Clinical Usefulness of Transtubular Potassium Gradient(TTKG) and Urine Ammonium in Differential Diagnosis of Hypokalemia

Han, Jin Suk; Chang, Se Ho; Lee, Jung Sang

Abstract

Hypokalemia is frequently encountered in clinical medicine, the cause of which can usually be determined from the history such as with diuretic use, vomiting, or diarrhea. And measurement of urinary indices such as excretory rate of K+, random urine K+ and fractional excretion of K+ and assessment of acid-base balance have been applied to the pathophysiologic diagnosis without fruitful success. To investigate the clinical usefulness of TTKG and urine ammonium in differential diagnosis of hypokalemia, we evaluated serum electrolytes and osmolality, random(spot) urine electrolytes, osmolality and ammonium, total CO2, UNa/K, plasma aldosterone and TTKG in 7 patients with diarrhea, 6 patients with vomiting, 7 patients with 3 primary hyperaldosteronism and 4 renovascular hypertension, 6 patients with diuretic uses. With the comparison to 7 overnight fasting and acid-loaded normal controls, we obtained the following results. 1) Random measurement of the urine potassium concentration did not accurately reflect potassium wasting if the urine became concentrated. So clinical usefulness of random urine potassium concentration was limited. 2) UNa/K was useful in the diagnosis of mineralocorticoid excess such as primary hyperaldosteronism and renovascular hypertension rather than the other causes of hypokalemia. 3) TTKG was very useful in differential diagnosis of hypokalemia between potassium losses through the gastrointestinal tract (below 2) and kidney (above 10), and it had a good correlation with the aldosterone activity in the hypokalemic patients due to renal loss. 4) Random urine ammonium was a useful index in differential diagnosis of hypokalemia with acid-base disturbances. In conclusion, TTKG and random urine ammonium were very useful indices in differential diagnosis of the causes of hypokalemia.