Creatinine excretion rates for renal clearance studies.

A total of 637 timed-urine collections for creatinine excretion rates obtained from 295 children over 14 years have been analyzed. The children ranged in age from 2.8 to 21.7 years at the time of the clearance study. The data analyzed included only one study from a child during any 6-month period. The objective is to provide data defining the expected range of creatinine excretion for renal clearance studies. One hundred forty-two studies were conducted on children not pretreated with cimetidine and 495 on those pretreated with cimetidine. Analysis showed that pretreatment with cimetidine for creatinine clearance studies does not alter creatinine excretion rates (P=0.080; 95% CI -0.03 to 1.61). Creatinine excretion rates in urine collections obtained at home (roughly 24-h collections) were compared with 2-h supervised collections in the Children's Kidney Center. The supervised urine collections resulted in creatinine excretion rates 1.38 mg/kg/24 h greater than home collections (P=0.001; 95% CI 0.76-2.00). Using regression equations for creatinine excretion rate with age, tables have been prepared showing the expected rate of creatinine excretion for renal clearance studies in children 3-21 years of age.

Creatinine clearance following cimetidine for estimation of glomerular filtration rate.

Simultaneous inulin (C[in]) and creatinine clearance (C[Cr]) studies were performed on 53 pediatric renal patients using a cimetidine protocol. Since cimetidine blocks the tubular secretion of creatinine, it was hypothesized that C(Cr) measured following cimetidine would closely approximate the C(in). C(in) was compared with C(Cr) with the latter calculated from: (1) a 24-h urine collection, (2) plasma creatinine, height, and a proportionality constant, (3) the same plasma and urine specimens used for calculating C(in), and (4) from the plasma and urine specimens of the four 30-min clearance periods treated as a single 2-h clearance. The C(in) was very closely approximated by the C(Cr) calculated from the same specimens used for the C(in) and by the 2-h clearance. The cimetidine protocol, with C(Cr) derived from a 2-h urine collection obtained under supervision in the office or clinic, provides a convenient and inexpensive procedure for estimation of glomerular filtration rate in a clinical setting.

The renal clearance and infusion clearance of inulin are similar, but not identical.

Thirty-eight simultaneous renal (R-Cin) and infusion (INF-Cin) clearances of inulin were done. The equilibration period preceding the clearance studies was of at least two hours duration. The R-Cin on each subject was based on two clearance periods during which the plasma inulin concentration ([P(in)]) varied by 1.0 mg/dl or less and the rate of inulin excretion by less than 10%. There was excellent correlation between the R-Cin and the INF-Cin (r = 0.976), but the INF-Cin consistently exceeded the R-Cin (mean difference = 13.8 +/- 8.8 ml/min/1.73 m2, t = 9.7163 and P = < 0.001). Complete equilibration of inulin in body fluids has been assumed when [P(in)] levels were relatively constant (variation < 10%). However, complete equilibration of inulin would not be present, even with relatively constant P(in) levels, if the rate of infusion of inulin were equal to the rate of excretion plus the rate of penetration of inulin into less permeable components of the extracellular fluid compartment (that is, dense connective tissue solids). Estimation of glomerular filtration rate using the INF-Cin requires complete equilibration of inulin in body fluids, a process probably requiring 12 to 15 hours or longer.