Background: Ultrafiltration failure associated with peritoneal membrane dysfunction is one of the main complications for patients on long-term peritoneal dialysis (PD). The dialysate-to-plasma concentration ratio (D/P) of creatinine is widely used to assess peritoneal membrane function. However, other small-sized solutes have not been studied in detail as potential indicators of peritoneal permeability. Methods: We studied the D/Ps of small, middle-sized and large molecules in peritoneal equilibration tests in 50 PD patients. We applied metabolomic analysis of comprehensive small molecular metabolites using capillary electrophoresis time-of-flight mass spectrometry. Results: D/Ps of middle-sized and large molecules correlated positively with D/P creatinine. Most D/Ps of small molecules correlated positively with D/P creatinine. Among 38 small molecules contained in the dialysate, urea, citrulline and choline showed significantly lower ability to permeate than creatinine. In the relationship between D/Ps of creatinine and small molecules, regression coefficients of three molecules were less than 0.3, representing no correlation to D/P creatinine. Five molecules showed negative regression coefficients. Among these molecules, hippurate and 3-indoxyl sulfate showed relatively high teinpro binding rates, which may affect permeability. Serum concentrations of two molecules were higher in the Low Kt/V group, mainly due to high protein binding rates. Conclusions: D/Ps of some molecules did not correlate with D/P creatinine. Factors other than molecular weight, such as charge and protein binding rate, are involved in peritoneal transport rates. Metabolomic analysis appears useful to analyze small molecular uremic toxins, which could accumulate in PD patients, and the status of peritoneal membrane transport for each molecule.
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