Are We Underestimating the Problem of Ultrafiltration in Peritoneal Dialysis Patients?

Abstract

Accurate measurement of ultrafiltration (UF) is important to improve the morbidity and mortality of peritoneal dialysis (PD) patients. The introduction of "flush-before-fill" PD systems has led to improved peritonitis rates. Partly to compensate for dialysate lost during flush-before-fill, extra dialysate was added to each PD bag. A 2-L PD bag now contains a mean volume of 2.225 L. That overfill volume might be erroneously measured as UF. We previously studied how this confounding factor might be affecting the diagnosis of UF failure and found that almost all units were overestimating daily UF by 900 mL. We now repeat the study to determine if the accuracy of UF estimation has improved. We conducted a telephone survey of PD units in the UK to determine how drain bags are weighed and how UF is calculated during formal assessment of adequacy and the peritoneal equilibrium test (PET). We also retrospectively analyzed our last 100, 24-hour dialysate collections and PET results to determine the potential clinical impact of overestimating UF. There has been an improvement since our last study, but 70% of PD units in the UK are still overestimating daily UF in patients on continuous ambulatory PD (CAPD). Half the surveyed units also inaccurately calculate UF during the PET, and 85% were reporting results of PET and 24-hour dialysate collections through the software provided by Baxter Healthcare. By including the overfill volume, 73% of patients with daily UF <750 mL would not be diagnosed as having inadequate daily UF (assuming that all were fluid overloaded and anuric). Similarly, 73% with potential UF failure during the PET (4-hour UF <100 mL) would be missed if overfill volume was misrepresented as UF. For patients undergoing CAPD, there requires standardization on when drain bags are weighed. Awareness that calculation of UF must exclude overfill volumes has improved but remains poor. The PD Adequest software (Baxter Healthcare, Compton, UK) is widely adopted in the UK and perhaps it could draw attention of users to the potential of UF overestimation in CAPD patients.