular resistance, rather than excess intake or cell breakdown. That distinction changes the workup completely.
Reading Your Result
| TTKG in hyperkalemia | Meaning |
|---|---|
| Above 7 | Appropriate renal response. The kidneys are excreting; look for extrarenal causes like intake, cell lysis, or acidosis shifts. |
| 5 to 7 | Borderline. Repeat and correlate with the clinical picture. |
| Below 5 | Impaired potassium excretion. Consider aldosterone deficiency, blocking drugs, or tubular dysfunction. |
My take: TTKG is most useful when it surprises you. Hyperkalemia with a high gradient redirects the hunt away from the kidney, while a low gradient in someone on spironolactone simply confirms what the drug list already said. Context first, number second.
Limits
TTKG assumes steady state and falls apart with heavy diuretic use, very high urine flow, or dilute urine, which is why the osmolality condition exists. It is also less reliable in advanced kidney failure where few nephrons remain. Like every derived index, it organizes thinking; it does not diagnose.
Related Tools
Potassium and sodium physiology overlap, so the sodium deficit calculator and daily water intake calculator round out the electrolyte picture. The blood pressure checker is relevant too, since aldosterone disorders move pressure and potassium together.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
What does TTKG measure?
The transtubular potassium gradient estimates potassium secretion in the kidney's collecting duct, corrected for water reabsorption. It tells whether the kidneys are responding appropriately to high blood potassium. A low value points to aldosterone deficiency or resistance.
What is a normal TTKG?
It depends on context. During hyperkalemia, a TTKG above 7 indicates an appropriate renal response, while below 5 suggests impaired excretion. There is no single normal value outside hyperkalemia because the expected gradient follows the potassium level.
Why must urine osmolality exceed serum osmolality?
The formula assumes ADH is acting and water is being reabsorbed downstream of the potassium-secreting segment. In dilute urine that assumption fails, and the correction factor would distort rather than fix the estimate. Valid TTKG requires concentrated urine.
What causes a low TTKG?
Aldosterone deficiency (adrenal insufficiency, hyporeninemic hypoaldosteronism in diabetes), drugs that block the pathway (ACE inhibitors, ARBs, spironolactone, trimethoprim, NSAIDs), and tubular resistance as in obstructive uropathy or interstitial disease.
What does a high TTKG with hyperkalemia mean?
The kidneys are doing their job, so the potassium is coming from elsewhere: excess intake or supplements, cell breakdown, metabolic acidosis shifting potassium out of cells, or pseudohyperkalemia from a traumatic blood draw or clenched fist.
Can TTKG diagnose adrenal insufficiency?
No, it only suggests impaired renal potassium handling that fits the pattern. Confirming adrenal insufficiency needs cortisol testing such as a cosyntropin stimulation test. TTKG is a clue that shapes the workup, not a diagnosis.
Does TTKG work in kidney failure?
Poorly. With few functioning nephrons the per-nephron dynamics change and the gradient loses meaning. It is most informative with relatively preserved kidney function and concentrated urine.
How is TTKG different from fractional excretion of potassium?
Both assess renal potassium handling, but fractional excretion needs a timed urine collection or creatinine pairing, while TTKG uses spot labs with an osmolality correction. TTKG is the quicker bedside estimate when spot values are available.
Can medications affect TTKG?
Yes, substantially. Anything altering aldosterone or distal sodium delivery changes it: diuretics, ACE inhibitors, ARBs, potassium-sparing diuretics, trimethoprim, and NSAIDs. Always read the gradient against the medication list.
Who should interpret a TTKG result?
A clinician managing the hyperkalemia. The number is an educational estimate that guides which tests come next. Hyperkalemia itself can be dangerous, so abnormal potassium always deserves prompt medical attention regardless of the gradient.