TTKG Calculator - Renal Potassium Gradient With Bedside Interpretation

A TTKG calculator (transtubular potassium gradient) turns four lab values - serum potassium, urine potassium, serum osmolality, and urine osmolality - into the gradient the kidney is generating for potassium at the distal tubule, paired with a bedside interpretation band.

• • Working up unexplained hyperkalemia: Tell whether a high serum potassium is from a renal excretion problem (low TTKG) or an external shift, and look for hypoaldosteronism or type 4 RTA.
• • Sorting out hypokalemia: Confirm that a low serum potassium is being handled by the kidney appropriately (TTKG under 3) or that the kidney is wasting potassium (TTKG above 3, suggesting Gitelman, Bartter, or diuretic use).
• • Teaching the formula at the bedside: Show trainees where the water-reabsorption ratio comes from, why urine osmolality must be at least 300 mOsm/kg H2O, and how the same numbers change the interpretation band.

Transtubular potassium gradient is the bedside workaround when fractional excretion is the wrong tool. The standard equation treats filtered and excreted substance as a ratio, but almost all potassium filtered at the glomerulus is reabsorbed upstream, so urine potassium reflects what the distal nephron secreted.

A TTKG calculator normalizes that distal secretion for the water the tubule has already pulled out, returning a single unitless number that answers: is the kidney responding to the serum potassium the way it should?

When fractional excretion of sodium is the better answer - for example, in oliguria or to separate pre-renal from intrinsic acute kidney injury - the FENa Calculator covers that side of the renal workup, while TTKG is reserved for potassium.

The TTKG calculator reads four laboratory values from the same sample window, builds the two ratios, divides one by the other, and returns the transtubular potassium gradient. The interpretation band is paired with the serum potassium range to give a bedside read.

• Serum K+: Serum or plasma potassium in mmol/L. Sets the denominator of the potassium ratio and the band (hypo-, normo-, or hyperkalemia) for interpretation.
• Urine K+: Urine potassium in mmol/L on a spot or timed sample drawn at the same time as the serum sample.
• Serum osmolality: Serum osmolality in mOsm/kg H2O. Used to estimate how much water the collecting duct has reabsorbed.
• Urine osmolality: Urine osmolality in mOsm/kg H2O. Must be at least 300 mOsm/kg H2O; below that, ADH activity is too variable.

The osmolality ratio is the part most people skip. It corrects for the fact that urine potassium gets concentrated as water is reabsorbed under antidiuretic hormone, so 35 mmol/L on a dilute sample represents less distal secretion than the same 35 mmol/L on a concentrated sample.

That is why the formula is unreliable when urine osmolality drops below 300 mOsm/kg H2O: ADH activity is too low and the calculated gradient no longer matches the true gradient.

According to JASN - The utility of the transtubular potassium gradient in the evaluation of hyperkalemia (Choi & Ziyadeh, 2008), the transtubular potassium gradient is urine potassium divided by serum potassium divided by urine osmolality divided by serum osmolality, and the result is only valid when urine osmolality is at least 300 mOsm/kg H2O and urine sodium is at least 25 mmol/L.

If you only have the lab's osmolality and want to double-check it against the sodium, glucose, and BUN inputs, the Plasma Osmolality Calculator returns that side of the math in a single screen.

Four ideas make TTKG click: renal anatomy, the math correction, and two bedside preconditions that decide whether to trust the number.

TTKG was developed for potassium specifically because fractional excretion is the wrong ratio for a substance that is mostly reabsorbed before the distal nephron.

At the bedside, the calculator is a tie-breaker when the serum potassium and the clinical picture disagree, and the answer changes which medications, fluids, or workups come next.

When the patient is already on diuretics and fractional excretion of sodium stops being reliable, the FEUrea Calculator reroutes the same pre-renal versus intrinsic AKI question through urea.

Run the inputs in the order a real lab slip arrives - serum potassium, urine potassium, then the two osmolalities - and use the same sample window for all four values.

