Creatinine excretion in continuous peritoneal dialysis: A systematic errorof the Cockroft-Gault formula

We investigated the hypothesis that the rate of loss of creatinine excretio n with age in peritoneal dialysis (PD) patients differs from the rate predi cted from the Cockroft-Gault formula (Cr-Pred) by analyzing creatinine excr etion data obtained from clearance studies of 925 patients on continuous am bulatory PD therapy with an age range of 12 to 91 years. Measured creatinin e generation (Cr-Meas) is the sum of creatinine excretion In urine plus dia lysate (Cr-Excr) plus an estimated metabolic degradation of creatinine. The effect of age on Cr-Excr and the differences Cr-Excr - Cr-Pred and Cr-Meas - Cr-Pred were analyzed by linear regression. In 373 women, Cr-Excr = W(16 .9360 - 0.084A), r = -0.342, P < 0.001 (where W is weight in kilograms and A is age in years). The regression slope was one half of the slope In the C ockroft-Gault formula. Cr-Excr-Cr-Pred = -413.91 + 4.78A, r= 0.300, P < 0.0 01. Cr-Maas - Cr-Pred = -176.36 + 4.37A, r= 0.278 P < 0.001. In 552 mend Cr -Excr W(21.079 - 0.108A), r = -0.338, P < 0.001. The regression slope was a pproximately one half of the slope in the Cockroft-Gault formula. Cr-Excr - Cr-Pred = -493.25 + 6.28A, r = 0.267, P < 0.001. Cr-Meas - Cr-Pred = -66.4 1 + 3.63A, r = 0.143, P = 0.001. The rate of loss of creatinine excretion w ith age is one half of the rate predicted by the Cockroft-Gault formula in both women and men on PD therapy. Therefore, the difference between excreti on (or measured generation) of creatinine and creatinine generation predict ed by the Cockroft-Gault formula is not constant, but increases with age. T he Cockroft-Gault formula systematically overestimates the effect of age on creatinine excretion in PD patients and is not suitable for predicting cre atinine excretion in these subjects. (C) 2001 by the National Kidney Founda tion, Inc.