Creatinine excretion in continuous peritoneal dialysis: a systematic error of the Cockroft-Gault formula.

We investigated the hypothesis that the rate of loss of creatinine excretion with age in peritoneal dialysis (PD) patients differs from the rate predicted from the Cockroft-Gault formula (Cr(Pred)) by analyzing creatinine excretion data obtained from clearance studies of 925 patients on continuous ambulatory PD therapy with an age range of 12 to 91 years. Measured creatinine generation (Cr(Meas)) is the sum of creatinine excretion in urine plus dialysate (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 Cockroft-Gault formula. Cr(Excr) - Cr(Pred) = -413.91 + 4.78A, r = 0.300, P < 0.001. Cr(Meas) - Cr(Pred) = -176.36 + 4.37A, r = 0.278, P < 0.001. In 552 men, Cr(Excr) = W(21.079 - 0.108A), r = -0.338, P < 0.001. The regression slope was approximately 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) = -66.41 + 3.63A, r = 0.143, P = 0.001. The rate of loss of creatinine excretion with 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 excretion (or measured generation) of creatinine and creatinine generation predicted by the Cockroft-Gault formula is not constant, but increases with age. The Cockroft-Gault formula systematically overestimates the effect of age on creatinine excretion in PD patients and is not suitable for predicting creatinine excretion in these subjects.