Transtubular Potassium Gradient (TTKG)
TTKG estimates potassium concentration in the cortical collecting duct, helping differentiate renal vs extrarenal causes of potassium disorders.
Why This Health Metric Matters
Why: TTKG differentiates hypoaldosteronism from extrarenal causes in hyperkalemia, and renal wasting from GI losses in hypokalemia.
How: TTKG = (Urine K / Serum K) ÷ (Urine Osm / Serum Osm). Urine osmolality must exceed serum for valid result.
- ●HyperK: TTKG >10 = appropriate; <6 = renal cause
- ●HypoK: TTKG <3 = appropriate; >4 = renal wasting
- ●Urine Osm >300 mOsm/kg ideal for accuracy
🧪 Sample Clinical Scenarios — Click to Load
Laboratory Values
Clinical Context
⚠️For informational purposes only — not medical advice. Consult a healthcare professional before acting on results.
🏥 Health Facts
TTKG reflects aldosterone-mediated K+ secretion in cortical collecting duct
— Nephrology
Osmolality correction accounts for water reabsorption concentrating urine K+
— Physiology
📋 Key Takeaways
- • TTKG estimates potassium concentration in the cortical collecting duct
- • Hyperkalemia: TTKG >10 = appropriate renal response; <6 = renal cause
- • Hypokalemia: TTKG <3 = appropriate conservation; >4 = renal wasting
- • Urine osmolality must exceed serum osmolality for valid TTKG
- • Diuretics significantly affect TTKG interpretation
💡 Did You Know?
📖 How It Works
TTKG estimates the potassium gradient in the cortical collecting duct, correcting for medullary water reabsorption.
Step 1: Collect Simultaneous Samples
Serum and spot urine for K+ and osmolality. Urine Osm must exceed serum Osm.
Step 2: Calculate Osmolality Ratio
Corrects for water reabsorption in medullary collecting duct.
Step 3: Apply TTKG Formula
TTKG = (UK/SerumK) ÷ (UrineOsm/SerumOsm)
🎯 Expert Tips
Validity Check
Always verify urine Osm > serum Osm before interpreting TTKG.
Sample Timing
Obtain serum and urine samples at the same time.
Medication Effects
Consider ACEi, ARBs, K-sparing diuretics when interpreting.
Follow-up
Aldosterone/renin testing if mineralocorticoid disorder suspected.
⚖️ TTKG vs Alternatives
| Tool | Renal vs Extrarenal | Spot Urine |
|---|---|---|
| TTKG | ✅ | ✅ |
| 24h urine K+ | ✅ | ❌ |
| FENa | Prerenal vs ATN | ✅ |
❓ FAQ
When is TTKG not valid?
When urine osmolality < serum osmolality (dilute urine). Ideally urine Osm >300 mOsm/kg.
What causes low TTKG in hyperkalemia?
Hypoaldosteronism, type 4 RTA, ACEi/ARBs, K-sparing diuretics, distal nephron dysfunction.
What causes high TTKG in hypokalemia?
Hyperaldosteronism, diuretics, Bartter/Gitelman, RTA types 1 and 2, hypomagnesemia.
How do diuretics affect TTKG?
Loop and thiazide increase TTKG; potassium-sparing decrease it.
Can TTKG diagnose hyperaldosteronism?
TTKG suggests it but is not diagnostic. Elevated TTKG in hypokalemia with HTN warrants aldosterone-to-renin ratio.
📊 Reference Ranges
⚠️ Disclaimer: This calculator is for educational purposes only. TTKG has limitations. Always consult a qualified healthcare provider.