Nephrol Dial Transplant (2000) 15: 554-555
© 2000 European Renal Association-European Dialysis and Transplant Association
Letters
Consistent timing of the post-dialysis blood sample is necessary to prevent undertreatment in single needle dialysis
Renal Unit, Wellcome Wing, Leeds General Infirmary 1 St James's University Hospital, Leeds, UK
Sir,
The efficiency of haemodialysis is related to prognosis [1,2], and recommendations now exist for the minimum acceptable Kt/V [3,4]. A local audit revealed some patients with unusually high or variable values for Kt/V. Several of these patients were on single-needle dialysis (SND) because of poorly-developed AV fistulae. We were concerned that the variable results might reflect inconsistencies in the post-dialysis blood sampling technique and subsequently explored this possibility with a prospective cross-over study.
Five of the 120 patients attending our unit regularly used a single needle for access (four females and one male, age range 30–73). These patients had the two-pool Kt/V estimated during a series of treatments [5]. Subjects were randomized to either routine post-dialysis sampling (RT) or sampling according to a novel protocol used by four selected members of staff (SP) for four treatments. After this, the subjects underwent four further dialyses using the other technique. All patients were undergoing thrice weekly bicarbonate dialysis with samples drawn over the 2-day interdialytic interval. No changes in dialysate flow rate or other variables were introduced during the study. Four patients used Polysulfone membranes (F6 or F8, Fresenius Medical Care, St Wendel, Germany) and one used a Polyethylene glycol-grafted cellulose membrane (Bio-750-wet, Asahi Medical Co Ltd, Tokyo, Japan).
Pre-dialysis samples were drawn from the access needle after insertion. For RT, the post-dialysis sample was taken from the arterial limb of the access needle when the machine operator stopped the blood pump. For SP, the pump was stopped at the end of the `arterial' cycle, in order to ensure that the smallest possible amount of dialysed blood was in the access. The sample was drawn from the `arterial' limb with the `venous' limb clamped.
The results are shown in Figure 1
. There was considerable variation using the normal sampling method and markedly less using the protocol (Coefficient of Variation range 37.1–61.8% vs 6.6–19.8%; P=0.002). The overall mean Kt/V was 1.72±0.88 for RT and 1.07±0.23 for SP (P=0.0035). The mean and median Kt/V were lower for all but one patient (patient 4 medians both 1.3) using SP than RT. There was a negative correlation between the coefficient of variation obtained by RT and the Kt/V derived from the SP samples, indicating that the patients with the lowest `true' Kt/V had the most inconsistent results (r=-0.88, P=0.048).
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Our results reinforce the point that SND can lead to inaccuracy and overestimation of the Kt/V. The protocol improved the reproducibility of the results and unmasked significant underdialysis in at least one subject (patient 5).
The logic of using Kt/V as a prognostic indicator has recently been called into question [6], but it remains a cornerstone of current treatment standards. Our patients would have consistently exceeded the UK target Kt/V of 1.2 on RT despite having low values when measured by SP. The concern, of course, is that patients might be underdialysed for a substantial period, with the attendant risks of malnutrition and uraemic toxicity. The systematic limitations of SND make it particularly important to assess Kt/V without error. The simple, cheap and effective tactic of increasing the blood pump speed to compensate for a low Kt/V is often not possible with the poor fistulae that make SND necessary. Even if it were, a 4-h SND treatment with a pump speed of 400 ml/min cannot deliver adequate dialysis for a patient weighing much more than 50 kg, so the recognition of undertreatment by careful sampling is crucial. Generally, fistulae accepting only one needle should be revised to make two-needle treatment possible.
In summary, we have demonstrated the effect of eliminating access recirculation from post-dialysis blood samples in SND. Incorrect sampling disguised low efficiency and produced unpredictable results. The risk of access recirculation is greater in SND due to the high blood flows used. We suggest that, if single needle treatment cannot be avoided, careful adequacy measurements should be carried out using a protocol similar to that described to confirm that an appropriate dialysis dose is delivered.
References
- Hornberger JC and the Renal Physicians working committee on clinical practice guidelines. The haemodialysis prescription quality-adjusted life expectancy. J Am Soc Nephrol 1993; 4: 1004–1020[Abstract]
- Parker TF Husni L Huang W et al. Survival of haemodialysis patients in the United States is improved with a greater quantity of dialysis. Am J Kidney Dis 1994 23: 670–680[Web of Science][Medline]
- Haemodialysis Adequacy Work Group: NKF-DOQI Clinical Practice Guidelines for haemodialysis adequacy. Am J Kidney Dis 1997; 30: S15–S66[Web of Science]
- Standards Subcommittee of the Renal Association: Treatment of adult patients with renal failure: recommended standards audit measures. Royal College of Physicians of London, London 1997: 23
- Standards Subcommittee of the Renal Association: Treatment of adult patients with renal failure: recommended standards audit measures. Royal College of Physicians of London, London 1997: 103
- Lowrie EG Zhu X Lew NL. Primary associates of mortality among dialysis patients: trends reassessment of Kt/V urea reduction ratio as outcome-based measures of dialysis dose. Am J Kidney Dis 1998 32 [Suppl 4]: S16–S31[Medline]
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