Optimizing iohexol plasma disappearance curves to measure the glomerular filtration rate in children with chronic kidney disease.

Measuring the glomerular filtration rate (GFR) by iohexol plasma disappearance in children requires optimization of duration and sampling times since shortening the study may overestimate GFR. To determine these parameters, we analyzed iohexol plasma disappearance curves using multiple sampling points in each of 27 children in the Chronic Kidney Disease in Children (CKiD) study. The GFR measured after 5 h was comparable to that after 6 h, but shortening to 4 h resulted in a significant 3% overestimation of GFR. We also sought to reduce the number of blood samples to determine GFR. This was done by adapting the Brochner-Mortensen equations to derive the relationship between a single-compartment two-sample (slow GFR model) disappearance curve and that from a double exponential analysis (two-compartment four-sample model) in the first clinic visit of 489 children. Using polynomial regression methods, we developed coefficients to accurately measure GFR from a single-compartment model. These coefficients were employed to recalculate the GFR and compare these to values measured with the two-compartment four-sample model in 361 of these children in their second clinic visit. There was excellent correlation (r=0.999) and no bias or change in between-individuals' dispersion. Hence, the GFR can be accurately measured in children with chronic kidney disease using the slow component of the iohexol plasma disappearance curve, provided the duration of study is at least 5 h.