1. 1 Enter the serum potassium: Type the serum or plasma potassium in mmol/L from the chemistry panel; mEq/L is the same unit for potassium.
2. 2 Enter the urine potassium: Use the urine potassium from a spot or timed collection drawn at the same time as the serum sample.
3. 3 Add serum osmolality: Enter the serum osmolality in mOsm/kg H2O. The lab's value is fine, or compute it from sodium, glucose, and BUN if needed.
4. 4 Add urine osmolality: Enter the urine osmolality in mOsm/kg H2O. A value below 300 triggers a low-confidence warning on the result.
5. 5 Read the gradient and interpretation: Read the TTKG as a unitless gradient and pair it with the bedside band to decide whether the kidney is part of the problem.

A 64-year-old on lisinopril and spironolactone comes in with serum potassium 5.6 mmol/L. The lab returns urine potassium 40 mmol/L, serum osmolality 290 mOsm/kg, and urine osmolality 600 mOsm/kg. The potassium ratio is 7.14, the osmolality ratio is 2.07, and the TTKG is about 3.45. The interpretation flags an inappropriately low gradient and points the team toward aldosterone activity, type 4 RTA, or medication effects.

Pair the gradient with a current estimate of kidney function, since reduced GFR shifts the TTKG interpretation toward the lower end of each band. The GFR Calculator returns that estimate from serum creatinine, age, and sex.

The TTKG calculator pays off when the serum potassium and the clinical picture disagree, when medications confound other tests, or when a trainee needs to see why fractional excretion is the wrong tool for potassium.

• • Surfaces a renal cause hidden behind a normal-looking value: A TTKG that is too low for the patient's serum potassium band is the earliest bedside signal that aldosterone activity or distal sodium delivery is off.
• • Saves a 24-hour urine collection: A spot urine potassium with a concurrent serum sample gives a usable TTKG in minutes.
• • Works alongside diuretic exposure: Unlike fractional excretion of sodium, TTKG stays useful when a patient is on loop or thiazide diuretics.
• • Pairs cleanly with the rest of the kidney panel: The same lab slip that returns serum creatinine, BUN, sodium, and urine sodium covers several kidney calculators in this category.

The calculator supports a clinical workup, it does not replace one. The TTKG is one of several pieces - serum potassium trend, acid-base status, medications, adrenal function - to look at together before any treatment plan.

When the workup needs an estimate of filtering capacity, the Creatinine Clearance Calculator uses a 24-hour urine creatinine or the Cockcroft-Gault equation to give that number alongside the TTKG result.

Three things change the TTKG for the same patient; the two caveats below are the limits that should always travel with the result.

• • TTKG is a ratio of two concentrations, not a direct measurement of distal potassium secretion. A very low urine flow rate can change the relationship between the calculated and the true gradient.
• • The bedside cutoffs are population averages, not individual thresholds. A borderline result should be read with the serum potassium trend, the acid-base picture, and the medications before treatment is changed.

In practice, dilute urine and a low urine sodium are the two most common reasons for a misleading TTKG, both ruled out from the same urinalysis that supplied the urine potassium.

According to Merck Manual Professional Edition - Hypokalemia, hypokalemia is a serum potassium below 3.5 mEq/L (3.5 mmol/L) caused by a deficit in total body potassium stores or by abnormal movement of potassium into cells, and once intracellular shifts are excluded, a 24-hour urine potassium above 15 mEq/L points to a renal cause of loss.

According to National Kidney Foundation - Potassium, the kidneys help keep the right amount of potassium in the body, the safe serum potassium zone is 3.5 to 5.0 mmol/L, and a sudden or severe shift in either direction is a life-threatening condition that requires immediate medical care.

Acid-base status shifts potassium between the cells and the serum, so the Anion Gap Calculator belongs next to TTKG whenever a metabolic acidosis or alkalosis is in the background.