Why Post-Dialysis BUN Measurement Is Challenging But Necessary
Post-dialysis BUN measurements are not routinely performed after every dialysis session because of technical challenges in obtaining accurate samples, but they remain essential for periodic assessment of dialysis adequacy through calculation of Kt/V and URR. 1
Technical Challenges with Post-Dialysis BUN Sampling
Timing and Methodology Issues
- Post-dialysis BUN samples must be drawn using specific techniques to ensure accuracy:
- The slow flow/stop pump technique is recommended but technically demanding 1
- Samples must be drawn from the arterial port or needle tubing
- Precise timing is required to prevent sample dilution with recirculated blood
- Healthcare teams in busy dialysis units may struggle to execute this technique with proper rigor
Urea Rebound Phenomenon
- After dialysis ends, urea rebounds due to compartment equilibration 1
- Alternative sampling methods like the blood reinfusion technique allow for urea rebound, resulting in:
- Higher post-dialysis BUN levels
- Partially equilibrated samples that don't meet the requirements of single-pool urea kinetics models
- Variable rebound times depending on reinfusion procedures
Dilution Effects
- Saline reinfused at the end of treatment (150-500 mL) can dilute plasma and lower post-dialysis BUN 1
- This effect is particularly significant in smaller patients with diminished intravascular distribution volume
Proper Collection Techniques for Post-Dialysis BUN
When post-dialysis BUN is measured, it must follow these protocols:
Slow Flow/Stop Pump Technique (Recommended)
- Reduce blood pump speed to 50-100 mL/min
- Stop dialysate flow or bypass dialysate
- Wait 10-15 seconds for cardiopulmonary recirculation to clear
- Draw sample from arterial blood line port
- This technique prevents dilution with recirculated blood and minimizes urea rebound effects 1
Alternative Methods (Less Accurate)
- Blood reinfusion sampling technique: drawing sample after blood has been completely reinfused
- Bypassing dialysate technique: obtaining sample after bypassing dialysate for 3 minutes 2
Modern Approaches to Reduce Post-Dialysis BUN Sampling
Several methods have been developed to reduce the need for frequent post-dialysis BUN measurements:
Conductivity-Based Clearance Monitoring
- Many dialysis machines can compute dialyzer sodium clearances using conductivity 3
- This allows estimation of Kt/V without requiring post-dialysis BUN for every session
- Once a patient-specific value for urea distribution volume (V) is established, subsequent treatments can be monitored using conductivity-based clearance measurements
- Benefits include:
- Reduced staff time and laboratory costs
- Increased patient and staff safety
- Decreased errors from improper sampling techniques
Mathematical Prediction Models
- Artificial neural networks can predict equilibrated BUN and Kt/V using pre-dialysis and limited post-dialysis samples 4
- Logarithmic extrapolation using a 15-minute post-dialysis BUN can accurately predict equilibrated BUN 5
- These models help reduce the frequency of full post-dialysis sampling while maintaining accuracy
Recommended Monitoring Schedule
According to guidelines, dialysis adequacy should be measured at least monthly 1, which requires:
- Pre-dialysis BUN sample (drawn before dialysis begins)
- Post-dialysis BUN sample (using proper technique)
- Both samples must be drawn during the same dialysis session
- Samples should be analyzed by the laboratory at the same time to minimize variability
Common Pitfalls to Avoid
- Dilution of pre-dialysis sample with saline or heparin (causes falsely low Kt/V)
- Improper timing of post-dialysis sample (affects urea rebound measurement)
- Variable blood reinfusion times between sessions (introduces inconsistency)
- Failure to analyze both pre and post samples from the same session
- Using samples from different dialysis sessions (provides meaningless information)
By understanding these challenges and implementing appropriate techniques, dialysis adequacy can be monitored effectively while minimizing the need for frequent post-dialysis BUN measurements.