NKF K/DOQI GUIDELINES 2000
GUIDELINES FOR HEMODIALYSIS ADEQUACY
III. Blood Urea Nitrogen (BUN) Sampling
Blood Urea Nitrogen (BUN) Sampling (Evidence)
Predialysis and postdialysis blood samples for measurement of BUN levels must be drawn at the same hemodialysis session.
Rationale Evaluation of the dose of dialysis delivered during a hemodialysis session must be based on data from that particular treatment. Incomplete information derived from separate hemodialysis treatments provides little meaningful information about the delivered hemodialysis dose. Similarly, the laboratory that is processing the blood samples should analyze both specimens at the same time. This strategy will minimize the variability in the BUN concentration that arises because of interassay variability. In an analysis of individual variability in measures of hemodialysis adequacy that standardized for blood sampling technique (4.0% and 2.4% coefficient of variation for Kt/V and URR, respectively), most of the variation was attributable to variation in the laboratoryūs BUN measurement.183
Acceptable Methods for BUN Sampling (Evidence)
Blood samples for BUN measurement must be drawn in a particular manner. Predialysis BUN samples should be drawn immediately prior to dialysis, using a technique that avoids dilution of the blood sample with saline or heparin. Postdialysis BUN samples should be drawn using the slow flow/stop pump technique that prevents sample dilution with recirculated blood and minimizes the confounding effects of urea rebound.
Rationale Accurate measurement of the change in BUN concentration that results from a hemodialysis treatment can only be made when blood samples are collected properly. Predialysis and postdialysis sampling techniques must control for the site of the blood draw, needle or catheter preparation, blood and dialysate pump flow rates, ultrafiltration rate, and the timing of the blood sampling with respect to the initiation and completion of hemodialysis.139 The ideal and accurate measurement of the delivered Kt/V, URR, and NPCR requires that:
1. Predialysis BUN be measured before hemodialysis begins and be obtained without dilution of the blood sample.194
2. Postdialysis BUN be measured after hemodialysis ends and angioaccess recirculation has resolved.195,196
3. Laboratory processing of BUN samples is accurate.183
Predialysis blood sampling procedures. The predialysis BUN must be drawn before dialysis is started to prevent this sample from reflecting any impact of dialysis.194 Dilution of the predialysis sample with saline or heparin must be avoided or the predialysis BUN will be artificially low, resulting in a falsely low Kt/V and/or URR, an incorrectly elevated V, and a falsely reduced K.
Recommended method when utilizing an arteriovenous fistula or graft:
1. Obtain the blood specimen from the arterial needle prior to connecting the arterial blood tubing or flushing the needle. Be sure that no saline and/or heparin is in the arterial needle and tubing prior to drawing the sample for BUN measurement.
Purpose: Prevents dilution of the blood sample.
2. Do not draw a sample for use as a predialysis measure of BUN if hemodialysis has been initiated, or if saline or heparin is present in the lines.
Purpose: Prevents sampling of dialyzed blood or dilution of sample, respectively.
Recommended method when utilizing a venous catheter:
1. Withdraw any heparin and saline from the arterial port of the catheter, following the dialysis unitūs protocol.
Purpose: Prevents dilution of the blood sample.
2. For adult patients, using sterile technique, withdraw 10 mL of blood from the arterial port of the catheter. For pediatric patients, withdraw 3 to 5 mL, according to the fill volume of the catheter. Do not discard this blood if the intent is to reinfuse it after the sampling is complete (see step 4).
Purpose: To ensure that the blood sample will not be diluted by heparin. Ideally, all the contaminating diluent is removed in step 1. Step 2 provides an additional margin of security. Because pediatric patients and their catheters are smaller, recommended volumes are reduced.
3. Connect a new syringe or collection device and draw the sample for BUN measurement.
Purpose: Prevents dilution of the sample, and preserves the blood from step 2 for reinfusion, if so desired.
4. Complete initiation of hemodialysis per dialysis unit protocol. (Optional step: reinfuse the blood drawn from step 2.)
Purpose: Reinfusion minimizes blood loss and may be particularly desirable in pediatric patients who have significantly smaller blood volumes.
