LAUSR

Background Dialysis in children as well as adults is prescribed to achieve a target spKt/V urea , where V urea is the volume of distribution of urea. Waste solute production may however be more closely correlated with body surface area (BSA) than V urea which rises in proportion with body weight. Plasma levels of waste solutes may thus be higher in smaller patients when targeting spKt/V urea since they have higher BSA relative to body weight. This study measured levels of pseudouridine (PU), a novel marker solute whose production is closely proportional to BSA, to test whether prescription of dialysis to a target spKt/V urea results in higher plasma levels of PU in smaller children. Methods PU and urea nitrogen (ureaN) were measured in plasma and dialysate at the midweek hemodialysis session in 20 pediatric patients, with BSA ranging from 0.65–1.87m 2 . Mathematical modeling was employed to estimate solute production rates and average plasma solute levels. Results The dialytic clearance (K d ) of PU was proportional to that of ureaN (average K dPU /K dUreaN 0.69 ± 0.13, r 2 0.84, p  < 0.001). Production of PU rose in proportion with BSA (r 2 0.57, p < 0.001). The pretreatment plasma level of PU was significantly higher in smaller children (r 2 0.20, p  = 0.051) while the pretreatment level of ureaN did not vary with size. Conclusions Prescribing dialysis based on urea kinetics may leave uremic solutes at higher levels in small children. Measurement of a solute produced proportional to BSA may provide a better index of dialysis adequacy than measurement of urea.