Urine anion gap in patients with chronic renal insufficiency

We investigated the urinary acid excretion and urine anion gap (AG) (Na+ + K(+)-Cl-) during NH4Cl-induced metabolic acidosis in 38 normal subjects and 53 patients with chronic renal diseases in order to clarify the significance of the urine AG as a useful marker of the ammonium (NH4+) excretion even in a state of chronic renal insufficiency. The urine pH became higher, and the urinary excretions of titratable acid (TA) and NH4+ decreased significantly, in parallel with a reduction of the creatinine clearance (Ccr). The urinary electrolyte excretion, especially the chloride excretion, also decreased significantly as Ccr fell. As a result, the urine AG increased from negative to positive values, in proportion to the decrease in Ccr with statistical significance. The urine AG showed the most significant correlation with the urine NH4+ excretion (r = -0.707, p less than 0.001). We conclude that the urine AG provides a significant marker of the urine NH4+ excretion even in a state of moderate to severe renal insufficiency.