Wednesday May 10, 2006
The TransTubular Potassium Gradient - TTKG
TTKG is an index reflecting conservation of potassium by Kidney.
TTKG = (Urine-K / Plasma-K) / (Urine-Osm / Plasma-Osm)
Normal value is 8-9 in normokalemic patient and should be 10-11 in hyperkalemic patient.
Clinical significance:
1. In hyperkalemia if TTKG remains less than 7, most probable cause is hypoaldosteronism. Diagnosis can be confirmed by challenging patient with 0.05 mg of fludrocortisone, which should increase the TTKG. (Mineralcorticoid increases TTKG).
2. In hypokalemia - kidney try to conserve potassium and TTKG should fall to less than 2 (like in GI source). If TTKG remains higher, it suggests renal loss of potassium (like in diuretics).
Limitations:
1. Always check urine sodium simultaneouly. TTKG is unreliable if urine sodium is less than 25 mmol/l.
2. TTKG is unreliable if urine osmolality is less than the serum osmolality.
References: click to get abstracts/articles
1.Potassium excretion indices in the diagnostic approach to hypokalaemia - Q J Med 2000; 93: 318-319
2. Transtubular potassium concentration gradient (TTKG) and urine ammonium in differential diagnosis of hypokalemia - J Nephrol. 2000 Mar =Apr;13(2):120-5
3. Diseases of Potassium metabolism: Atlas of disease of the kidney by Robert W. Schrier, Professor and Chairman, Department of Medicine, University of Colorado School of Medicine, Denver, Colorado
