Derived equations are not precise enough to predict the adequacy of creatinine clearance in peritoneal dialysis patients

Background:Most renal units assess dialysis adequacy in peritoneal dialysis (PD) patients by formal 24-hour collections of urine and effluent dialysate. We sought a reliable method of predicting dialysis adequacy that allows a decrease in the frequency of these formal and cumbersome measurements. Methods:We created a formula for estimating total creatinine clearances, then assessed the clinical utility of this formula and other published formulae in predicting adequate and inadequate dialysis in PD patients. We collected data over a 6-month period in 2001 from 288 PD patients from 9 centers in Scotland. Four out of every 5 patients were selected at random to create a formula for estimating total creatinine clearance per week, and the fifth patient was used to form a validation group. We plotted creatinine excretion against age, and the resultant linear regression equation was transformed to produce a formula for predicting total creatinine clearance per week, based on patient sex, weight, and serum creatinine. We used the data from the validation subgroup to calculate predictive values for our derived formula and data from all of the patients to calculate predictive values for the Cockcroft and Gault, Jones, and Modification of Diet in Renal Disease Study formulae. Results:Neither our derived formula nor the three published formulae were sufficiently powerful to predict accurately either adequate or inadequate PD clearance. Receiver operator characteristic curves showed that no significant improvement in these predictive values could be achieved by altering either the sensitivity or the specificity. Conclusion:Prediction formulae are not accurate enough to detect underdialysis in PD patients. Am J Kidney Dis40:1036-1044. © 2002 by the National Kidney Foundation, Inc.