The difference between delivered and prescribed dialysate sodium in haemodialysis machines

BACKGROUND: The choice of dialysate sodium (DNa) for haemodialysis (HD) patients remains controversial, with some studies reporting that a lower DNa improves blood pressure control and reduces intradialytic weight gain. Studies on DNa depend on the alignment of programmed to delivered DNa. We wished to determine whether there were differences between programmed and delivered DNa. METHODS: Dialysate samples were obtained from three dialysis machines: Fresenius 4008H (F4008H) and 5008S (F5008S) and B-Braun hemodiafiltration (HDF) Dialog+(BB). DNa was measured by indirect ion-selective electrode (ISE), flame photometry (FP) and ion chromatography (IC) at different DNa concentrations. RESULTS: We tested 18 F5008S, 18 F4008H and 31 BB machines over 153 HD treatments. The median measured minus programmed DNa was significantly greater with the BB machine [ISE, 7 (6–8); FP, 7 (6–8); IC, 6 (5–7)], followed by the F4008H [ISE, 5.5 (5–7); FP, 4 (2.25–5.75); IC, 4 (2–5)]and F5008S [ISE, 4 (2–5); FP, 1 (−1–1.75); IC, 1 (−0.5 to 2)] mEq/L (P < 0.05). At higher programmed DNa (140–145 mEq/L), measured DNa was greater for the BB and F4008 machines by all methods (P < 0.05), but only by ISE for the F5008 (P < 0.05). CONCLUSIONS: We noted a systematic bias in DNa delivery with measured DNa being greater than that programmed by our HD machines. The magnitude of the bias varied between machines and with DNa. Our results may help explain the diverse results reported in studies of DNa.