Definitions for the levels of evidence (I–IV) can be found at the end of the "Major Recommendations" field.
Guidelines
No recommendations possible based on Level I or II evidence.
Suggestions for Clinical Care
(Suggestions are based on Level III and IV sources)
- Pre- and post-dialysis samples for urea measurement must be drawn at the same midweek treatment.
- The pre-dialysis sample should be drawn from the arterial needle without saline or heparin contamination. If there is a suggestion that the blood is contaminated, first withdraw 10 mL of blood before taking the sample (e.g., central venous catheter).
- It is suggested that the slow flow/stop pump sampling technique (described below) be used where possible to provide uniformity across dialysis units.
- The post-dialysis sample should be drawn from the arterial line port or arterial needle at 20 seconds after initiating slow (or zero) blood pump flow with attention to accuracy of timing. The 2–3 minute post-dialysis sampling practice is an acceptable alternative. Unit practice should be consistent, documented, and compared with standards with similar methodology.
Pre-dialysis Urea Sampling
- It is self-evident that sampling immediately before (not after) the commencement of dialysis is necessary to ensure valid dialysis dose estimates. Distorting factors for urea levels may be saline or heparin contamination of the sample, taken from the arterial needle or the central venous catheter limb.
Post-dialysis Urea Sampling
- Sampling after 20 seconds will include unpredictable effects on rebound from cardiopulmonary recirculation and regional blood distribution, and hence underestimate urea clearance to an unknown and inconsistent degree. Nevertheless, this may work in the patient's favour by ensuring that the dose delivered is at least that calculated.
- Increased use of high efficiency/high flux dialysers of larger surface area and high blood flow rates delivering rapid urea clearance require the use of equilibrated Kt/V standards or of Kt/V and URR standards that recognise the inherent overestimation of single pool urea clearance arising from increased urea rebound with these techniques.
Pre-dialysis Sampling Technique
The sample is drawn:
- Directly from arterial needle before introduction of any saline or heparin or
- From central venous catheter after withdrawing at least 10 mL of blood.
Post-dialysis Sampling Technique
Slow/stop pump techniques:
- Decrease ultrafiltration rate (UFR) to zero, set lowest dialysate flow rate, turn blood flow to 50–100 mL/minute
- After 20 seconds draw sample from arterial port or stop pump after 20 seconds and draw sample from arterial needle or arterial port after clamping arterial and venous lines.
An alternative technique is to withdraw a sample from the arterial port or needle after 2 minutes of slow flow. This results in a higher post-dialysis urea level with lower Kt/V values.
Definitions:
Levels of Evidence
Level I: Evidence obtained from a systematic review of all relevant randomized controlled trials (RCTs)
Level II: Evidence obtained from at least one properly designed RCT
Level III: Evidence obtained from well-designed pseudo-randomized controlled trials (alternate allocation or some other method); comparative studies with concurrent controls and allocation not randomized, cohort studies, case-control studies, interrupted time series with a control group; comparative studies with historical control, two or more single arm studies, interrupted time series without a parallel control group
Level IV: Evidence obtained from case series, either post-test or pretest/post-test
