Metabolic acidosis in advanced renal failure: differences between diabetic and nondiabetic patients

Metabolic acidosis is almost invariably a consequence of advanced renal failure, although its severity can vary widely. To evaluate the determinants of the severity of metabolic acidosis, with special interest in determining if there is any difference in the prevalence and severity of metabolic acidosis between patients with and without diabetes, 113 predialysis patients with renal failure were studied. Criteria for inclusion onto the study were: creatinine clearance (Ccr)/1.73 m2 less than 30 mL/min, no alkali therapy within the previous 30 days, and the absence of respiratory diseases. Forty-eight patients had diabetes (33 patients with diabetic nephropathy). The following data were analyzed: demographics; cause of renal failure; hematocrit; serum urea, creatinine, uric acid, albumin, glucose, hemoglobin A1c, bicarbonate, sodium, potassium, chloride, calcium, phosphorus, and alkaline phosphatase levels; anion gap; urinary protein excretion; Ccr/1.73 m2; half of the sum of creatinine and urea clearances (Ccr-Cu); protein-equivalent nitrogen appearance (PNA); and whether the patients received diuretics (75 patients), angiotensin-converting enzyme inhibitors (54 patients), and/or calcium channel blockers (55 patients). After the exclusion of eight patients because of hypochloremia (three patients with and five patients without diabetes), mean serum bicarbonate levels were significantly greater in patients with diabetes than in the rest of the patients (20.7 +/- 2.3 v 18.2 +/- 2. 3 mmol/L; P = 0.0001). The mean anion gap (mmol/L) was also significantly less in patients with than without diabetes (19.70 +/- 3.65 v 22.35 +/- 3.64; P = 0.003). Eleven of 105 patients had serum bicarbonate levels of 23 mmol/L or greater (9 patients with and 2 patients without diabetes). Pure elevated anion gap followed by mixed (high anion gap and hyperchloremia) were the most common types of metabolic acidosis observed in both groups. There were no differences in PNA, diuretic treatment, or vomiting history between patients with and without diabetes. By multiple logistic regression analysis, the best determinants for a serum bicarbonate level greater than 19 mmol/L were: the diagnosis of diabetic nephropathy (odds ratio, 0.107; P = 0.0002), Ccr-Cu (odds ratio, 0.824; P = 0. 014), and age (odds ratio, 0.966; P = 0.046). In conclusion, patients with diabetes with advanced renal failure showed a less severe metabolic acidosis, which cannot be explained by gastrointestinal hydrogen ion losses, drugs, or reduced protein catabolic rate. Patients with diabetes may have a more efficient extrarenal generation of bicarbonate than end-stage renal failure patients without diabetes.