Transtubular potassium gradient (TTKG)

Background Info

Transtubular potassium gradient (TTKG) -
Key points:

• The TTKG reflects the conservation of potassium in the cortical collecting ducts (CCD) of the kidneys.  It also estimates the ratio of potassium in the lumen of the CCD to that in the peritubular capillaries  which may be useful in diagnosing the causes of hyperkalemia or hypokalemia.
  
  
• TTKG values  less than 6 suggests a renal cause of hyperkalemia (aldosterone deficiency or lack of response).
  
  
• TTKG values greater than 6 suggests an extrarenal cause of hyperkalemia such as increased potassium intake, hemolysis, acidosis, or rhabdomyolysis.
  
  
• If TTKG is less than 6 with  a normal serum aldosterone --> possible renal tubular deficit.  Other scenarios:  low aldosterone, nml serum renin --> hypoaldosteronism.  Low aldosterone and low serum renin --> hyporeninemic hypoaldosteronism.
  
  
• Typical TTKG in a normal person on a normal diet is 8-9.
  
  
• During hyperkalemia or high potassium intake, more potassium should be excreted in the urine and the TTKG should be above 10.
  
  
• Low levels (<7) during hyperkalemia may indicate mineralocorticoid deficiency, especially if accompanied by hyponatremia and high urine Na.
  
  
• During potassium depletion or hypokalemia, the TTKG should fall to less than 3, indicating appropriately reduced urinary excretion of potassium.
  
  
• TTKG > 2 and hypokalemia --> renal loss.
  
  
• Useful in gauging the correctness of renal K excretion during disorders in serum potassium
  
  
•  Index of potassium secretory activity in the distal tubule  (gauges renal potassium secretion by the cortical collecting duct (CCD), indirectly assessing mineralocorticoid activity in patients who have elevated or low potassium levels).
  

References

1. Choi MJ, Ziyadeh FN. The utility of the transtubular potassium gradient in the evaluation of hyperkalemia.  J Am Soc Nephrol 19: 424–426, 2008.   PMID 18216310.
   
   
2. Ethier JH, Kamel KS, Magner PO, Lemann J, Halperin ML (April 1990). "The transtubular potassium concentration in patients with hypokalemia and hyperkalemia". Am. J. Kidney Dis. 15 (4): 309–15,1990.  PMID 2321642
   
   
3. West ML, Marsden PA, Richardson RM, et. al., New clinical approach to evaluate disorders of potassium excretion, Mineral Electrolyt Metab, 12:234,1986., PubMed ID number 3762510