Impact of total body water errors on Kt/V estimates in children on peritoneal dialysis.

Determining Kt/V in peritoneal dialysis (PD) requires estimation of total body water (TBW). The Dialysis Outcomes Quality Initiative (DOQI) guidelines recommend use of the Mellits and Cheek (MC) formulas for the estimation of TBW in children. However, the MC formulas were developed from healthy children and may not apply to children on PD. Re-evaluating the MC data with additional, recent data from healthy infants has led to the development of new formulas. In addition, and as part of a prospective study of children initiating PD, the Pediatric Peritoneal Dialysis Study Consortium (PPDSC) has directly measured TBW using H2[18O]. To assess the impact of various TBW estimates, KPDt/V values prospectively collected in 24 children were calculated using H2[18O]-measured TBW (O18), MC-derived TBW (MCD), and new-formula TBW (NEW). The mean weekly KPDt/V by O18 was 2.2; by MCD, it was 2.0; and by NEW, it was 2.0. The results derived using the O18 method varied from both the MCD and the NEW results (p < 0.001). The mean deviation from the measured KPDt/V using O18 was 9.5% (maximum: 16%) using the MCD estimate and 7.8% (maximum: 18%) using the NEW formulas. Determinations of KPDt/V are significantly affected by the method of estimating TBW. The PPDSC formulas for children on PD based on the use of H2[18O] offer the most accurate means of calculating TBW and should replace formulas derived from healthy children. The use of Kt/V itself as a marker of adequacy in children will be validated only in prospective studies.