Postdialysis blood sampling procedure. Proper timing for acquisition of the postdialysis BUN sample is critical.194 Immediately upon completion of hemodialysis, if vascular access recirculation was present, some of the blood remaining in the angioaccess and extracorporeal circuit is actually recirculated blood. That is, some of the just-dialyzed blood has been routed through the angioaccess and the extracorporeal circuit for hemodialysis without that blood first having passed through waste product-rich tissues. If the blood sample is drawn immediately upon completion of dialysis, just-dialyzed blood that has recirculated into the angioaccess will dilute the sample. The consequence of sampling this admixture is a falsely reduced BUN value and artificially elevated Kt/V and URR.196 In this situation, the V will be falsely low and the K falsely elevated. Therefore, the amount of dialysis delivered will be overestimated.
Early urea rebound (≤3 minutes postdialysis) may
be viewed as a two-component process.58,197,198
The first component is secondary to blood recirculation within the angioaccess
or catheter and is not present in patients without access recirculation. If
access recirculation is present, urea rebound from recirculation begins immediately
upon completion of hemodialysis and resolves in less than 1 minute, usually
approximately 20 seconds. The second component of early urea rebound is cardiopulmonary
recirculation that begins approximately 20 seconds after the completion of hemodialysis
and is completed 2 to 3 minutes after slowing or stopping of the blood pump.198
Cardiopulmonary recirculation refers to the routing of just-dialyzed blood through
the veins to the heart, through the pulmonary circuit, and back to the angioaccess
without the passage of the just-dialyzed blood through any urea-rich tissues.169,198,199
The late phase of urea rebound (>3 minutes) is completed within 30 to 60
minutes after the cessation of dialysis. The late phase is a consequence of
flow-volume disequilibrium (perfusion or parallel flow model)175
and/or of delayed transcellular movement of urea (diffusion model)67,169
(see Guideline 2: Method of Measurement of Delivered
Dose of Hemodialysis, Single-Pool Versus Double-Pool Effects in Adult and Pediatric
Patients). These components of urea rebound are schematically presented in Fig
|Fig I-2. Components of urea rebound. This illustration shows a total of 65% urea rebound of which over half is secondary to access recirculation (A→B). The contribution from cardiopulmonary recirculation is 15% (B→C), and the remaining 31% (C→D) is a consequence of flow and diffusion limitations. Reprinted with permission.58|
Because of urea rebound, the post-BUN concentration and
the resultant Kt/V, URR, and NPCR will vary greatly depending upon the timing
of the acquisition of the blood sample.194
For example, the postdialysis BUN concentration will be higher, and the resultant
Kt/V, URR, and NPCR will be relatively lower, with increased time after the
completion of hemodialysis (see Table I-4). Although
the most accurate way to resolve this problem would be to uniformly wait 30
to 60 minutes after the completion of hemodialysis before drawing a postdialysis
BUN sample, this approach is impractical for patients and busy dialysis facilities.
|Table I-4. Effect of Timing of Postdialysis Sampling on BUN and Kt/V or URR|
|Time After Dialysis Ends (min)||Potential Effect on BUN||Potential Effect on Kt/V or URR||Physiology||Comments|
|0||Major reduction||Major increase because of sampling of recirculated blood||BUN will be falsely decreased secondary to recirculated blood still present in the angioaccess*||Inappropriate time for BUN sampling for any form of urea kinetic modeling|
|0.25-0.50||Index||Index||Angioaccess recirculation resolved; urea rebound starting to occur secondary to cardiopulmonary recirculation, flow volume disequilibrium, and delayed diffusion||Most accurate for blood drawing to support formal UKM based on single-pool model; sample timing corresponds to slow flow/stop pump technique|
|2-3||Increased||Decreased because urea rebound is occurring||Urea rebound is occurring; cardiopulmonary recirculation completely dissipated||Sample timing corresponds to reinfusion technique|
|5-10||Increased more||Decreased more||Significant, but incomplete resolution of urea rebound from compartment effects and flow/volume disequilibrium||Sample timing corresponds to reinfusion technique|
|30||Greatest increase||Greatest decrease||Complete resolution of urea rebound||Correlates with double-pool model, but time for sampling is clinically impractical|
*This is true only if angioaccess recirculation is present.
Because of the variable timing of blood reinfusion at the end of hemodialysis, the sampling typically occurs 2 to 10 minutes after the completion of hemodialysis.
