What is the optimal serum bicarbonate level for patients on dialysis (dialysis) and how can metabolic acidosis be managed in these patients?

Optimal Serum Bicarbonate Management in Dialysis Patients

For patients on maintenance dialysis, serum bicarbonate should be maintained at ≥22 mmol/L through regular monitoring and adjustment of dialysate bicarbonate concentration, with measurements obtained at least monthly. 1

Target Bicarbonate Level and Monitoring

The evidence-based target for predialysis serum bicarbonate is ≥22 mmol/L, which has been established to prevent multiple complications including bone disease, protein catabolism, and progression of metabolic derangements. 1, 2

• Serum total CO2 (bicarbonate) should be measured at least monthly in all maintenance dialysis patients. 1, 3
• Approximately 30% of hemodialysis patients have metabolic acidosis (bicarbonate <22 mmol/L) despite dialysis therapy. 4
• Patients with normal bone biopsy results have been shown to have serum bicarbonate levels around 23 mmol/L, while those with osteodystrophy have levels below 20 mmol/L. 1

Clinical Consequences of Untreated Acidosis in Dialysis

Chronic metabolic acidosis in dialysis patients causes multiple adverse outcomes that directly impact morbidity and mortality:

• Bone disease: Acidosis exaggerates bone dissolution by altering calcium-PTH-vitamin D homeostasis, contributing to renal osteodystrophy and increased fracture risk. 1
• Protein catabolism: Increased protein breakdown leads to muscle wasting, malnutrition, and negative nitrogen balance. 1, 2, 3
• Secondary hyperparathyroidism: A study of 21 dialysis patients showed that those with acidosis (total CO2 15 mmol/L) had progression of secondary hyperparathyroidism over 18 months compared to controls with total CO2 24 mmol/L. 1

Management Strategy: Dialysate Bicarbonate Adjustment

The primary method for correcting metabolic acidosis in dialysis patients is through dialysate bicarbonate concentration:

• Use "high" dialysate bicarbonate (40-42 mmol/L) as a safe, well-tolerated standard for better correction of metabolic acidosis. 3
• Acid-base homeostasis in hemodialysis depends on: net acid production, alkali delivered by dialysate, interdialytic period duration, and residual kidney function if present. 5, 6
• The amount of bicarbonate gained during dialysis is the primary determinant of predialysis bicarbonate levels in the subsequent session. 5

Key Determinants of Acidosis in Dialysis Patients

Three independent factors determine metabolic acidosis severity in hemodialysis patients:

1. Protein breakdown (increased protein nitrogen appearance increases acidosis risk, OR 1.60 per 0.2 g/kg/day). 4
2. Dialysis dose (higher Kt/V reduces acidosis risk, OR 0.61 per 0.20 increase). 4
3. Phosphate binders (calcium carbonate use reduces acidosis risk, OR 0.38 per 2 g/day, likely due to its alkalinizing effect). 4

Important Clinical Caveats

Avoid citrate-containing alkali supplements in dialysis patients exposed to aluminum salts, as citrate increases aluminum absorption both before and during dialysis, potentially worsening bone disease. 1, 2

Be cautious of post-dialysis metabolic alkalosis: Recent data suggest that excessively high bicarbonate levels after dialysis may contribute to adverse outcomes. 5, 6 However, the goal remains to maintain predialysis bicarbonate ≥22 mmol/L while avoiding excessive alkalosis.

Interpret high bicarbonate levels carefully: In dialysis patients, a high predialysis bicarbonate (>26 mmol/L) may paradoxically indicate poor nutritional status and inadequate protein intake rather than optimal acid-base control, and should prompt nutritional assessment. 7

Treatment Algorithm

1. Measure predialysis bicarbonate monthly in all maintenance dialysis patients. 1, 3
2. If bicarbonate <22 mmol/L: Increase dialysate bicarbonate concentration to 40-42 mmol/L. 3
3. Optimize dialysis adequacy: Ensure Kt/V targets are met, as inadequate dialysis dose independently contributes to acidosis. 4
4. Consider calcium carbonate as the phosphate binder of choice when appropriate, as it provides additional alkalinizing benefit. 4
5. Assess protein intake: Ensure adequate nutrition while recognizing that higher protein intake increases acid load. 4, 7
6. Avoid citrate-based supplements in patients with potential aluminum exposure. 1, 2