Transtubular potassium concentration gradient (TTKG) and urine ammonium indifferential diagnosis of hypokalemia

Background: Hypokalemia is a common and sometimes serious clinical problem, whose etiological diagnosis can frequently be based on the patient's histo ry and the clinical setting. Measurement of urinary indices such as excreto ry rate of K+, random urine K+ concentrations and blood acid-base parameter s have been employed in the pathophysiological diagnosis, though with some pitfalls. Methods: To investigate the diagnostic usefulness of the transtubular potas sium concentration gradient (TTKG) and urine ammonium in the differentiatio n of hypokalemia, we measured serum K+ and osmolality, random urine electro lytes, osmolality and ammonium, the urinary [Na]/[K] ratio (U-Na/K), plasma aldosterone and TTKG in 7 patients with diarrhea, 6 with vomiting, 7 with mineralocorticoid excess, 6 with diuretic usage, and compared them with tho se of 7 overnight fasted and acid-loaded healthy volunteers. Results: The urine K+ concentrations did not reflect urinary loss of potass ium according to the subjects hydration status. U-Na/k in the hypokalemic p atients with mineralocorticoid excess (1.4 +/- 0.5) was lower than in norma l subjects (2.3 +/- 0.4) (p < 0,05). TTKG was higher in hypokalemic patient s with mineralocorticoid excess (13.3 +/- 4.4) and diuretic usage (8.6 +/- 1.3) and lower in those with diarrhea (1.6 +/- 0.3) than in the normal cont rols (5.0 +/- 0.7) (p < 0.5). TTKG in the patients with vomiting (3.5 +/- 0 .6) was the same as in normal controls. TTKG was stronger correlated with t he plasma aldosterone levels in the hypokalemic patients due to renal potas sium loss. Urine ammonium concentrations of the acid-loaded normal subjects (73.3 +/- 5.0 mEq/L), patients with diarrhea (74.4 +/- 2.0 mEq/L) and pati ents with mineralocorticoid excess (68.7 +/- 6.9 mEq/L) were higher than in overnight-fasted normal subjects (31.3 +/- 4.9 mEq/L). Conclusion: TTKG and random urine ammonium were useful in the pathophysiolo gical differential diagnosis of hypokalemia.