The HD Adequacy Work Group identified a preferred method for postdialysis BUN sampling that is sufficiently reproducible and simple to be implemented by different dialysis care teams in varied dialysis settings. This method, called the slow flow/stop pump sampling technique, supports the use of formal UKM to quantitate the delivered dose of hemodialysis.
Recommended method for blood sampling using the slow flow/stop pump sampling technique.
1. At the completion of hemodialysis, turn off the dialysate flow and decrease the ultrafiltration rate (UFR) to 50 mL/h, to the lowest transmembrane pressure (TMP)/UFR setting, or off. If the dialysis machine does not allow for turning off the dialysate flow, or if doing so violates unit policy, decrease the dialysate flow to its minimum setting.
Purpose: Stop the hemodialysis treatment without stopping the blood flow completely. The risk of clotting the extracorporeal circuit is low.
2. Decrease the blood flow to 50 to 100 mL/min for 15 seconds. To prevent pump shut-off as the blood flow rate is reduced, it may be necessary to manually adjust the venous pressure limits downward.
Purpose: Fills the arterial needle tubing and the arterial blood line with non-recirculated blood (in case there is any access recirculation) by clearing the dead space in the arterial needle tubing and the arterial blood line.
At this point, proceed with either the slow flow or stop pump technique:
Slow flow sampling technique.
3. With the blood pump still running at 50 to 100 mL/min, draw the blood sample for postdialysis BUN measurement from the arterial sampling port closest to the patient.
Purpose: Drawing the blood from the arterial sampling port ensures the postdialysis BUN measurement is performed on undialyzed blood.
4. Stop the blood pump and complete the patient disconnection procedure as per dialysis unit protocol.
Stop pump sampling technique.
3. Immediately stop the blood pump.
4. Clamp the arterial and venous blood lines. Clamp the arterial needle tubing.
5. Blood for postdialysis BUN measurement may be sampled by needle aspiration from the arterial sampling port closest to the patient. Alternatively, blood may be obtained from the arterial needle tubing after disconnection from the arterial blood line and attaching a vacutainer or syringe without a needle.
6. Blood is returned to the patient and the patient disconnection procedure proceeds as per unit protocol.
Successful application of the slow flow/stop pump sampling technique has several advantages. There is minimal technical variability between blood drawing sessions, so that the calculation of the delivered dose of hemodialysis is impacted less by this operation. Therefore, longitudinal comparisons of hemodialysis adequacy more accurately reflect delivered dialysis dose. Furthermore, the use of a single-pool model for formal urea kinetic modeling mandates that postdialysis BUN be measured without the effects of access recirculation and before a significant amount of urea rebound has occurred. The relatively precise timing of the blood sampling (shortly after the cessation of hemodialysis) meets this requirement by minimizing the confounding effects of urea rebound.199 Lastly, the two recommended formulae for converting the single-pool value of Kt/V to a double-pool measurement require that the postdialysis BUN sample be obtained before urea rebound is completed (see Guideline 2: Method of Measurement of Delivered Dose of Hemodialysis).
There are modest challenges associated with the use of the slow flow/stop pump sampling technique, especially the relative precision required in timing the blood draws and setting the blood and dialysate pumps. Some members of the Work Group expressed concern that in busy hemodialysis units, where the healthcare team is performing multiple tasks at the completion of dialysis, the rigor needed to execute the slow flow/stop pump sampling technique cannot be provided routinely. In addition, the necessity that the slow flow/stop pump sampling technique blood samples be drawn from the arterial port or the arterial needle tubing increases the likelihood of error. Nevertheless, the reproducibility of this technique and its support of the formulae to calculate Kt/V greatly outweigh these potential operational issues.
The HD Adequacy Work Group recognizes the widespread use of an alternative method of postdialysis BUN sampling, the blood reinfusion sampling technique, in which the postdialysis BUN sample is drawn after the patientūs blood has been completely reinfused. It is much less reproducible than the slow flow/stop pump sampling technique. The HD Adequacy Work Group identified several potential limitations with this technique. These are:
1. Based on the experience of the Work Group members, it takes at least 5 minutes to return a patients blood at the end of hemodialysis. Therefore, the blood reinfusion sampling technique allows for the postdialysis BUN sample to be obtained after resolution of cardiopulmonary recirculation (which takes 2 to 3 minutes).197,198 By this time, some urea rebound has occurred secondary to compartment equilibration. In effect, the postdialysis BUN sample is a partially equilibrated determination and has a higher BUN level. This relative inaccuracy in V will work in the patientūs favor, ie, the measured Kt/V and URR will be lower than they would be otherwise.
