Recommended Rate of BUN Drop During CRRT
There is no specific recommended rate of BUN drop over 24 hours during CRRT; instead, focus on achieving a target steady-state BUN level of 40-80 mg/dL by delivering adequate CRRT intensity of 20-25 mL/kg/hr, which will naturally result in BUN declining exponentially over 3-4 days to reach plateau. 1
Understanding BUN Kinetics During CRRT
The key concept is that BUN does not drop at a fixed rate per 24 hours. Instead, when CRRT is initiated at a constant intensity in patients with relatively stable urea generation rates, BUN falls in an exponential fashion, approaching a plateau level after 3-4 days of continuous treatment 2. The rate of decline is steepest initially and gradually slows as it approaches the steady-state target.
Target BUN Levels Rather Than Rate of Change
- The goal is to achieve and maintain a steady-state BUN of 40-80 mg/dL, not to control the speed of decline 2
- The CRRT clearance (K) necessary to achieve a desired plateau BUN can be calculated as: K = G/BUN_goal, where G is the urea generation rate 2
- For a target BUN of 60 mg/dL with moderate hypercatabolism, a practical formula is: K (L/hr) = 2 × body weight (kg) / 100 2
Recommended CRRT Intensity to Control Uremia
Deliver a minimum effluent flow rate of 20-25 mL/kg/hr to adequately control uremia and achieve target BUN levels. 1
Evidence-Based Dosing Thresholds
- The lower limit of delivered intensity to control uremia is approximately 10-15 mL/kg/hr, below which BUN and creatinine levels will continue to rise rather than decline 3
- A prescribed intensity of approximately 15 mL/kg/hr may be adequate to prevent uremia progression, but higher doses are recommended for optimal outcomes 3
- Intensities of 20-25 mL/kg/hr are recommended based on large randomized trials showing no benefit from higher doses (35-40 mL/kg/hr) 1
Accounting for Delivered vs. Prescribed Dose
- Prescribed dose should be 20-25% higher than target dose to account for treatment interruptions and filter efficiency decline 1
- In clinical practice, patients typically receive only 68-85% of prescribed dose due to circuit clotting, imaging, surgery, and declining membrane porosity 1
- Monitor delivered dose rather than relying solely on prescribed settings 1
Monitoring BUN During CRRT
Frequency of Monitoring
- Check BUN levels every 6-12 hours initially to assess response to therapy 4
- BUN measurements should be interpreted in the context of their rate of change over time, not as isolated values 1
- Rapid increases in BUN suggest severe renal dysfunction or inadequate CRRT intensity 1
Relationship Between BUN and Nitrogen Balance
- BUN correlates strongly with nitrogen loss (r = 0.804, P < 0.0001) during CRRT 5
- Median nitrogen loss during CRRT is approximately 10.6 g/day, with total nitrogen loss potentially reaching 25 g/day 1, 5
- Higher protein intake (1.5-2.5 g/kg/day) increases BUN production, requiring adjustment of CRRT intensity to maintain target BUN levels 1
Common Pitfalls and Caveats
Avoid Overly Aggressive BUN Reduction
- Do not aim for rapid normalization of BUN - the exponential decline over 3-4 days is physiologically appropriate 2
- Excessively high CRRT intensities (>35-40 mL/kg/hr) do not improve outcomes and increase costs 1
Consider Urea Generation Rate
- Urea generation rate (G) varies widely in critically ill patients and affects BUN trajectory 4
- Normalized protein catabolic rate ranges from 1.2-2.1 g/kg/day in AKI patients on CRRT, with mean values around 1.75 g/kg/day 1, 4
- Patients receiving higher protein intake (2.5 g/kg/day) require increased CRRT dose to manage elevated BUN production 1
Account for Residual Kidney Function
- Residual kidney function contributes to urea clearance and should be factored into total clearance calculations 1
- Urine output and urinary urea nitrogen should be measured when present 4