What dialysate bicarbonate concentration should be set on a hemodialysis machine for an adult chronic hemodialysis patient with a pre‑dialysis serum bicarbonate of 18 mmol/L?

Dialysate Bicarbonate Setting for Serum CO2 of 18 mmol/L

For a hemodialysis patient with a pre-dialysis serum bicarbonate (CO2) of 18 mmol/L, set the dialysate bicarbonate concentration to 35-38 mmol/L to correct the metabolic acidosis and achieve a target pre-dialysis bicarbonate of ≥22 mmol/L. 1, 2

Understanding the Clinical Context

A serum CO2 (bicarbonate) of 18 mmol/L represents metabolic acidosis that requires pharmacological intervention in chronic kidney disease and hemodialysis patients. 1 This level falls below the critical threshold of 22 mmol/L, where treatment is strongly recommended to prevent:

• Protein catabolism and muscle wasting 1
• Bone demineralization and renal osteodystrophy 1
• CKD progression 1
• Increased mortality risk 3

Dialysate Bicarbonate Prescription Algorithm

Step 1: Calculate Required Dialysate Bicarbonate

For a pre-dialysis bicarbonate of 18 mmol/L, start with a dialysate bicarbonate of 35-38 mmol/L. 4 The specific concentration depends on:

• Acid generation rate between treatments (higher protein intake requires higher dialysate bicarbonate) 4
• Dialyzer bicarbonate dialysance (efficiency of bicarbonate transfer) 4
• Treatment time and blood flow rate (longer sessions and higher blood flow allow lower dialysate concentrations) 5, 4

Step 2: Target Bicarbonate Levels

Your treatment goals should be:

• Pre-dialysis bicarbonate: 19-25 mmol/L (ideally ≥22 mmol/L) 1, 6, 7
• Post-dialysis bicarbonate: ≤29 mmol/L (to avoid post-dialysis alkalosis) 6, 7
• Mid-dialysis bicarbonate: approximately 25 mmol/L 4

Step 3: Initial Prescription

Begin with dialysate bicarbonate of 35-37 mmol/L for this patient. 4, 7 This higher concentration is necessary because:

• The patient's baseline bicarbonate (18 mmol/L) is significantly below target 1
• A gradient of 17-19 mmol/L (35-37 dialysate minus 18 serum) is needed to achieve adequate correction 5
• Most patients require individualized concentrations ranging from 29-38 mmol/L, with many needing 35 mmol/L or higher 4

Monitoring and Adjustment Protocol

First Month Follow-Up

Measure pre- and post-dialysis bicarbonate at 2 weeks and 1 month after initiating the new dialysate concentration. 6, 7

• If pre-dialysis bicarbonate remains <22 mmol/L, increase dialysate bicarbonate by 2-3 mmol/L 7
• If post-dialysis bicarbonate exceeds 29 mmol/L, decrease dialysate bicarbonate by 2-3 mmol/L 7
• If pre-dialysis bicarbonate is 19-25 mmol/L AND post-dialysis is ≤29 mmol/L, maintain current prescription 6

Ongoing Monitoring

Check pre- and post-dialysis bicarbonate monthly for the first 3 months, then every 3 months once stable. 1, 2 Also monitor:

• Serum albumin and normalized protein nitrogen appearance (nPNA) – initial decreases may occur but should stabilize after 3 months 5
• Body weight – transient decreases may occur with dialysate bicarbonate changes 5
• Serum potassium – post-dialysis hypokalemia may worsen with higher bicarbonate 7
• PTH levels – may improve with better acidosis control 7

Critical Clinical Considerations

Avoid One-Size-Fits-All Approach

The standard dialysate bicarbonate of 32 mmol/L used in many centers is insufficient for patients with pre-dialysis bicarbonate <20 mmol/L. 6, 7 Studies show that:

• Only 67.9% of patients achieve target bicarbonate ranges with fixed dialysate concentrations 6
• Individualized prescriptions result in nearly 100% of patients meeting targets 6
• 75% of patients ultimately require dialysate bicarbonate of 32-34 mmol/L or higher 6

Pitfall: Lowering Dialysate Bicarbonate Too Much

Reducing dialysate bicarbonate from 32 to 28 mmol/L in patients with borderline acidosis increases pre-dialysis acidosis from 11.9% to 23.8% of sessions. 5 For your patient with baseline bicarbonate of 18 mmol/L, using dialysate bicarbonate ≤30 mmol/L would worsen acidosis. 5

Pitfall: Post-Dialysis Alkalosis

While correcting acidosis, avoid creating post-dialysis alkalosis (bicarbonate >29 mmol/L), which is associated with increased mortality. 3, 6 The bicarbonate increase during dialysis depends on:

• Bicarbonate gradient (dialysate minus serum) – strongest predictor 5
• Session time – longer sessions increase bicarbonate transfer 5
• Protein intake (nPNA) – higher protein intake increases acid generation and bicarbonate consumption 5

Complementary Oral Therapy

If dialysate bicarbonate adjustment alone is insufficient, add oral sodium bicarbonate 2-4 g/day (25-50 mEq/day) divided into 2-3 doses. 1, 2 This is particularly important for patients with:

• High protein intake (nPNA >1.0 g/kg/day) 5
• Shorter dialysis sessions (<4 hours) 5
• Persistent pre-dialysis bicarbonate <22 mmol/L despite optimized dialysate 1

Monitor for sodium overload, hypertension, and fluid retention when adding oral bicarbonate. 2 Consider increasing fruit and vegetable intake as an alternative or adjunct, which provides alkali without sodium load and may reduce blood pressure. 1

Practical Implementation

Set the hemodialysis machine dialysate bicarbonate to 35-37 mmol/L initially, then titrate based on pre- and post-dialysis bicarbonate measurements over the next 1-3 months to achieve pre-dialysis bicarbonate 19-25 mmol/L and post-dialysis bicarbonate ≤29 mmol/L. 6, 4, 7 This individualized approach eliminates both pre-dialysis acidosis and post-dialysis alkalosis in >95% of patients. 6