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.

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.

The determination ofthe so-called 'ethereal sulphate content' of urine is a simple analytical procedure, and there are many reports in the literature which show that the administration of phenols to experimental animals is followed by an increased excretion ofethereal sulphates in the urine. On the other hand,

Glomerular filtration rate (GFR) and urine and serum concentrations of cystatin C and creatinine were measured in 40 boys and 42 girls. The fractional excretion of cystatin C (FE Cyst C) increased in proportion to the decrease in GFR. Since serum creatinine concentration (S-Creatinine) in the numerator of the fractional excretion equation and serum cystatin C concentration (S-Cystatin C) in the denominator have similar numerical values, they cancel out. The result is an equation in which the FE Cyst C is equal to the ratio of urinary cystatin C to urinary creatinine (u[cystatin-C/Cr]). The ratio of u[cystatin C/Cr] was compared with GFR. Using a receiving operating characteristic (ROC) plot, the data showed that a ratio of u[cystatin C/Cr]*100 that is > or =0.100 has a sensitivity of 90.0% for identification of children with GFR < or =60 ml/min per 1.73 m(2). The false-positive rate is 16.1%. The u[cystatin C/Cr] ratio is a reliable screening tool for detecting decreased GFR that does not require a serum sample.