Spurious estimations of sodium removal during CAPD when [Na](+) is measured by Na electrode methodology

Background The aim of this study was to investigate the effect of pH and gl ucose concentration on sodium removal and the dialysate and plasma sodium r atio (D/PNa) as measured by means of a flame photometer (NaF) or direct ion -selective electrode (NaE) in continuous ambulatory peritoneal dialysis (CA PD). Methods. In vitro, glucose concentration, pH, NaF, and NaE were measured in fresh peritoneal dialysis solutions (PDSs) before and after the addition o f glucose or KOH. In vivo, 66 four-hour peritoneal equilibration tests were performed in 35 patients on CAPD using a low pH PDS with a glucose concent ration of 3.86%. Results. In vitro, NaF and NaE were significantly influenced by the glucose concentration and pH of the PDS. In vivo, in fresh PDS, there was a signif icant difference between the NaF and NaE results; the respective median val ues were 132.1 (interquartile range 129.3 to 137.5) versus 138.0 (134.4 to 141.5) mmol/L (P < 0.0001). The D/PNa ratio calculated by NaE was significa ntly lower than that calculated by NaF (0.88 +/- 0.03 vs. 0.91 +/- 0.04 and 0.90 +/- 0.03 vs. 0.94 +/- 0.04 at 60 and 240 min, respectively, P < 0.000 1), whereas there was no significant difference between the NaE and NaF val ues after correction for plasma water and a Donnan factor of 0.96 (0.88 +/- 0.03 vs. 0.88 +/- 0.04 and 0.90 +/- 0.03 vs. 0.91 +/- 0.04, P < 0.3473). S odium removal was significantly lower when calculated as NaE than when calc ulated as NaF (43.9 +/- 32.7 vs. 61.0 +/- 32.2 mmol, P < 0.0001). Conclusions. The fresh PDS sodium concentration can be corrected using a gl ucose concentration-related factor. The D/PNa ratio calculated as NaE or Na F is not different after correction for plasma water and a Donnan factor of 0.96. Sodium removal must be measured by means of NaF rather than NaE. Thi s could have an important clinical impact.