2. Obtaining the postdialysis BUN by the blood reinfusion sampling technique does not meet the rigorous requirements of the single-pool, variable volume model of urea kinetics. This is because the BUN obtained ≥5 minutes after the completion of hemodialysis is a partially equilibrated sample.
3. The highly variable time necessary to reinfuse the patientūs blood between different dialysis sessions, and the lack of uniformity of reinfusion times by different caregivers, introduces unacceptable variability in the postdialysis BUN. The variable time necessary to perform blood reinfusion from different dialysis sessions means that variable amounts of rebound will have occurred.198 Indeed, variability in the timing of blood reinfusion may introduce enough variation in the postdialysis BUN results to make it difficult to interpret changes in Kt/V or URR. Although this inaccuracy is in the patientūs favor because the measured Kt/V and URR will be lower than the unequilibrated values, continuous quality improvement efforts and longitudinal comparisons of delivered dialysis dose will be much less rigorous.63 However, for patients receiving hemodialysis characterized by a high K/V, who therefore are at risk for significant urea rebound, the blood reinfusion sampling technique partially compensates for the reduced effective delivered dose of hemodialysis, attributable to single urea pool underestimation of the postdialysis BUN.
4. The HD Adequacy Work Group noted that the saline reinfused into the patient at the end of the treatment (approximately 150 to 500 mL, depending upon the volume of the hemodialyzer and blood lines and the rinsing protocol in use) will dilute the plasma and can lower the postdialysis BUN concentration. The extent of lowering may be insignificant if the patient is large. However, for small patients, with a relatively diminished intravascular distribution volume for urea, the impact of the saline infusion can be noteworthy.
The principal advantage of the blood reinfusion sampling technique for obtaining the postdialysis BUN sample is its relative simplicity, which reduces the likelihood of operational error. Unlike the slow flow/stop pump sampling technique, it is not necessary in the blood reinfusion sampling technique for the postdialysis BUN sample to be drawn with the blood pump, dialysate pump, and ultrafiltration controller or TMP/UFR rate at precise settings. Similarly, critical timing and location of the blood draw, the cornerstones of the slow flow/stop pump sampling technique, are not required for the blood reinfusion sampling technique. For example, because the extracorporeal circuit no longer exists after blood reinfusion, the postdialysis blood sample does not have be drawn from the arterial sampling port or arterial needle tubing. However, based on experience, the Work Group found these issues to be of little concern, if the step by step approach that we have described for the slow flow/stop pump sampling technique is followed.
The HD Adequacy Work Group recognizes that hemodialysis facilities using the blood reinfusion sampling technique to obtain their postdialysis BUN samples may have Kt/V and URR values that are systematically lower than dialysis units using the slow flow/stop pump sampling technique, even when the amount of delivered dialysis is the same.195 In addition, because the postdialysis BUN is higher with the blood reinfusion sampling technique, the NPCR will be relatively lower when the blood reinfusion sampling technique is used. Continuous quality improvement initiatives that use the delivered dose of hemodialysis and the NPCR as clinical performance measurements must realize that apparent performance differences between patients and facilities may be due solely to differences in the blood sampling method alone.62
Standardization of BUN Sampling Procedure (Opinion)
Hemodialysis facilities should adopt a single BUN sampling method. If several different methods are used, the sampling method should be routinely recorded. The sampling method used for a given patient should remain consistent. The predialysis and postdialysis BUN samples for a given patient should be processed in the same batch analysis at the laboratory.
Rationale Implementing a standardized protocol for blood sampling within each hemodialysis facility will promote reproducibility of the measured hemodialysis dose. The use of consistent methodology for a given patient permits longitudinal analysis of treatment effectiveness. In addition, rigor in sampling procedures greatly enhances quality assurance programs related to the dose of hemodialysis.
The HD Adequacy Work Group notes that inaccurate measures of hemodialysis adequacy also can result from improper processing of the BUN samples after they are drawn. Interassay variability in the laboratory measurement of BUN is approximately 3%. This variability can be minimized by requesting that all predialysis and postdialysis BUN samples from a given patient be processed in the same batch analysis at the laboratory.
© 2001 National Kidney Foundation, Inc