Timing of Blood Sample Collection After Dialysis
For post-dialysis blood urea nitrogen (BUN) sampling to assess dialysis adequacy, blood should be drawn within 15-30 seconds after reducing blood flow to 50-100 mL/min at the completion of dialysis, using the slow flow/stop pump technique. 1
Recommended Sampling Method: Slow Flow/Stop Pump Technique
The NKF-K/DOQI guidelines establish this as the gold standard approach for post-dialysis blood sampling: 1
Step-by-Step Protocol
At dialysis completion: Turn off dialysate flow and decrease ultrafiltration rate to 50 mL/h or minimum setting 1
Reduce blood flow: Decrease to 50-100 mL/min for 15 seconds to clear the dead space (approximately 10-15 mL) in the arterial needle tubing and blood line 1
Draw the sample: Obtain blood from the arterial sampling port closest to the patient while blood pump runs at 50-100 mL/min (slow flow technique) OR immediately after stopping the pump (stop pump technique) 1
This timing (within 15-30 seconds of slowing flow) represents the optimal window—designated as 0.25-0.50 minutes post-dialysis—when access recirculation has resolved but before significant urea rebound occurs. 1
Why Timing Matters: The Urea Rebound Phenomenon
Post-dialysis urea rebound occurs in three distinct phases that dramatically affect measurement accuracy: 1
Phase 1: Access Recirculation (0 seconds)
- Sampling immediately causes major underestimation of BUN due to recirculated dialyzed blood still in the access 1
- Results in falsely elevated Kt/V and URR, potentially masking inadequate dialysis 1
- Critical pitfall: Patients with true Kt/V <0.8 (associated with increased mortality) may appear to have adequate Kt/V of 1.4 or higher 1
Phase 2: Cardiopulmonary Recirculation (1-2 minutes)
- Resolves within 1-2 minutes after stopping dialysis 1
- Contributes approximately 15% to total urea rebound 1
Phase 3: Remote Compartment Rebound (30-60 minutes)
- Complete equilibration takes 30-60 minutes as urea redistributes from peripheral tissues 1
- Accounts for 31% of total rebound due to flow-volume disequilibrium and delayed transcellular urea movement 1
- While waiting 30-60 minutes would provide the most accurate double-pool measurement, this is clinically impractical 1
Alternative Method: Stop Dialysate Flow Technique
An acceptable alternative is stopping dialysate flow for 5 minutes before sampling, which allows equilibration across the dialyzer membrane. 1, 2
- Blood can be drawn from any part of the extracorporeal circuit between 4-6 minutes after stopping dialysate flow 2
- This method compensates for both access and cardiopulmonary recirculation 2
- Simpler to perform in busy dialysis units as it doesn't require precise timing within seconds 2
- Results in slightly lower URR (69.1% vs 71.7%) compared to immediate post-dialysis sampling, providing more accurate dialysis dose assessment 2
Impact of Sampling Time on Dialysis Adequacy Measurements
The timing dramatically affects calculated dialysis adequacy: 1
| Time After Dialysis | Effect on BUN | Effect on Kt/V/URR | Clinical Implication |
|---|---|---|---|
| 0 minutes | Major decrease | Major increase | Inappropriate—overestimates dose [1] |
| 0.25-0.50 minutes | Index value | Index value | Optimal for single-pool modeling [1] |
| 2-3 minutes | Increased | Decreased | Reinfusion technique timing [1] |
| 5-10 minutes | Increased more | Decreased more | Significant rebound occurring [1] |
| 30 minutes | Greatest increase | Greatest decrease | Complete rebound—impractical [1] |
Common Pitfalls to Avoid
Dilution errors: Never allow saline or heparin to contaminate the sample, as this falsely lowers BUN and overestimates dialysis adequacy 1
Catheter sampling: When using double-lumen catheters, discard the initial aspirate (dead space volume of 1-2 mL) before drawing the sample 1
Timing imprecision: The slow flow/stop pump technique requires relatively precise timing—blood must be drawn within 15 seconds of reducing flow to avoid both recirculation effects and premature rebound 1
Laboratory error consideration: If hemoglobin or other values show unexpected changes post-dialysis, consider sampling technique issues including inadequate equilibration time 3
For Other Laboratory Tests
While the above guidelines specifically address BUN sampling for dialysis adequacy monitoring, routine chemistry panels and other blood work not related to urea kinetics can be drawn at different times based on clinical need. The critical timing requirements apply specifically to post-dialysis BUN measurement for Kt/V and URR calculations. 1