Objectives Since the introduction of the peritoneal equilibration test (PET), the 4-hour dialysatelplasma creatinine (DIP Cr) has been used by several authors for determining continuous ambulatory peritoneal dialysis (CAPD) prescriptions. However, the results have been unsatisfactory because the 4-hr DIP Cr does not accurately reflect the DIP Cr in 24-hr collections. The PET and the 24-hr dialysate collections differ in the duration of dwell and the tonicity and volume of dialysate, all of which influence the equilibrated DIP Cr. It can be assumed that the DIP Cr in 24-hr collections in these patients is closer to a 6-hr DIP Cr. Because a 6hr PET is inconvenient, we developed a mathematical model to calculate the 5 and 6-hr DIP using the results of a standard PET. Design In a retrospective analysis, DIP Cr ratios in 24-hr collections and DIP Cr ratios calculated from a mathematical formula were correlated. Using a mathematical model, the data collected fit an exponential relation of the type DIP = a(1 -e-t/τ). The values of a and τ are unique for a given patient and were determined using a nonlinear regression technique. The formula performed well on our published data -the true and predicted 6-hr DIP Cr being 0.696 and 0.71, respectively. Setting The University Hospital and Clinics, Dalton Cardiovascular Research Center and Dialysis Clinic, Inc., Columbia, Missouri. Patients All CAPD patients on four 2-L exchangeslday at the time of the 24-hr collections were included. Interventions None. Main Outcome Measures Closeness of 4-hr and 6-hr DIP Cr values to those of 24-hr ratios. Results The study group comprised 74 patients (age, mean ± SEM: 56.4 ± 1.8 yr) with 80 PETs and 145 (24-hr) collections. The interval between the two tests was 8. 3 ± 0.9 months (0 48.7 months). The median 24-hr DIP Cr of 0.760 did not differ significantly from the predicted median 6-hr DIP Cr of 0.755. A subgroup analysis, based on transport type, showed that this relationship was most precise in the high-average transporters. The predicted 6hr DIP Cr was within 100;0 of the 24-hr DIP Cr in 48% of patients and within 20% in 77% of patients. The margin of error was greatest in the low transporters. Conclusions To conclude, the 4-hr DIP Cr from a PET cannot be used interchangeably with the DIP Cr in the 24-hr dialysate collections, hence, the clearances calculated thereof will be inaccurate. Using the proposed model, it is feasible to use the 4 -hr PET results to obtain 5 and 6 -hr DIP Cr values. In our study, using this model, the extrapolated 6-hr DIP Cr is similar to the DIP Cr in 24-hr dialysate collections only in the high-average transporters. Hence, the best way to determine clearances in peritoneal dialysis patients is still by collecting 24-hr dialysates.
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