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How to Assure Correct Pre- and Postdialysis Blood Sampling of Blood Urea Nitrogen.

Q: At our unit, we recently had an audit of how we draw our blood urea nitrogens (BUNs) pre- and postdialysis. We were told that it was part of collecting clinical performance measures (CPMs) for adequancy. Can you explain what the recommended method is and how it relates to adequacy of dialysis (Kt/V or URR)?

A: Accurate measurement of changes in the pre/post BUN concentration that results from a dialysis treatment is dependent upon accurate blood sample collection. Guideline 8 in the NKF-DOQI Guidelines for Hemodialysis Adequacy describes acceptable methods for collecting BUN samples (National Kidney Foundation [NKF], 1997). The guideline states that blood samples for BUN measurement must be drawn in a particular manner. Predialysis BUN samples should be drawn immediately prior to the start of dialysis using a technique that avoids diluting the blood with saline or heparin. The postdialysis BUN samples should be drawn using the "Slow Flow/Stop Pump Technique that prevents dilution of the blood with recirculated blood and minimizes the effects of urea rebound" (NKF, 1997).

How do you meet the recommendations of this guideline? Your facility should have a standardized procedure for drawing blood samples that is consistently used by all patient care staff.

Predialysis BUN should be measured before hemodialysis begins to prevent the sample from being affected by the dialysis procedure. If drawn after the start of dialysis, the predialysis BUN will be lowered by the dialysis treatment and not reflect the true predialysis BUN level. The specimen must not be contaminated with saline or heparin. Draw the predialysis sample from the arterial needle prior to flushing with saline or connecting to the arterial tubing. If using a catheter, withdraw any heparin or saline from the arterial port, following facility protocol. If protocol allows, in adults, using sterile technique, withdraw 10 ml of blood from the arterial port and save for reinfusion. Draw the predialysis BUN sample. Do not draw a predialysis sample if hemodialysis has been initiated or if saline or heparin is present in the lines.

Postdialysis BUN sampling requires proper timing in relation to the end of the dialysis session. It is critical that the specimen be drawn immediately upon completion of dialysis. Care must be taken to avoid drawing a sample that contains just-dialyzed blood that will result in a falsely low BUN. It is recommended that the Slow Flow/Stop Pump Method (see Table 1) be used for postdialysis sampling. This sampling method will reduce the likelihood of recirculated blood contaminating the post BUN sample.

Table 1. Slow Flow or Stop Pump Technique

1. At the completion of hemodialysis, turn off the dialysate flow and decrease the ultrafiltration rate (UFR) to 50 mL/hr, 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 500-100 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 (see below).

Slow Flow Sampling Technique

3. With the blood pump still running at 50-100 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.

Note: From National Kidney Foundation (NKF). (1997). NKF-DOQI clinical practice guidelines for hemodialysis adequacy (p. 57). New York: Author.

Blood that is drawn more than 2-3 minutes after stopping the dialysis treatment may result in an increased BUN. The longer the time period after dialysis is discontinued, the greater the increase in the BUN. This increase is caused by the urea rebound that occurs within minutes of ending the dialysis treatment. Using the proper technique in drawing BUN samples will provide accurate blood sampling necessary for the calculation of Kt/V or URR.


National Kidney Foundation (NKF). (1997). NKF-DOQI clinical practice guidelines for hemodialysis adequacy. New York: Author.

Evelyn Butera, MS, RN, CNN, is quality improvement manager; Satellite Health Care, Redwood City, C.A.
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Author:Butera, Evelyn
Publication:Nephrology Nursing Journal
Article Type:Brief Article
Geographic Code:1USA
Date:Apr 1, 2001
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