Introduction
The Transtubular Potassium Gradient (TTKG) Calculator checks how well the kidneys are handling potassium. It compares the potassium in urine to the potassium in blood, then adjusts for how much water the kidney pulled back. The result is one simple number.
Doctors use TTKG when a patient has high potassium (hyperkalemia) or low potassium (hypokalemia). The number helps show if the problem comes from the kidneys or from somewhere else in the body, like the gut or the diet.
To use it, enter four lab values: urine potassium, serum potassium, urine osmolality, and serum osmolality. You can pick the units you have on your lab report. The result and every calculation step appear right away.
A TTKG of about 8 to 9 is typical on a regular diet. The expected value depends on the blood potassium level, so the result is always read against it.1 The calculator marks where your number falls, explains what it may mean, and plots it on two charts. It also reminds you that TTKG only works when urine sodium is above 25 mEq/L and urine osmolality is at least as high as blood osmolality.
This tool is for learning and clinical support. It does not replace advice from a doctor.
How to use our TTKG Calculator
Enter four lab values (urine potassium, serum potassium, urine osmolality, and serum osmolality), and the TTKG Calculator gives you your transtubular potassium gradient, a step-by-step solution, and what the number means for kidney potassium handling.
Urine Potassium (UK): Type the potassium level from your spot urine test. Pick mEq/L or mmol/L in the dropdown next to the box.
Serum Potassium (PK): Type the potassium level from your blood test. The normal range is 3.7 to 5.2.4 Choose mEq/L or mmol/L.
Urine Osmolality (Uosm): Type the measured urine osmolality. In a random sample, normal values are 50 to 1,200 mOsm/kg.5 Choose mOsm/kg, mOsm/L, or mOsm/dL.
Serum Osmolality (Posm): Type the measured blood osmolality. Normal is about 275 to 295.6 Choose mOsm/kg, mOsm/L, or mOsm/dL.
Urine Sodium (UNa) > 25 mEq/L? Pick "Yes" or "No". TTKG only works when enough sodium reaches the distal nephron. If you pick "No", the tool warns you that the result may not be valid.
Decimal Places: Choose 0 to 3 decimals for the TTKG result. The number updates right away.
Click Calculate to see your TTKG, the potassium ratio, the osmolality ratio, and two charts showing where your value falls. Click Reset to go back to the sample values.
What Is the Transtubular Potassium Gradient (TTKG)?
The transtubular potassium gradient, or TTKG, is a simple number that shows how well your kidneys are getting rid of potassium. Potassium is a mineral that helps your heart beat and your muscles work. Too much or too little can be dangerous.
TTKG looks at one spot in the kidney called the cortical collecting duct.2 This is where a hormone named aldosterone tells the kidney to dump potassium into the urine. The TTKG number helps doctors guess if aldosterone is doing its job.
What the TTKG Formula Uses
The TTKG needs four lab values taken at the same time:
- Urine potassium (UK): potassium in a spot urine sample
- Serum potassium (PK): potassium in the blood (normal is 3.7 to 5.2 mEq/L)4
- Urine osmolality (Uosm): how packed the urine is with particles
- Serum osmolality (Posm): how packed the blood is with particles
The potassium ratio alone can trick you, because the kidney pulls water back out of the urine and makes the potassium look stronger than it really is. Dividing by the osmolality ratio cancels out that water effect. The final answer has no units. It is just a ratio.
Normal TTKG Values
A TTKG of about 8 to 9 is typical on a regular diet. What counts as "good" or "bad" depends on the blood potassium level.1
- High blood potassium (hyperkalemia): TTKG above 10 means the kidney is working hard and doing the right thing. In healthy people given a potassium drink, TTKG rose to about 13.1.1 A TTKG under 6 points to a kidney or aldosterone problem.2
- Low blood potassium (hypokalemia): A TTKG of 2 or less means the kidney is saving potassium, so the loss is likely from the gut, such as vomiting or diarrhea. A TTKG above 2 during low potassium points to the kidney losing potassium.2 In one study, TTKG was about 0.9 when low potassium came from outside the kidney and about 6.7 when it came from too much aldosterone.1
When TTKG Does Not Work
TTKG is only useful if two rules are met:
- Urine sodium must be above 25 mEq/L. If not enough sodium reaches that part of the kidney, potassium cannot be swapped out, and the number means nothing.
- Urine osmolality must be equal to or higher than blood osmolality. Watery, dilute urine breaks the math.
Two of the doctors who helped create the TTKG later wrote that its main assumption, that little is reabsorbed after the cortical collecting duct, is not valid, because a large amount of urea is reabsorbed further down.3 Many clinics now prefer the urine potassium-to-creatinine ratio. TTKG is still taught and used as a quick clue, but it is one piece of the puzzle, never the whole answer.
Why It Matters
Potassium problems can cause weak muscles, cramps, and unsafe heart rhythms. Finding the cause fast changes the treatment. TTKG helps sort a kidney cause from a non-kidney cause in minutes, using labs most hospitals already run. Always go over your results with a doctor or kidney specialist.