The Use of the Urinary Anion Gap in the Diagnosis of Hyperchloremic Metabolic Acidosis

We evaluated the use of the urinary anion gap (sodium plus potassium minus chloride) in assessing hyperchloremic metabolic acidosis in 38 patients with altered distal urinary acidification and in 8 patients with diarrhea. In seven normal subjects given ammonium chloride for three days, the anion gap was negative (-27±9.8 mmol per liter) and the urinary pH under 5.3 (4.9±0.03). In the eight patients with diarrhea the anion gap was also negative (-20±5.7 mmol per liter), even though the urinary pH was above 5.3 (5.64±0.14). In contrast, the anion gap was positive in all patients with altered urinary acidification, who were classified as having classic renal tubular acidosis (23±4.1 mmol per liter, 11 patients), hyperkalemic distal renal tubular acidosis (30±4.2, 12 patients), or selective aldosterone deficiency (39±4.2, 15 patients). When the data on all subjects studied were pooled, a negative correlation was found between the urinary ammonium level and the urinary anion gap. We conclude that the use of the urinary anion gap, as a rough index of urinary ammonium, may be helpful in the initial evaluation of hyperchloremic metabolic acidosis. A negative anion gap suggests gastrointestinal loss of bicarbonate, whereas a positive anion gap suggests the presence of altered distal urinary acidification. (N Engl J Med 1988; 318:594–9.)