To the best of our knowledge, our study is the first real-world study with a large amount of precise data obtained by using Homechoice Claria with Sharesource, which empowers us to accurately analyze the UF under different conditions such as the dextrose concentration and PET results. Between the two groups, D2.5% had a longer PD duration and less residual urine than D1.5%. This is reasonable because as time passes, the longer the PD duration is, the greater the renal function decrease, which may cause a decrease in residual urine. Our study emphasizes precise UF, its predictive factors, and influential factors.
Few trials have investigated UF at different dextrose concentrations, not to mention the real-world treatment details. Net UF is composed of transcapillary UF and lymphatic absorption from the peritoneal cavity. Transcapillary UF is mainly driven by osmotic pressure through the dialysate glucose gradient but is also affected by transcapillary hydrostatic pressure, whereas lymphatic absorption is mainly governed by intraperitoneal hydrostatic pressure and remains almost unchanged throughout the dialysis period 30–32. In our study, the mean night UF per cycle was 95.8 ml in D1.5% and 220.3 ml in D2.5% (Table 2), which was significantly different. The mean night UF/FV per cycle was 5.5% (ranging from − 7.1–12.0%) in D1.5% and 12.0% (ranging from 3.4–20.7%) in D2.5%. Corrected by the DT, the mean night UF/FV/DT per cycle was 5.0 ‱/min in D1.5% and 11.6 ‱/min in D2.5% (Table 4). This is in line with the theory that the greater the osmotic pressure caused by the glucose gradient is, the greater the UF increase. Previous trials reported that the UF percentage ranged from − 3.50–16.50% with 1.5% dextrose 33–36, 10.50–18.76% with 2.5% dextrose 31, 34, 35, 37–39, 30.60–51.40% with 4.25% dextrose 33–35, and 11.7–12.62% with mixed dextrose concentrations 40, 41, with the same FV (2 liters) and different DTs (ranging from 1.5 hours to 6 hours). The UF/FV/DT values ranged from 0.97 to 6.07 ‱/min with 1.5% dextrose 35, 36 and 2.93 to 11.85 ‱/min with 2.5% dextrose 31, 35, 37–39. Our results were all in accordance with previous trials.
Notably, among the studies mentioned above, the lowest UF (3.50% with 1.5% dextrose, 10.50% with 2.5% dextrose, and 30.60% with 4.25% dextrose) was noted in a 6-hour dwell study by Heimburger O et al. 35. In this study, the intraperitoneal dialysate volume versus time curve and net UF rate over time demonstrated an initial positive net UF (mainly driven by dextrose osmotic pressure), then an isovolemic period (15 to 120 minutes, 90 to 240 minutes, and 120 to 240 minutes for 1.5%, 2.5%, and 4.25% dextrose, respectively), and finally, a fluid reabsorption period that was similar for all three solutions. This highlights the relationship between the DT and the UF, which is not linear. The maximal net UF rate occurs within the first few minutes of the dialysate dwell (ranging from 4.3 to 6 ml/min, 8 ml/min, and 12.8 to 14 ml/min during the first 15 minutes, 90 minutes, and 90 minutes for 1.5%, 2.5%, and 4.25% dextrose, respectively 35, 42), and the maximal net UF is achieved when the transcapillary UF rate equals the lymphatic absorption rate, that is, dynamic equilibrium is achieved between osmotic pressure, transcapillary hydrostatic pressure, and lymphatic absorption 31, 43, which occurs within 85 to 140 minutes with 1.5% dextrose, 140 to 160 minutes with 2.5% dextrose, and 197 to 254 minutes with 4.25% dextrose, according to previous studies 31, 33, 34, 42, 43. The net UF decreases after equilibrium is broken, and lymphatic absorption plays a dominant role. These studies give an explanation for our result. In the univariate linear regression model for mean night UF per cycle in D2.5%, we found a significant difference in mean night DT per cycle, in which a 1-minute increase in mean night dwell time per cycle would result in a 2.24 ml increase in UF (p = 0.003, not shown in the table). This may be because the mean night DT per cycle of the D2.5% group was 105.1 minutes, which is below the range of the time to reach the maximal net UF, as mentioned above (140 to 160 minutes); thus, before reaching equilibrium, the net UF increases as the DT gets longer. For D1.5%, there was no significant difference. We presume this is because the mean DT per cycle of D1.5% was 112.8 minutes and was within the range of the time to reach equilibrium (85 to 140 minutes), which means that the maximal net UF is achieved and even enters to the fluid reabsorption period.
In addition to the dextrose concentration, which was not considered in the studies above, peritoneal membrane characteristics were evaluated. Similar to what was suggested in other studies 25, 32, 41, there was a trend of correlation between high peritoneal transport status and decreased UF in both D1.5% and D2.5% in our results (Figs. 1a & 1b). Additionally, the dialysate DT to reach the maximal net UF was longer in patients with lower peritoneal permeability than in those with higher peritoneal permeability. In a study by Alp Akonur et al. using 2.5% dextrose dialysate 44, the peak DT according to UF was 2.5 hours in the high peritoneal transport type and 4 hours in the low-average peritoneal transport type. This gives an explain for our result—UF increased in the group with low average transport function after a 120-minute dwell at both the 1.5% and 2.5% dextrose concentrations, while in the group with high average transport function, UF decreased after a 120-minute dwell at the 1.5% dextrose concentration (Figs. 1c & 1d).
UF is also affected by intraperitoneal pressure (IPP) and the dialysate FV 45, 46. In a comparison of the UF between dialysis with a 2- and 3-liter exchange of 1.5% dextrose dialysate, Krediet RT et al. revealed that the UF was lower in the 3-liter exchange due to the increased water reabsorption rate, which is related to IPP 47, while another study demonstrated that the maximal net UF was achieved when the dialysate FV was 2286 ml and the UF then decreased secondary to the increased IPP 37. This explains our result revealing that the mean night FV per cycle was positively correlated with mean night UF per cycle (p = 0.006) because our mean night FV was 1803.1 ml with a median of 1799.0 ml, which had not yet reached the maximal UF. This also indicates that there is still room for improvement in our dialysis prescription.
In conclusion, our study presents precise UF measurement with two solutions at different dextrose concentrations and four peritoneal transport levels. UF is positively correlated with the DT and FV of the dialysate within a reasonable range. High peritoneal permeability is associated with decreased UF, and low peritoneal permeability needs a longer dwell time to reach the maximal UF.
Due to the retrospective nature of our study, there are some limitations. First, the number of enrolled patients was not large enough to counterbalance the effect of some extrema, especially in the different PET groups. Second, because the kinetics of fluid transport during PD are not available, we used the average method to correct UF with the FV and DT, although the UF versus time curve was not linearly correlated, as indicated in the studies mentioned above. Finally, we did not take intraperitoneal residual volume into account. Different dialysate dextrose concentrations will affect intraperitoneal residual volume 35 and further influence the UF calculated by FV minus the drained volume.