Ultrafiltration is clearly important for patient survival in peritoneal dialysis. It is also an important parameter for peritoneal membrane function. It was thought to be the easiest variable to measure. Not until post 2000, we started to recognize overfill and flush before fill cause significant measurement error for ultrafiltration. 4–6 The current daily practice is to weight the “whole” drain bag and minus the overfill volume. Some carefully designed clinical trials measured dialysate bags before and after. However, the controversy finding around ultrafiltration still exists to some degree. In the current study, we clarified several potential measurement errors for ultrafiltration.
Overfill exists in all brands, but different in each brand
We knew overfill exists in all brands. But how big the difference is was not clear to the public. Theoretically, overfill should be different between brand, type and even batch. We picked the 2L, lactate buffered, 2.5% dextrose dialysate from four different brands. It was just to get a rough idea of how big the difference was. Ideally, the manufacturer should be encouraged to publish regular audits of overfill for each type of dialysate.
Storage condition does make difference for long storage duration
According most dialysate storage instruction, lower than 0 degree should be avoided. The expiration date is 24 months. From the current study, it was clear the storage duration and condition did make difference in dialysate volume. In practices, we definitely suggest dialysate should not put in incubator for long time, which would cause significant evaporation. It could also be a problem in clinical trials. Because dialysate supplement in clinical trials may not be as frequent as routine clinical use. This problem could be more significant in the investigation arm than the control arm, which may give systemic error when comparing ultrafiltration. For clinical trials, weight dialysate bags before and after is strongly suggested. The fact that temperature and humidity had effect on dialysate volume may also contribute to the center effect of ultrafiltration and sodium removal.
PVC and non PVC package show difference in evaporation
We also noticed the different character in evaporation between PVC and non PVC package. So far, there is no clinical data on ultrafiltration comparing PVC and non PVC package. An ongoing clinical trial from China may give us some useful information. 12 The problem of storage duration and difference in evaporation character should be carefully treated.
Neglecting the effect of specific gravity leading to overestimate of ultrafiltration in CAPD
It is not surprise that the specific gravity of dialysate is slightly higher than pure water. However, it is supposed to be neglectable by almost the whole PD society. Ultrafiltration in manual exchange is measured by weight and transform to volume by divided by 1g/ml (specific gravity of pure water). While in APD, ultrafiltration is directly measured by machine in volume. Taking the average specific gravity from our dextrose only cohort; the potential over estimation of UF in CAPD with 8L prescribe input volume and 1L UF was calculated as following.
Reported UF (L, misleading by kg)
= [8L (input volume) + 1L (UF)] * 1.0136 (g/ml) − 8L (input volume)
= 1.122(L, misleading by kg) (over estimate for 0.122L)
For icodextrin, the specific gravity is even higher than dextrose solution. As we don’t have icodextrin in Shanghai, Prof Simon Davies kindly shared the information with us. From three long dwell icodextrin dialysate samples in Stoke on Trent, the mean specific gravity was 1.026 ± 0.006g/ml. In another word, the potential over estimation of UF in CAPD in a single icodextrin dwell (2L) with 0.4L UF was calculated as following.
Reported UF (L, misleading by kg)
= [2L (input volume) + 0.4L (UF)] * 1.026 (g/ml) − 2L (input volume)
= 0.461(L, misleading by kg) (over estimate for 0.061L)
The gap between weight and volume is big enough to give systemic error when comparing ultrafiltration between CAPD and APD. However, measuring dialysate volume manually is not feasible. It may cause even bigger measurement error and also increase the risk of contamination. Weight instead of volume measurement is still a reasonably way for daily practice. Mobile volume measuring tool may help with this problem in clinical trial scenario.
Limitation: In the current study, we got the dialysate bags from market. We had tried our best to get the freshest dialysate available in the market for the study. The time between manufactory to baseline measurement was still slightly different between brands (from 43 to 105 days). On the other hand, theoretically, overfill should be different between brand, type and even batch. It may give some bias when comparing between brands. However, the study was design to clarify the difference itself rather than report the exact figure of the difference.
There is argument that in real life dialysate is not likely to store in that extreme conditions as in the current study. The study was simply to clarify the fact that storage condition (temperature and humidity) and duration would make difference on dialysate weight rather than working out an exact figure of how big the difference is.