A population-specific formula predicting creatinine excretion in continuous peritoneal dialysis.

Objective: The Cockroft-Gault formula was shown to systematically overestimate the decline in creatinine excretion with age in continuous peritoneal dialysis (CPD) patients and is, therefore, not suitable for studying creatinine excretion. The purpose of the present study was to develop and test a population-specific formula predicting average creatinine excretion in CPD.
Methods: Creatinine excretion in urine plus dialysate was measured in 925 CPD patients. Forty patients were excluded because of evidence of noncompliance. The remaining 885 subjects were randomly grouped into a derivation group (n = 432) and a validation group (n = 453). Stepwise multiple linear regression models were used to predict creatinine excretion in the derivation group. The candidate variables, chosen because they were previously shown to be predictors of creatinine excretion in CPD, included weight (W), age (A), gender (G), diabetes (D), and interaction terms between these four variables. Estimates of creatinine excretion from the best-fit regression formula (CrExcr1) and from the Cockroft-Gault formula (CrExcr2) were compared to creatinine excretion (CrExcr) in the validation group.
Results: The best-fit regression model in the derivation group included all four candidate variables (W, A, G, D), but no interaction terms. This model was as follows: CrExcr1 = 302.150 - 4.380A + 171.234G - 39.041D + 11.730W (r2 = 0.477, p < 0.001). In the validation set, CrExcr = -15.795 + 0.988CrExcr1 (r2 = 0.447, p < 0.001), and CrExcr = -303.823 + 0.732CrExcr2 (r2 = 0.340, p < 0.001). When the differences between measured and predicted creatinine excretion did not take into account the sign of each individual difference, CrExcr - CrExcr1 = 201 +/- 156 mg/24 hours, and CrExcr - CrExcr2 = 235 +/- 174 mg/24 hr (p < 0.001) in the validation group. When the sign of the difference was taken into account, CrExcr - CrExcr1 = -28 +/- 149 mg/24 hr, and CrExcr - CrExcr2 = 63 +/- 295 mg/24 hr (p < 0.001).
Conclusions: A population-specific formula predicting creatinine excretion in CPD was derived. This formula has greater accuracy than the Cockroft-Gault formula and can be used in studies of creatinine excretion in CPD.