Acid excretion and serum electrolyte patterns in patients with advanced chronic renal failure

Hyperchloremic metabolic acidosis can be seen in advanced chronic renal failure (CRF). To study this further, we measured acid excretion under controlled conditions in 19 patients with severe but stable CRF. Twelve patients had a hyperchloremic metabolite acidosis, 3 had an elevated anion gap acidosis ('delta acidosis'), while the 4 remaining patients had a slightly decreased tCO2 with a normal anion gap and serum chloride level ('mild acidosis'). Ammonium excretion was markedly reduced in CRF patients (11 +/- 1 vs. 32 +/- 2 mEq/day in normal subjects, p less than 0.005), and likewise urinary titratable acid was diminished (18 +/- 2 mEq/day in patients, vs. 28 +/- 3 mEq/day in normal subjects; p less than 0.005). When expressed per 100 ml GFR, ammonium and titratable acid excretion were markedly increased in all groups of patients compared to normal subjects. The Tm for bicarbonate was 22.5 +/- 3.7 mEq/l for all CRF patients and 24 +/- 1 mEq/l for the patients with a hyperchloremic metabolic acidosis. We conclude that the hyperchloremic metabolic acidosis in advanced renal failure is due to diminished excretion of ammonium and titratable acid and is not due to an increased bicarbonate leak. As renal failure advances and renal mass declines, the remaining functioning nephrons hypersecrete acid.