bjectives : The clinical usefulness of the urine anion gap as anndirect index of theammonium excretion was reported only in patients with hyperchloremic metabolicacidosis.
And the significance of the urine anion gap and its relation with other indices of urine
acidification are not clearly defined yet. We are aimed to evaluate the clinical usefulness of
the urine anion gap and its relations with other indices of urine acidification in normal and
high anion gap metabolic acidosis.
Methods : We measured the urine acidification indices (ammonium, titratable acid, net acid)
and the urine anion gap in 24 patients with chronic renal failure, 7 patients with distal
renaltubular acidosis, and 8 healthy adults with acid loading (normal controls), whose arterial
blood bicarbonate concentrations were 14.0 mmol/L (7.0 to 20.0 mmol/L)(median;range), 18.1
mmol/L(15.0 to 20.0 mmol/L), 19.5 mmol/L(16.8 to 22.0 mmol/L), respectively.
Results : The urinary excretion of ammonium in chronic renal failure(4.5 mmol/day; 1.6 to
11.8 mmol/day) and renal tubular acidosis (19.8 mmol/day; 6.9 to 27.2 mmol/day) were lower
than in normal controls (52.5 mmol/day; 37.3 to 69.4 mmol/day)(p<0.05). The urinary excretion
of titratable acid in chronic renal failure (4.9 mmol/day; 0.1 to 19.7 mmol/day) and renal
tubular acidosisi (2.8 mmol/day; 0.1 to 20.2 mmol/day) were lower than in normal controls
(6.2 mmol/day; 20.6 to 36.9 mmol/day)(p<0.05). The urinary excretion of net acid in chronic
renal failture(8.8 mmol/day; 0.1 to 28.2 mmol/day) and renal tubular acidosis (12.9 mmol/day;
0.1 to 33.6 mmol/day) were also lower than in normal controls (77.9 mmol/day; 62.7 to 98.9
mmol/day)(p<0.05). The urine anion gap in chronic renal failure (22.9 mmol/L; 13.0 to 43.2
mmol/L) and renal tubular acidosis (36.0 mmol/L; 7.0 to 82.0 mmol/L) were higher than in
normal controls (-14.6 mmol/L ; -40.7 to 2.2 mmol/L)(p<0.05), and had inverse relation
with urine ammonium (r=-0.71, p<0.01), titratable acid (r=-0.76, p<0.01), and had inverse
relation with urine ammomnium (r=-0.83, p<0.01), repectively. The urine anion gap in chronic
renal failure and renal tubular acidosis were all above 5.0 mmol/L.
Conclusion : We concluded that the urine anion gap in chronic metabolic acidosis would be
a good clinical index of the impairment of urine acidification in the distal nephron, and
reflect not only urine ammonium excretion but also urinary excretion of net acid.
|