Calculation of 6-Hour Dip Creatinine Ratio from the 4-Hour Peritoneal Equilibration Test. The Effect of Dwell Duration on the Results

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.

Prescription and Assessment of Tidal Peritoneal Dialysis Delivery with a Specific Equilibration Test

Objective

To propose a simplified equilibration test specific for tidal peritoneal dialysis (TPD) that will overcome the inconveniences of the measurement of TPD peritoneal solute clearances through whole dialysate collection. This will enable the prediction of peritoneal creatinine and urea clearances, the suitability of patients for TPD, and routine assessment of TPD delivery.

Design

In a prospective study, patients had a standardized TPD run, and dialysate-to-plasma (DIP) ratios for creatinine and urea were calculated at various TPD and peritoneal equilibration test (PET) time points and on total TPD dialysate. Solute clearances were estimated and measured, and correlation coefficients were obtained among all these variables.

Setting

Dialysis unit of a pediatric nephrology department and patients’ homes.

Patients

Eleven pediatric patients with end-stage renal disease in stable clinical conditions treated with TPD.

Interventions

Dialysate and blood sample collections.

Main Outcome Measures

DIP ratios for creatinine and urea at the fifth and seventh TPD exchanges, at 15–,30–,60, and 120-minute PETtimes, and on total TPD dialysate and TPD peritoneal creatinine and urea clearances.

Results

Correlation coefficients between PET -derived and total TPD dialysate-derived DIP ratios, and those between PET -derived and measured creatinine and urea clearances were more significant at the 120-minute PET time point compared with the other PET time points. Best correlations were obtained at the fifth and seventh TPD exchanges. DIP ratios for creatinine and urea of the fifth and seventh TPD exchanges correlated significantly with the DIP ratios calculated from total TPD dialysate. A significant correlation was also found between peritoneal creatinine and urea clearances on total dialysate volume (measured clearances) and those derived from the dialysate collection of the fifth and seventh TPD exchanges (estimated clearances) -that based on the seventh exchange being slightly more significant. Moreover, the estimated clearances derived from the seventh exchange were within 10% of the measured value in 90.9% of patients both for creatinine and urea.

Conclusion

The significant correlation between measured and estimated peritoneal creatinine and urea clearances and the low percentage of underestimates of measured clearances obtained using the seventh TPD exchange-derived indices confirm the accuracy of the DIP ratios for creatinine and urea derived from any exchange after the fifth (preferably the seventh) of a standardized TPD run in estimating peritoneal creatinine and urea clearances. This method could represent a simple and accurate means for prescribing TPD and routinely assessing TPD delivery.

Evaluation of peritoneal membrane permeability

Total removal of fluid and solutes during peritoneal dialysis depends on both the dialysis prescription (ie, the number, length, and timing of the dwells and the volume and contents of the dialysis solution) and the permeability of the peritoneal membrane. Peritoneal membrane permeability determines the rate of solute equilibration between body fluids and the solution within the peritoneal cavity and is, therefore, a significant determinant of the solute removal rate. The relationship between fluid removal during peritoneal dialysis and the properties of the peritoneal membrane is more complex; peritoneal ultrafiltration is inversely related to the permeability of the peritoneal membrane to osmotic solutes but is directly related to the hydraulic conductivity of the peritoneal membrane. The peritoneal equilibration test (PET) is widely used as the standard method for evaluating peritoneal membrane permeability, and results from the PET can be used to determine the type of peritoneal dialysis therapy optimal for the permeability characteristics of the patient's peritoneal membrane. Simple extrapolations from the PET can only provide qualitative estimates of the dialysis dose required for various types of peritoneal dialysis. More accurate estimates of the required dialysis dose can be provided by mathematical models of peritoneal fluid and solute transport, but these models require calculation of the permeability-area product or mass transfer-area coefficient of the peritoneal membrane. Although the latter approach shows promise for improving peritoneal dialysis prescriptions, determination of the delivered dose of peritoneal dialysis can, at present, only be assessed by directly measuring total 24-hour solute and fluid removal.