For healthcare professionals
TTKG Calculator (Transtubular Potassium Gradient)
The transtubular potassium gradient (TTKG) reflects the ability of the collecting duct to secrete potassium. It compares urine and serum potassium, adjusted for urine and serum osmolality. In a patient with a high potassium, it is used as an indirect sign of how much aldosterone is acting on the kidney tubule. This tool is written for clinicians and is not a diagnosis.
Urine and serum values
Use a spot urine sample and a blood sample taken at about the same time. Nothing you enter leaves this page.
What you will see
The TTKG with the working, whether the two validity conditions are met, and how a value under or over 7 is read in hyperkalemia.
Educational only, not medical advice. Not a diagnosis. Decisions belong to the treating team.
Source
The formula and the reading in hyperkalemia as printed in the mini review by Ertek S and Caglar K, Frontiers in Clinical Diabetes and Healthcare 2025, and the two validity conditions as printed in Kanai H and Sato H, Cureus 2026 (Table 2). Both are open access (CC BY 4.0). The tests check the formula, both conditions at their edges, the reading on each side of 7 and at exactly 7, and that a value near 7 is never rounded onto it.
Validated in
No validated cut-off for an outcome. Ertek 2025 is a narrative review of type IV renal tubular acidosis in diabetes; Kanai 2026 is a single pediatric case report. A study summarized by Alamilla-Sanchez and colleagues 2025 found the TTKG insufficient in older people with hyperkalemia.
Limits
The reading applies only to a patient with hyperkalemia, and only when urine osmolality is greater than serum osmolality and urine sodium is above 25 mmol/L (Kanai 2026); otherwise the tool shows the value but gives no reading. Sources differ on the threshold: Ertek 2025 uses 7, while Kanai 2026 gives a reference of more than 6 during hyperkalemia; the tool uses 7 and gives no reading at exactly 7. To separate aldosterone deficiency from aldosterone resistance, Ertek 2025 describes repeating the TTKG after fludrocortisone.
The formula
TTKG = (urine potassium x serum osmolality) / (serum potassium x urine osmolality) (Ertek 2025).
Worked example: urine potassium 60 mmol/L, serum osmolality 300 mOsm/kg, serum potassium 6.0 mmol/L and urine osmolality 600 mOsm/kg give (60 x 300) / (6 x 600) = 5.0.
When it can be read
The TTKG is interpretable only when urine osmolality is greater than plasma osmolality and urine sodium is above 25 mmol/L (Kanai 2026).
Reading the result
The TTKG reflects the ability of the collecting duct to secrete potassium (Ertek 2025). In the presence of hyperkalemia:
- Above 7: the expected response, reflecting compensatory hyperaldosteronism (Ertek 2025).
- Lower than 7: seen in hyperkalemic renal tubular acidosis (type IV) (Ertek 2025).
After fludrocortisone, an early return of the TTKG to normal (2 to 4 hours, physiological dose) suggests aldosterone deficiency, and a late return (over 24 hours, higher doses) suggests aldosterone resistance (Ertek 2025). In older people with hyperkalemia, one study found the TTKG insufficient (Alamilla-Sanchez 2025).
Related tools
For the acid-base side of a normal anion gap acidosis, see the urine anion gap calculator and the urine osmolal gap calculator. For acute kidney injury, the FENa calculator uses urine and serum sodium. Other bedside scores are listed under For healthcare professionals.
Sources
- Ertek S, Caglar K. Underestimated reason of hyperkalemia in diabetic patients: type IV renal tubular acidosis, mini review. Front Clin Diabetes Healthc. 2025;6:1570868. doi:10.3389/fcdhc.2025.1570868. PMID 40717832. Open access (CC BY 4.0). TTKG formula, reading in hyperkalemia and fludrocortisone response. pmc.ncbi.nlm.nih.gov/articles/PMC12289468/
- Kanai H, Sato H. Do not dismiss incidental hyperkalemia in childhood: early recognition of pseudohypoaldosteronism type II. Cureus. 2026;18:e103496. doi:10.7759/cureus.103496. PMID 41841078. Open access (CC BY 4.0). Table 2 (TTKG prerequisites: urine osmolality greater than plasma osmolality, urinary sodium over 25 mmol/L; reference more than 6 during hyperkalemia). pmc.ncbi.nlm.nih.gov/articles/PMC12989117/
- Alamilla-Sanchez M, Alcala Salgado MA, Ulloa Galvan VM, Yanez Salguero V, Yama Estrella MB, Morales Lopez EF, et al. Understanding renal tubular function: key mechanisms, clinical relevance, and comprehensive urine assessment. Pathophysiology. 2025;32(3):33. doi:10.3390/pathophysiology32030033. PMID 40700075. Open access (CC BY 4.0). Potassium section (TTKG found insufficient in older people with hyperkalemia). pmc.ncbi.nlm.nih.gov/articles/PMC12285995/
Written and maintained by the uncovermyrisk.com editorial team. We do not have a medical reviewer; every claim links to its primary source above. Last reviewed: 2026-11-02. Spotted an error? See About.