Calculate the transtubular potassium gradient (TTKG) as a renal-potassium-handling worksheet for hyperkalemia or hypokalemia review, with validity checks and limitation notes.
The transtubular potassium gradient (TTKG) is a traditional bedside estimate of distal potassium handling. It uses serum and urine potassium together with osmolality to ask whether the kidney is appearing to conserve potassium or secrete it in the expected direction for the clinical scenario.
The index is most useful as a structured review aid rather than as a stand-alone answer. It has practical limitations, including its dependence on concentrated urine and adequate distal sodium delivery, and many nephrology workflows now pair it with spot urine potassium indices or 24-hour urine data when available.
This calculator computes the TTKG, checks the usual validity prerequisites, and keeps the hyperkalemia and hypokalemia interpretation bands visible as a worksheet for broader electrolyte review.
TTKG can be helpful when you want a quick, structured look at whether the kidney appears to be responding to a potassium disorder in the expected direction. The calculation is fast, but it works best as part of the larger electrolyte picture rather than as a single decisive test.
TTKG = (U_K / S_K) / (U_Osm / S_Osm) = (U_K × S_Osm) / (S_K × U_Osm) Validity: requires U_Osm > S_Osm and U_Na > 25 mEq/L Hyperkalemia: TTKG < 5 = impaired excretion; > 7 = appropriate Hypokalemia: TTKG < 2 = appropriate conservation; > 3 = renal wasting
Result: TTKG = 2.3 — Impaired renal K⁺ excretion
TTKG = (25 × 290) / (6.2 × 500) = 7250 / 3100 = 2.3. In the setting of hyperkalemia (K⁺ 6.2), a TTKG of 2.3 is inappropriately low (expected > 7). This indicates the kidneys are not adequately secreting potassium, suggesting hypoaldosteronism, K⁺-sparing diuretic effect, or tubular defect.
TTKG uses the urine-to-serum potassium relationship, adjusted by osmolality, to approximate whether distal potassium secretion is low, appropriate, or excessive for the scenario. In practice it is mainly a quick organizing tool for potassium-disorder review.
The calculation depends on concentrated urine and adequate distal sodium delivery. If the urine is dilute or distal sodium delivery is very low, the TTKG can look reassuring or abnormal for the wrong reason. That is why the page surfaces the prerequisite checks before leaning on the interpretation bands.
Borderline values should usually push the review back toward the full potassium workup: medication list, acid-base status, kidney function, blood pressure pattern, renin-aldosterone data when relevant, and direct urine potassium measurements. The TTKG is most useful when it supports that broader pattern rather than when it is asked to settle the question by itself.
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This calculator computes TTKG as (urine potassium × serum osmolality) / (serum potassium × urine osmolality). It also surfaces the common validity checks by reminding the user that concentrated urine and adequate distal sodium delivery are needed before the number is interpreted with confidence.
The page keeps the hyperkalemia and hypokalemia cutoffs visible as worksheet context, but it does not treat the index as a stand-alone diagnostic or treatment rule.
The TTKG requires concentrated urine (U_Osm > S_Osm) and adequate sodium delivery to the distal nephron (U_Na > 25 mEq/L). In the setting of water diuresis (dilute urine) or very low sodium intake, the TTKG is unreliable. Some nephrologists argue the TTKG has fundamental limitations and prefer 24-hour urine K⁺ or urine K/creatinine ratio.
Type 4 renal tubular acidosis (hypoaldosteronism, very common in diabetic nephropathy), ACE inhibitors, ARBs, K⁺-sparing diuretics (spironolactone, amiloride, triamterene), TMP-SMX, heparin, calcineurin inhibitors, and adrenal insufficiency. Use this as a practical reminder before finalizing the result.
Indirectly. In hypokalemia with a TTKG > 3, the kidney is wasting potassium, which can be from excess aldosterone. However, the aldosterone-to-renin ratio (ARR) is the specific screening test. TTKG distinguishes renal from GI causes but does not specify the renal mechanism.
Borderline values are common in chronic kidney disease where nephron mass reduction impairs K⁺ excretion even with adequate aldosterone. Consider checking an aldosterone level and evaluating GFR. A fludrocortisone trial may help differentiate aldosterone deficiency from tubular resistance.
Some researchers (including the original authors) have questioned whether the TTKG accurately reflects collecting duct K⁺ secretion because urea recycling in the medulla can alter the osmolality correction. Despite this, TTKG remains widely used in clinical practice because no simple alternative exists for bedside assessment of renal K⁺ handling.
A 24-hour urine potassium collection is more robust: > 30 mEq/day in hypokalemia indicates renal wasting. A spot urine K⁺/creatinine ratio > 15 mEq/g also suggests renal loss. These avoid the osmolality assumptions inherent in the TTKG calculation.