Recommended BUN Drop During CRRT
There is no specific recommended daily drop in BUN during CRRT; instead, focus on achieving a target steady-state BUN level of approximately 40-60 mg/dL through appropriate CRRT dosing at 20-25 mL/kg/h, rather than targeting a specific rate of decline. 1, 2
Understanding BUN Kinetics in CRRT
The approach to BUN management during CRRT differs fundamentally from targeting a specific daily reduction rate:
- BUN falls exponentially during CRRT, approaching a plateau level after 3-4 days of continuous treatment when urea generation rate remains relatively constant 3
- The plateau BUN level achieved depends on the balance between urea generation (G) and CRRT clearance (K), following the relationship: plateau BUN = G/K 3
- Target a steady-state BUN of 40-60 mg/dL rather than a specific daily drop, as this reflects adequate solute control without excessive clearance 3
Optimal CRRT Dosing Strategy
The KDIGO guidelines provide Level 1A evidence recommending an effluent volume of 20-25 mL/kg/h for CRRT, which will naturally control BUN levels appropriately 1, 2:
- This dose is based on large randomized controlled trials (RENAL and ATN studies) showing no benefit from higher intensity therapy 2
- Higher doses (35-40 mL/kg/h) did not improve survival or kidney recovery compared to 20-25 mL/kg/h 2
- The prescribed dose should exceed the target delivered dose, as actual delivery often falls short 2
Practical Prescription Approach
For initial CRRT prescription without knowing the patient's urea generation rate, use this simplified formula 3:
- K (L/hr) = 2 × body weight (kg) ÷ 100 to achieve a goal BUN of approximately 60 mg/dL
- This assumes moderate hypercatabolism (normalized protein catabolic rate = 2.0 g/kg/day) and urea distribution volume of 60% body weight 3
- For lower target BUN (e.g., 40 mg/dL), increase the clearance proportionally 3
Monitoring and Adjustment
Rather than tracking daily BUN drops, focus on these parameters:
- Monitor BUN trends over 3-4 days to assess approach to steady state 3
- Nitrogen loss correlates strongly with BUN levels (r = 0.804), with median nitrogen loss of approximately 10.58 g/day in CRRT patients 4
- Avoid excessively rapid BUN reduction, as this may indicate inadequate protein intake or excessive catabolism 4
- The relationship between BUN and nitrogen balance is more clinically relevant than absolute BUN reduction rate 4
Critical Pitfalls to Avoid
- Do not use single BUN thresholds alone to guide CRRT initiation or intensity; consider the broader clinical context and trends 1
- Excessively aggressive BUN lowering (targeting very low levels) provides no survival benefit and may indicate protein malnutrition 2
- Protein intake of 1.0-1.7 g/kg/day is recommended during CRRT, with higher needs in hypercatabolic states, which will influence BUN levels 1
- Poor correlation exists between protein intake and nitrogen balance in CRRT patients, so BUN must be interpreted alongside nutritional assessment 4
Clinical Context
The focus should be on adequate CRRT dose delivery (20-25 mL/kg/h effluent rate) rather than achieving a specific daily BUN reduction 1, 2. This approach ensures appropriate solute control while avoiding the complications of excessive clearance, including metabolic derangements and unnecessary protein losses 1.